Working on SD and netowrking

docs/audio_logging.md

@@ -0,0 +1,350 @@
+# Audio Logging Design
+
+This document defines how the Hub writes the audio it collects from the
+Nodes to the SD card. It is the contract that the future Node-collection task
+(producer) and the existing `StorageTask` (consumer) are written against.
+
+The deliverable for this design is:
+
+* `src/storage/data_logger.h` - the public interface (skeleton).
+* `src/storage/data_logger.cpp` - stub bodies + fully implemented WAV metadata
+  helpers. The chunk-pool/queue/file logic is marked `TODO` and is the
+  remaining work.
+* The constants in `src/storage/storage_config.h` under the "Audio logging"
+  section.
+
+---
+
+## 1. Data model
+
+| Parameter             | Value      |
+|-----------------------|------------|
+| Sample rate           | 48 kHz     |
+| Bit depth             | 16 bit (signed little-endian PCM) |
+| Microphones per node  | 4          |
+| Max nodes             | 10         |
+| **Total channels**    | **40**     |
+
+One microphone == one channel == one "track". A single file holds all of them
+as an interleaved multichannel WAV.
+
+## 2. Throughput budget
+
+```
+raw stream rate = 40 ch x 48 kHz x 2 B = 3,840,000 B/s  (3.66 MiB/s)
+```
+
+* SDIO 4-bit @ 40 MHz sustains roughly 8-12 MB/s -> the card write has ample
+  headroom at less than half of its budget.
+* WiFi (AP) carries the same 3.84 MB/s inbound as UDP payload, which the
+  design already budgets for.
+* The 128 KB chunk pool represents **~34 ms** of audio. The write must be able
+  to absorb bursts longer than that only by dropping (Section 8).
+
+## 3. File format: single multichannel WAV
+
+The stream is a **RIFF/WAVE file with 40 interleaved channels**. The 44-byte
+header is the only metadata; no per-chunk headers are written to the file.
+
+Header layout (all little-endian):
+
+| Offset | Size | Value                                      |
+|--------|------|--------------------------------------------|
+| 0      | 4    | `"RIFF"`                                   |
+| 4      | 4    | chunk size = file size - 8 (patched)       |
+| 8      | 4    | `"WAVE"`                                   |
+| 12     | 4    | `"fmt "`                                   |
+| 16     | 4    | 16 (PCM fmt chunk size)                    |
+| 20     | 2    | 1 (PCM)                                    |
+| 22     | 2    | 40 (channels)                              |
+| 24     | 4    | 48000 (sample rate)                        |
+| 28     | 4    | 3,840,000 (byte rate = rate x blockAlign)  |
+| 32     | 2    | 80 (block align = channels x 2)            |
+| 34     | 2    | 16 (bits per sample)                       |
+| 36     | 4    | `"data"`                                   |
+| 40     | 4    | data size = file size - 44 (patched)       |
+
+`buildWavHeader()` and `finalizeWavHeader()` in `data_logger.h/cpp` produce
+these bytes and the two 32-bit values patched at offsets 4 and 40.
+
+### Header finalization
+
+The sizes at offsets 4 and 40 are unknown while the file is being written.
+They are patched when the file is closed (rotation or session end):
+
+```
+file.seek(4);  file.write(riffSize, 4);
+file.seek(40); file.write(dataSize, 4);
+```
+
+### Crash tolerance
+
+If power is lost mid-file, the header still contains the placeholder sizes.
+PCM decoders that read to EOF (Audacity, ffmpeg, Python `wave`) play the valid
+audio regardless; the last partial sector may be zero-padded. This satisfies
+the design.md requirement that a mid-recording power loss still yields usable
+audio.
+
+### Opening the file on a PC
+
+* **Audacity** / **ffmpeg** / **Python `wave`**: handle 40 channels.
+* VLC and stock Windows players generally will not render 40 channels even
+  though the file is structurally valid. Convert with ffmpeg first if needed.
+
+## 4. Architecture: producer / consumer with a chunk pool
+
+```
+ Node collection task (core 1)                 StorageTask (core 0)
+   assembles one round of 40 channels              |  loop:
+   interleaved into frame order                    v
+   +----------------------+                 +----------------------+
+   | c = acquireChunk(5ms)|  <-- freeQ ---- |   pool: 8 x 16 KB    |
+   | fill c->data[...]    |                 |   (MALLOC_CAP_DMA)   |
+   | submitChunk(c)       | ---- filledQ -->|   nextChunk(1000ms)  |
+   +----------------------+                 |   writeChunk(c)      |
+                                            |   releaseChunk(c)    |
+                                            |   rotateIfNeeded()   |
+                                            |   updateStats()      |
+                                            +----------------------+
+
+ DataLogger owns both queues and the pool. Producers and the consumer never
+ touch the file or the card directly - only DataLogger does.
+```
+
+* **Pool**: 8 chunks x 16 KB = 128 KB, allocated once in `begin()` with
+  `heap_caps_malloc(MALLOC_CAP_DMA)`.
+* **Queues**: `freeQ` holds pointers to empty chunks, `filledQ` holds pointers
+  to chunks waiting to be written. Both are FreeRTOS queues of
+  `AudioChunk*`.
+* **Cores**: the producer runs on core 1 (network), the consumer on core 0
+  (the existing pinned `StorageTask`). SD card work never touches core 1.
+
+## 5. The interface
+
+Types (from `data_logger.h`):
+
+```cpp
+struct AudioChunk
+{
+    uint8_t* data;      // DMA-capable buffer owned by the pool
+    uint32_t capacity;  // buffer size in bytes (STORAGE_LOG_CHUNK_SIZE)
+    uint32_t length;    // valid bytes written by the producer
+    uint32_t sequence;  // monotonic order, for diagnostics only
+};
+
+class DataLogger
+{
+public:
+    bool begin(fs::FS& files);   // allocate pool, create log dir
+    void end();                  // finalize current file, free pool
+
+    bool openSession();          // create rec_<uptime>_<n>.wav
+    bool closeSession();         // finalize + close current file
+    bool rotateIfNeeded();       // called by the consumer
+
+    // Producer (core 1)
+    AudioChunk* acquireChunk(TickType_t timeout);
+    void        submitChunk(AudioChunk* chunk);
+
+    // Consumer (core 0)
+    AudioChunk* nextChunk(TickType_t timeout);
+    bool        writeChunk(AudioChunk* chunk);
+    void        releaseChunk(AudioChunk* chunk);
+
+    // Stats
+    uint32_t bytesWritten() const;
+    uint32_t chunksWritten() const;
+    uint32_t droppedChunks() const;
+    uint32_t writeSpeedBps() const;
+    bool     overflowing() const;
+};
+```
+
+### Producer contract (Node collection task, not yet written)
+
+```cpp
+AudioChunk* c = logger.acquireChunk(pdMS_TO_TICKS(5));
+if (c == nullptr) { /* collection is behind; drop this round */ return; }
+
+// Fill c->data[0..c->length) with interleaved PCM, frame order:
+//   frame0: ch0..ch39, frame1: ch0..ch39, ...
+// 40 samples of 2 bytes per frame = 80 bytes per frame.
+c->length = interleavedBytes;
+c->sequence = nextSeq++;
+logger.submitChunk(c);
+```
+
+* Chunks **must be submitted in strict stream order** (single producer, FIFO
+  queue -> order is preserved automatically).
+* A chunk is not required to end on a frame boundary, but `c->length` should
+  stay a multiple of 80 bytes so the file is always frame-aligned.
+* `acquireChunk` can return a chunk even when the pool is momentarily empty;
+  see the drop-oldest rule in Section 8.
+
+### Consumer contract (StorageTask on core 0, future)
+
+```cpp
+while (true)
+{
+    AudioChunk* c = logger.nextChunk(pdMS_TO_TICKS(1000));
+    if (c != nullptr)
+    {
+        logger.writeChunk(c);   // one file.write(c->data, c->length)
+        logger.releaseChunk(c); // returns the chunk to freeQ
+    }
+    logger.rotateIfNeeded();
+    logger.updateStats(storageState);  // bps + drops -> dashboard
+}
+```
+
+* The 1 s timeout lets the loop pulse for stats/capacity updates even when no
+  audio is flowing (this replaces the current 1 s `vTaskDelayUntil` heartbeat).
+* `writeChunk` is the only place the card is touched. It appends the chunk's
+  PCM directly to the WAV data section in one `file.write`.
+
+### Ownership rules
+
+* The pool owns the buffers; producers and the consumer borrow them.
+* A borrowed chunk is in exactly one place at a time: a producer (between
+  acquire and submit), a queue, or the consumer (between next and release).
+* `releaseChunk` returns it to `freeQ`. `writeChunk` never frees.
+
+## 6. Upstream interleaving contract
+
+The WAV must be written frame-interleaved, but the Nodes deliver one UDP
+packet per Node (4 contiguous mono channels). Frames cannot be interleaved
+until all Nodes in a round have been collected. That re-ordering is the job of
+the collection task, **not** the logger:
+
+1. Collect all 10 Node dumps for round `n`.
+2. A missing/offline Node contributes silence: zero-fill its 4 channels.
+3. Re-order into frame order: `frame j = node0[mic0..3], node1[mic0..3], ...`
+4. Feed the resulting bytes into chunks and submit them in order.
+
+Consequences the collection task must honor:
+
+* **Round size**: with the 128 KB pool, a round must stay <= ~96 KB
+  (~25 ms of audio) so the pool can hold more than one round. See the sizing
+  math in Section 7.
+* **Silence for offline Nodes** comes from this zero-fill; the logger never
+  invents data.
+
+## 7. Chunk pool and RAM budget
+
+```
+chunk size  = STORAGE_LOG_CHUNK_SIZE  = 16 KiB
+pool depth  = STORAGE_LOG_POOL_SIZE   = 8
+pool total  = 128 KiB  (MALLOC_CAP_DMA)
+
+buffering   = 128 KiB / 3.84 MB/s  ~= 34 ms
+round size  = 48 kHz x 2 B x 40 ch x round_s
+  25 ms round -> 96 KB  (~6 chunks)   [fits the pool with slack]
+  50 ms round -> 192 KB                [does NOT fit - must shrink]
+```
+
+The collection round duration is therefore bounded by the pool unless the pool
+grows. Keep rounds at <= 25 ms, or raise `STORAGE_LOG_POOL_SIZE`/chunk size and
+re-run the math.
+
+The round's interleave buffer lives in the **collection task's own RAM** (up
+to 96 KB), not in the logger pool; it is freed after the round is submitted.
+
+## 8. Overflow: drop-oldest + warning
+
+If the SD card cannot keep up, the pool drains and the producer has no chunk.
+Policy (chosen): **drop the oldest buffered chunk until caught up** - never
+block, never halt.
+
+Mechanism inside `acquireChunk`: on timeout, the logger pops one chunk off the
+back of `filledQ` (the oldest unwritten data), returns it to `freeQ`, hands it
+to the producer, increments `droppedChunks`, and raises the overflow warning.
+The stream keeps flowing with the newest data at the cost of a gap.
+
+The warning is visible two ways:
+
+* **Serial**: a rate-limited `[Logger] X chunks dropped, Y MB behind` line.
+* **LED on GPIO `STORAGE_WARN_LED_GPIO`** (default 4, active-high): on while
+  `overflowing()`, off once the queue drains below a low-water mark again.
+
+## 9. Rotation, naming, flush
+
+* **Rotate by size**: when `bytesThisFile >= STORAGE_LOG_ROTATE_BYTES`
+  (default 1 GiB, ~4.6 min at 3.84 MB/s) the consumer calls `rotateIfNeeded()`:
+  finalize + close the current file, open the next.
+* **Files**: created under `STORAGE_LOG_DIR` (`/sdcard/audio`), named
+  `rec_<uptimeSeconds>_<n>.wav`. `n` increments per rotation within a boot;
+  the uptime prefix keeps names unique across boots. If a name already exists,
+  skip forward until it does not (never overwrite).
+* **Flush**: `file.flush()` (f_sync) every `STORAGE_LOG_FLUSH_BYTES`
+  (default 16 MiB, ~4.3 s) so an unclean power-off loses at most that window
+  and never corrupts earlier data.
+* **Card full**: rotation cannot create a file -> report a fatal error, light
+  the warning LED solid, and halt (matches design.md's SD-failure stance).
+
+## 10. DMA requirements
+
+The SDIO path (production) uses the SDMMC controller's internal IDMA engine:
+block data moves to the card without CPU cycles. Two rules make this work:
+
+* Buffers must be in DMA-capable memory: allocate the pool with
+  `heap_caps_malloc(MALLOC_CAP_DMA)` (guarantees internal DRAM + alignment).
+* Never hand the card a buffer that lives in PSRAM or a stack array.
+
+The SPI path (`STORAGE_IFACE_SPI`, current bring-up) does **not** use DMA -
+the Arduino SPI driver busy-waits the FIFO. It is bring-up only and cannot
+sustain the 3.84 MB/s target; production logging must run on SDIO.
+
+## 11. Metering -> dashboard
+
+`writeChunk` accumulates `bytesWritten`/`chunksWritten`; `writeSpeedBps` is
+derived from a sliding 1 s window of real writes. The consumer publishes
+`writeSpeedBps` and `droppedChunks` into `StorageState` each loop, replacing
+the boot-time `measureWriteSpeed()` benchmark (whose interval is already 0).
+Add a `droppedChunks` field to `StorageSnapshot` and surface it on the
+dashboard when the logging loop lands.
+
+## 12. Future integration points (StorageTask)
+
+When the consumer loop is implemented, `StorageTask::run()`:
+
+1. After mount: `logger.begin(storage.fs())`, `logger.openSession()`.
+2. Replace the 1 s heartbeat loop with the consumer loop in Section 5.
+3. Stop calling `measureWriteSpeed()` (the logger provides real bps).
+
+## 13. SPI -> SDIO migration checklist
+
+Already documented in `src/storage/storage_config.h`. Restated for logging:
+
+1. `STORAGE_IFACE` SPI -> SDMMC.
+2. Wire SD to `STORAGE_SDMMC_*` pins (freely re-routable on classic ESP32).
+3. Keep 4-bit mode (`STORAGE_SDMMC_MODE_1BIT == false`) - 1-bit halves the
+   throughput margin.
+4. `MALLOC_CAP_DMA` buffers work unchanged; nothing else moves.
+
+## 14. Open TODOs (to finish the feature)
+
+- [ ] `data_logger.cpp`: chunk pool + queue creation in `begin()`.
+- [ ] `data_logger.cpp`: `acquireChunk` drop-oldest path + overflow warning
+      (Serial + LED on `STORAGE_WARN_LED_GPIO`).
+- [ ] `data_logger.cpp`: `writeChunk`/`rotateIfNeeded`/`closeSession` file
+      handling + header finalization + flush cadence.
+- [ ] `data_logger.cpp`: `writeSpeedBps` sliding window.
+- [ ] `StorageTask`: consumer loop (Section 5).
+- [ ] `StorageState`/dashboard: `droppedChunks` field.
+- [ ] Node collection task: round assembly + interleave + zero-fill
+      (Section 6).
+
+## Appendix: constants
+
+| Constant                          | Default      | Meaning                         |
+|-----------------------------------|--------------|---------------------------------|
+| `STORAGE_AUDIO_SAMPLE_RATE_HZ`    | 48000        | WAV sample rate                 |
+| `STORAGE_AUDIO_CHANNELS`          | 40           | = nodes x mics, WAV channels    |
+| `STORAGE_AUDIO_BITS`              | 16           | WAV bit depth                   |
+| `STORAGE_LOG_CHUNK_SIZE`          | 16 * 1024    | pool chunk size (bytes)         |
+| `STORAGE_LOG_POOL_SIZE`           | 8            | pool chunk count                |
+| `STORAGE_LOG_DIR`                 | "/sdcard/audio" | recording directory          |
+| `STORAGE_LOG_ROTATE_BYTES`        | 1 GiB        | rotate when a file reaches this |
+| `STORAGE_LOG_FLUSH_BYTES`         | 16 MiB       | f_sync cadence                  |
+| `STORAGE_WARN_LED_GPIO`           | 4            | overflow warning LED            |
+| `STORAGE_WARN_LED_ACTIVE_HIGH`    | true         | LED polarity                    |

src/core/diagnostics_state.cpp

@@ -12,12 +12,6 @@ void DiagnosticsState::update()
 
     current.cpuFrequency =
         ESP.getCpuFreqMHz();    
-
-    Serial.print("#update: ");
-    Serial.println(current.freeHeap);
-    Serial.println("Calling current from update");
-    getCurrent();
-    
 }
 
 
@@ -29,9 +23,5 @@ DiagnosticSample DiagnosticsState::getCurrent()
     sample.freeHeap = current.freeHeap;
     sample.minimumFreeHeap = current.minimumFreeHeap;
     sample.cpuFrequency = current.cpuFrequency;
-
-    Serial.print("#getCurrent: ");
-    Serial.println(sample.freeHeap);
-    Serial.println(sample.timestamp);
     return sample;
 }

src/core/storage_state.cpp

@@ -0,0 +1,67 @@
+#include "storage_state.h"
+
+
+void StorageState::begin()
+{
+    current.mounted = false;
+    current.timestamp = 0;
+    current.totalMB = 0;
+    current.usedMB = 0;
+    current.freeMB = 0;
+    current.writeSpeedBps = 0;
+    current.cardType[0] = '\0';
+}
+
+
+void StorageState::setMounted(bool mounted)
+{
+    current.mounted = mounted;
+    current.timestamp = millis();
+}
+
+
+void StorageState::setCardType(const char* type)
+{
+    size_t i = 0;
+    for (i = 0; i < sizeof(current.cardType) - 1 && type[i] != '\0'; i++)
+    {
+        current.cardType[i] = type[i];
+    }
+    current.cardType[i] = '\0';
+}
+
+
+void StorageState::setCapacity(uint64_t totalBytes, uint64_t usedBytes)
+{
+    uint64_t freeBytes = (totalBytes > usedBytes) ? (totalBytes - usedBytes) : 0;
+
+    current.totalMB = (uint32_t)(totalBytes >> 20);
+    current.usedMB  = (uint32_t)(usedBytes >> 20);
+    current.freeMB  = (uint32_t)(freeBytes >> 20);
+}
+
+
+void StorageState::setWriteSpeedBps(uint32_t bytesPerSecond)
+{
+    current.writeSpeedBps = bytesPerSecond;
+}
+
+
+StorageSnapshot StorageState::getCurrent()
+{
+    StorageSnapshot snapshot;
+
+    snapshot.mounted = current.mounted;
+    snapshot.timestamp = current.timestamp;
+    snapshot.totalMB = current.totalMB;
+    snapshot.usedMB = current.usedMB;
+    snapshot.freeMB = current.freeMB;
+    snapshot.writeSpeedBps = current.writeSpeedBps;
+
+    for (size_t i = 0; i < sizeof(snapshot.cardType); i++)
+    {
+        snapshot.cardType[i] = current.cardType[i];
+    }
+
+    return snapshot;
+}

src/core/storage_state.h

@@ -0,0 +1,35 @@
+#pragma once
+
+#include <Arduino.h>
+
+
+// Cross-task snapshot of the SD card, filled by the StorageTask (core 0) and
+// read by the WebService (core 1) for the dashboard broadcast. Values are
+// stored as 32-bit quantities (MB / Bps) so each field reads atomically.
+struct StorageSnapshot
+{
+    bool mounted;
+    uint32_t timestamp;
+    uint32_t totalMB;
+    uint32_t usedMB;
+    uint32_t freeMB;
+    uint32_t writeSpeedBps;
+    char cardType[16];
+};
+
+
+class StorageState
+{
+public:
+    void begin();
+
+    void setMounted(bool mounted);
+    void setCardType(const char* type);
+    void setCapacity(uint64_t totalBytes, uint64_t usedBytes);
+    void setWriteSpeedBps(uint32_t bytesPerSecond);
+
+    StorageSnapshot getCurrent();
+
+private:
+    volatile StorageSnapshot current;
+};

src/main.cpp

@@ -5,25 +5,30 @@
 
 #include "core/dashboard_state.h"
 #include "core/diagnostics_state.h"
+#include "core/storage_state.h"
 
 #include "services/service_manager.h"
 #include "services/wifi_service.h"
 #include "services/ota_service.h"
 #include "services/web_service.h"
 
+#include "storage/sd_manager.h"
+
 #include "tasks/system_task.h"
 #include "tasks/diagnostics_task.h"
+#include "tasks/storage_task.h"
 
 
 DashboardState dashboardState;
 DiagnosticsState diagnosticsState;
+StorageState storageState;
 
 // Simple service scheduler nice for grouping tasks
 ServiceManager services;
 
 WiFiService wifi;
 OTAService ota;
-WebService web(dashboardState, diagnosticsState);
+WebService web(dashboardState, diagnosticsState, storageState);
 
 
 // Actual FreeRTOS tasks that are scheduled
@@ -31,6 +36,9 @@ SystemTask systemTask(services);
 
 DiagnosticsTask diagnosticsTask(diagnosticsState);
 
+SDManager sdManager;
+StorageTask storageTask(sdManager, storageState);
+
 void setup()
 {
     Serial.begin(115200);
@@ -42,6 +50,7 @@ void setup()
 
     systemTask.start();
     diagnosticsTask.start();
+    storageTask.start();
 }
 
 

src/services/web_service.cpp

@@ -1,10 +1,11 @@
 #include "web_service.h"
 
-WebService::WebService(DashboardState& state, DiagnosticsState& diag_state)
+WebService::WebService(DashboardState& state, DiagnosticsState& diag_state, StorageState& storage_state)
     :
     Service("Web", 10),
     dashboardState(state),
     diagnosticsState(diag_state),
+    storageState(storage_state),
     server(80),
     webSocket(81)
 {
@@ -64,6 +65,11 @@ h1 {
   font-size: 28px;
   margin-top: 20px;
 }
+
+#sd_status {
+  font-size: 24px;
+  font-weight: bold;
+}
 </style>
 
 <script>
@@ -82,11 +88,13 @@ function connectWebSocket() {
     socket.onmessage = function(event) {
         const data = JSON.parse(event.data);
 
-        document.getElementById("uptime").innerHTML = data.system.uptime;
+        document.getElementById("uptime").innerHTML = "Uptime: " + data.system.uptime;
-        document.getElementById("firmware_version").innerHTML = data.system.version;
+        document.getElementById("firmware_version").innerHTML = "Firmware Version: " + data.system.version;
-        document.getElementById("free_heap").innerHTML = data.diagnostics.free_heap;
+        document.getElementById("free_heap").innerHTML = "Free Heap: " + data.diagnostics.free_heap;
-        document.getElementById("minimum_free_heap").innerHTML = data.diagnostics.minimum_free_heap;
+        document.getElementById("minimum_free_heap").innerHTML = "Minimum Free Heap: " + data.diagnostics.minimum_free_heap;
-        document.getElementById("cpu_frequency").innerHTML = data.diagnostics.cpu_frequency;
+        document.getElementById("cpu_frequency").innerHTML = "CPU Frequency: " + data.diagnostics.cpu_frequency + "MHz";
+
+        updateStorage(data.storage);
     };
 
     socket.onclose = function() {
@@ -95,6 +103,37 @@ function connectWebSocket() {
     };
 }
 
+function formatBytesMB(mb) {
+    const value = Number(mb);
+    if (value >= 1024) {
+        return (value / 1024).toFixed(2) + " GB";
+    }
+    return value.toFixed(0) + " MB";
+}
+
+function updateStorage(storage) {
+    const sdStatus = document.getElementById("sd_status");
+
+    if (storage && storage.mounted === "true") {
+        sdStatus.innerHTML = "SD card found";
+        sdStatus.style.color = "#00ff99";
+
+        document.getElementById("sd_type").innerHTML = "Card Type: " + storage.card_type;
+        document.getElementById("sd_free").innerHTML = "Space left: " + 
+            formatBytesMB(storage.free_mb) + " free of " + formatBytesMB(storage.total_mb) +
+            " (" + Math.round(storage.used_mb / storage.total_mb * 100) + "% used)";
+        document.getElementById("sd_speed").innerHTML = "Estimated write speed: " + 
+            (storage.write_speed_bps / 1048576).toFixed(2) + " MB/s";
+    } else {
+        sdStatus.innerHTML = "SD not found";
+        sdStatus.style.color = "red";
+
+        document.getElementById("sd_type").innerHTML = "-";
+        document.getElementById("sd_free").innerHTML = "-";
+        document.getElementById("sd_speed").innerHTML = "-";
+    }
+}
+
 window.onload = connectWebSocket;
 
 </script>
@@ -104,23 +143,27 @@ window.onload = connectWebSocket;
 <body>
 <div class="card">
 <h1>ESP32 Dashboard OTA</h1>
-<p>Device uptime:</p>
+<div id="uptime">Device uptime: Loading...</div>
-<div id="uptime">Loading...</div>
 </div>
 
 <div class="card">
 <h1>Hub Diagnostics:</h1>
-<p>Free heap:</p>
+<div id=free_heap>Free heap: Loading...</div>
-<div id=free_heap>Loading...</div>
+<div id=minimum_free_heap>Minimum free heap: Loading...</div>
-<p>Minimum free heap:</p>
+<div id=cpu_frequency>CPU Frequency: Loading...</div>
-<div id=minimum_free_heap>Loading...</div>
+</div>
-<p>CPU Frequency:</p>
+
-<div id=cpu_frequency>Loading...</div>
+<div class="card">
+<h1>SD Storage:</h1>
+<div id=sd_status>Status: Loading...</div>
+<div id=sd_type>Card type: Loading...</div>
+<div id=sd_free>Space left: Loading...</div>
+<div id=sd_speed>Estimated write speed: Loading...</div>
 </div>
 
 </body>
 <footer>
-Firmware Version: <div id="firmware_version">Loading...</div>
+<div id="firmware_version">Firmware Version: Loading...</div>
 </footer>
 </html>
 )rawliteral";
@@ -128,7 +171,7 @@ Firmware Version: <div id="firmware_version">Loading...</div>
 void WebService::broadcastState()
 {
     DiagnosticSample diagnostics = diagnosticsState.getCurrent();
-    Serial.println(diagnostics.freeHeap);
+    StorageSnapshot storage = storageState.getCurrent();
     String json = "{";
 
     // Opening system tag:
@@ -162,6 +205,35 @@ void WebService::broadcastState()
 
     json += "}"; // Clost diagnostic tag
 
+    // Opening storage tag:
+    json += ",\"storage\":{";
+
+    json += "\"mounted\":\"";
+    json += storage.mounted ? "true" : "false";
+    json += "\",";
+
+    json += "\"card_type\":\"";
+    json += storage.cardType;
+    json += "\",";
+
+    json += "\"total_mb\":\"";
+    json += storage.totalMB;
+    json += "\",";
+
+    json += "\"free_mb\":\"";
+    json += storage.freeMB;
+    json += "\",";
+
+    json += "\"used_mb\":\"";
+    json += storage.usedMB;
+    json += "\",";
+
+    json += "\"write_speed_bps\":\"";
+    json += storage.writeSpeedBps;
+    json += "\"";
+
+    json += "}"; // Close storage tag
+
 
     // Final close bracket
     json += "}";

src/services/web_service.h

@@ -7,12 +7,13 @@
 
 #include "../core/dashboard_state.h"
 #include "../core/diagnostics_state.h"
+#include "../core/storage_state.h"
 
 class WebService : public Service {
 
 public:
 
-    WebService(DashboardState& state, DiagnosticsState& diag_state);
+    WebService(DashboardState& state, DiagnosticsState& diag_state, StorageState& storage_state);
 
     void begin() override;
     void update() override;
@@ -24,6 +25,7 @@ private:
 
     DashboardState& dashboardState;
     DiagnosticsState& diagnosticsState;
+    StorageState& storageState;
 
     void handleWebSocketMessage(uint8_t clientNum, uint8_t *payload, size_t length);
     void broadcastState();

src/storage/data_logger.cpp

@@ -0,0 +1,175 @@
+#include "data_logger.h"
+
+#include <string.h>
+
+// ============================================================================
+//  Skeleton implementation. The WAV metadata helpers below are complete;
+//  everything else is stubbed with a TODO and the work is tracked in
+//  docs/audio_logging.md section 14.
+// ============================================================================
+
+
+// --- WAV metadata helpers (complete) -----------------------------------------
+
+void buildWavHeader(WavHeader& header,
+                    uint16_t numChannels,
+                    uint32_t sampleRate,
+                    uint16_t bitsPerSample,
+                    uint32_t dataSize)
+{
+    uint16_t blockAlign = numChannels * (bitsPerSample / 8);
+
+    memset(&header, 0, sizeof(header));
+
+    memcpy(header.riff, "RIFF", 4);
+    header.riffSize     = sizeof(header) + dataSize - 8;
+    memcpy(header.wave, "WAVE", 4);
+
+    memcpy(header.fmt, "fmt ", 4);
+    header.fmtChunkSize = 16;
+    header.audioFormat  = 1; // PCM
+    header.numChannels  = numChannels;
+    header.sampleRate   = sampleRate;
+    header.byteRate     = sampleRate * blockAlign;
+    header.blockAlign   = blockAlign;
+    header.bitsPerSample = bitsPerSample;
+
+    memcpy(header.data, "data", 4);
+    header.dataSize     = dataSize;
+}
+
+
+void finalizeWavHeader(uint32_t fileSize,
+                       uint32_t& riffSizeOut,
+                       uint32_t& dataSizeOut)
+{
+    riffSizeOut = fileSize - 8;
+    dataSizeOut = fileSize - sizeof(WavHeader);
+}
+
+
+// --- DataLogger ---------------------------------------------------------------
+
+DataLogger::DataLogger()
+:
+files(nullptr),
+pool(nullptr),
+freeQ(nullptr),
+filledQ(nullptr),
+sessionSeq(0),
+bytesThisFile(0),
+totalBytes(0),
+totalChunks(0),
+totalDropped(0),
+lastDropPrintMs(0),
+windowBytes(0),
+windowStartMs(0),
+bps(0),
+active(false),
+warning(false)
+{
+}
+
+
+bool DataLogger::begin(fs::FS& files)
+{
+    // TODO: allocate the chunk pool with heap_caps_malloc(MALLOC_CAP_DMA)
+    // (STORAGE_LOG_POOL_SIZE x STORAGE_LOG_CHUNK_SIZE), create freeQ with all
+    // chunks and filledQ empty (xQueueCreate), mkdir STORAGE_LOG_DIR.
+    this->files = &files;
+    return false;
+}
+
+
+void DataLogger::end()
+{
+    // TODO: closeSession(), free the pool (heap_caps_free), delete the queues.
+}
+
+
+bool DataLogger::openSession()
+{
+    // TODO: build "rec_<uptimeSeconds>_<sessionSeq>.wav" under
+    // STORAGE_LOG_DIR (skip forward if the name exists), open FILE_WRITE,
+    // write the 44-byte header with buildWavHeader(dataSize=0).
+    return false;
+}
+
+
+bool DataLogger::closeSession()
+{
+    // TODO: finalizeWavHeader(file.size(), ...) -> patch offsets 4 and 40,
+    // file.flush(), file.close().
+    return false;
+}
+
+
+bool DataLogger::rotateIfNeeded()
+{
+    // TODO: if active && bytesThisFile >= STORAGE_LOG_ROTATE_BYTES:
+    // closeSession(); sessionSeq++; openSession().
+    return false;
+}
+
+
+AudioChunk* DataLogger::acquireChunk(TickType_t timeout)
+{
+    // TODO: xQueueReceive(freeQ, &chunk, timeout). On timeout, call
+    // dropOldestChunk() and return its chunk so the producer keeps streaming.
+    return nullptr;
+}
+
+
+void DataLogger::submitChunk(AudioChunk* chunk)
+{
+    // TODO: xQueueSend(filledQ, &chunk, ...). Must preserve stream order.
+}
+
+
+AudioChunk* DataLogger::nextChunk(TickType_t timeout)
+{
+    // TODO: xQueueReceive(filledQ, &chunk, timeout).
+    return nullptr;
+}
+
+
+bool DataLogger::writeChunk(AudioChunk* chunk)
+{
+    // TODO: if (!active) return false;
+    //   n = file.write(chunk->data, chunk->length);
+    //   bytesThisFile += n; totalBytes += n; totalChunks++;
+    //   feed the writeSpeedBps window (windowBytes/windowStartMs);
+    //   file.flush() every STORAGE_LOG_FLUSH_BYTES;
+    //   return n == chunk->length;  (card failure -> fatal)
+    return false;
+}
+
+
+void DataLogger::releaseChunk(AudioChunk* chunk)
+{
+    // TODO: chunk->length = 0; xQueueSend(freeQ, &chunk, ...).
+}
+
+
+AudioChunk* DataLogger::dropOldestChunk()
+{
+    // TODO: xQueueReceive from the BACK of filledQ without writing, count it,
+    // setOverflowWarning(true). Called by acquireChunk on timeout.
+    return nullptr;
+}
+
+
+void DataLogger::setOverflowWarning(bool overflowing)
+{
+    // TODO: rate-limited Serial line with totalDropped + bytes behind;
+    // digitalWrite(STORAGE_WARN_LED_GPIO, ...) using
+    // STORAGE_WARN_LED_ACTIVE_HIGH.
+    (void)overflowing;
+}
+
+
+uint32_t DataLogger::bytesWritten() const  { return (uint32_t)totalBytes; }
+uint32_t DataLogger::chunksWritten() const { return totalChunks; }
+uint32_t DataLogger::droppedChunks() const { return totalDropped; }
+uint32_t DataLogger::writeSpeedBps() const { return bps; }
+bool     DataLogger::overflowing() const   { return warning; }

src/storage/data_logger.h

@@ -0,0 +1,174 @@
+#pragma once
+
+// ============================================================================
+//  Audio logging interface (see docs/audio_logging.md for the full design).
+//
+//  THIS FILE IS A SKELETON. The chunk-pool / queue / file logic in
+//  data_logger.cpp is stubbed with TODOs; only the WAV metadata helpers are
+//  implemented. The interface below is the contract the future Node
+//  collection task (producer) and the StorageTask (consumer) are written
+//  against.
+// ============================================================================
+
+#include <Arduino.h>
+#include <FS.h>
+
+#include <freertos/FreeRTOS.h>
+#include <freertos/queue.h>
+
+#include "storage_config.h"
+
+
+// ----------------------------------------------------------------------------
+//  WAV (RIFF) metadata helpers - fully implemented in data_logger.cpp.
+// ----------------------------------------------------------------------------
+
+// 44-byte PCM WAVE header. Layout and field meaning are documented in
+// docs/audio_logging.md section 3.
+struct WavHeader
+{
+    uint8_t  riff[4];        // "RIFF"
+    uint32_t riffSize;       // file size - 8
+    uint8_t  wave[4];        // "WAVE"
+    uint8_t  fmt[4];         // "fmt "
+    uint32_t fmtChunkSize;   // 16 (PCM)
+    uint16_t audioFormat;    // 1 (PCM)
+    uint16_t numChannels;    // STORAGE_AUDIO_CHANNELS
+    uint32_t sampleRate;     // STORAGE_AUDIO_SAMPLE_RATE_HZ
+    uint32_t byteRate;       // sampleRate * blockAlign
+    uint16_t blockAlign;     // numChannels * (bitsPerSample / 8)
+    uint16_t bitsPerSample;  // STORAGE_AUDIO_BITS
+    uint8_t  data[4];        // "data"
+    uint32_t dataSize;       // file size - 44
+} __attribute__((packed));
+
+static_assert(sizeof(WavHeader) == 44, "WAV header must be exactly 44 bytes");
+
+// Fills the header for a PCM stream of the given channels/rate/depth.
+// dataSize is typically 0 at file open and corrected on finalize.
+void buildWavHeader(WavHeader& header,
+                    uint16_t numChannels,
+                    uint32_t sampleRate,
+                    uint16_t bitsPerSample,
+                    uint32_t dataSize);
+
+// Returns the two little-endian values to patch at offsets 4 and 40 when a
+// file is closed/rotated: riffSize = fileSize - 8, dataSize = fileSize - 44.
+void finalizeWavHeader(uint32_t fileSize,
+                       uint32_t& riffSizeOut,
+                       uint32_t& dataSizeOut);
+
+
+// ----------------------------------------------------------------------------
+//  Chunk pool
+// ----------------------------------------------------------------------------
+
+// One slot of the write pool. Producers borrow a chunk, fill `data[0..length)`
+// with interleaved PCM (40 channels, frame order, 80 bytes/frame), and submit
+// it. Chunks must be submitted in strict stream order.
+struct AudioChunk
+{
+    uint8_t* data;       // DMA-capable buffer owned by the pool
+    uint32_t capacity;   // STORAGE_LOG_CHUNK_SIZE
+    uint32_t length;     // valid bytes, multiple of the WAV frame size
+    uint32_t sequence;   // monotonic order (diagnostics only)
+};
+
+
+// ----------------------------------------------------------------------------
+//  DataLogger
+// ----------------------------------------------------------------------------
+
+// Producer/consumer bridge between the Node collection task (core 1) and the
+// StorageTask (core 0). Owns the chunk pool, the two FreeRTOS queues, and the
+// open WAV file. Only the consumer touches the card.
+class DataLogger
+{
+public:
+    DataLogger();
+
+    // Lifecycle --------------------------------------------------------------
+
+    // Allocates the pool (MALLOC_CAP_DMA), creates the queues, creates
+    // STORAGE_LOG_DIR. Does not open a session file.
+    bool begin(fs::FS& files);
+
+    // Finalizes and closes the current session file, frees the pool/queues.
+    void end();
+
+    // Session control --------------------------------------------------------
+
+    // Creates the next rec_<uptimeSeconds>_<n>.wav under STORAGE_LOG_DIR,
+    // writes the 44-byte WAV header. false if the card is full or unwritable.
+    bool openSession();
+
+    // Finalizes (patches RIFF/data sizes) and closes the current file.
+    bool closeSession();
+
+    // Closes/opens when bytesThisFile >= STORAGE_LOG_ROTATE_BYTES. Called by
+    // the consumer on every loop; must be a no-op when idle.
+    bool rotateIfNeeded();
+
+    // Producer API (Node collection task, core 1) ----------------------------
+
+    // Pops a free chunk, or on timeout drops the oldest queued chunk (Section
+    // 8) and returns it so the producer can keep streaming. nullptr only if
+    // there is nothing to drop (empty pool + empty queue).
+    AudioChunk* acquireChunk(TickType_t timeout);
+
+    // Returns a filled chunk to the write queue. Must be called in order.
+    void submitChunk(AudioChunk* chunk);
+
+    // Consumer API (StorageTask, core 0) --------------------------------------
+
+    // Blocks up to `timeout` for the next filled chunk. nullptr on timeout
+    // (lets the consumer pulse for stats even when idle).
+    AudioChunk* nextChunk(TickType_t timeout);
+
+    // Appends chunk->data[0..length) to the WAV data section with one
+    // file.write(), updates the byte/chunk counters and the bps window.
+    // false on card failure (fatal per design.md).
+    bool writeChunk(AudioChunk* chunk);
+
+    // Returns a chunk to the free pool after it has been written or dropped.
+    void releaseChunk(AudioChunk* chunk);
+
+    // Stats -------------------------------------------------------------------
+
+    uint32_t bytesWritten() const;
+    uint32_t chunksWritten() const;
+    uint32_t droppedChunks() const;
+    uint32_t writeSpeedBps() const;
+    bool     overflowing() const;
+
+private:
+    // Pops the oldest unwritten chunk off filledQ without writing it,
+    // increments droppedChunks, raises the warning. Returns it to the caller
+    // (used by acquireChunk on timeout).
+    AudioChunk* dropOldestChunk();
+
+    // Serial (rate-limited) + LED (STORAGE_WARN_LED_GPIO) overflow indication.
+    void setOverflowWarning(bool overflowing);
+
+    fs::FS*        files;        // SD backend, injected by begin()
+    AudioChunk*    pool;         // STORAGE_LOG_POOL_SIZE chunks
+    QueueHandle_t  freeQ;        // empty chunks
+    QueueHandle_t  filledQ;      // chunks waiting to be written
+    File           file;         // open session file
+    char           filePath[96];
+    uint32_t       sessionSeq;   // rotation counter within this boot
+
+    uint64_t       bytesThisFile;
+    uint64_t       totalBytes;
+    uint32_t       totalChunks;
+    uint32_t       totalDropped;
+    uint32_t       lastDropPrintMs;
+
+    // bps metering window (writeChunk is the only writer)
+    uint32_t       windowBytes;
+    uint32_t       windowStartMs;
+    uint32_t       bps;
+
+    bool           active;
+    bool           warning;
+};

src/storage/sd_manager.cpp

@@ -0,0 +1,443 @@
+#include "sd_manager.h"
+
+#include "storage_config.h"
+
+#include <driver/gpio.h>
+
+// Both backends are compiled into the image so that the SDIO path is
+// compile-checked even while the system still runs SPI for bring-up.
+#include <SD.h>
+#include <SD_MMC.h>
+
+namespace
+{
+
+class SpiBackend : public StorageBackend
+{
+public:
+    bool begin() override
+    {
+        // Explicit SPI instance so the pin wiring is driven from
+        // storage_config.h rather than the board defaults. The second
+        // spi.begin() call made inside SDFS::begin() is a no-op because the
+        // bus is already started with these pins.
+        static SPIClass spi;
+
+        spi.begin(
+            STORAGE_SPI_SCK,
+            STORAGE_SPI_MISO,
+            STORAGE_SPI_MOSI,
+            STORAGE_SPI_CS
+        );
+
+        // Cheap breakout modules often omit the pull-up resistors the SD
+        // spec expects on the idle-high lines. The ESP32 SPI HAL clears the
+        // internal pull-ups when it attaches a pin (esp32-hal-spi.c), so a
+        // floating MISO/CS means the card never answers CMD0 during init
+        // ("Card Failed! cmd: 0x00"). Re-enable the pull-ups directly so we
+        // do not disturb the pin's peripheral function.
+        gpio_pullup_en((gpio_num_t)STORAGE_SPI_MISO);
+        gpio_pullup_en((gpio_num_t)STORAGE_SPI_CS);
+
+        // Give the card a moment to stabilize after power-on before the init
+        // handshake starts (the framework sends 74+ dummy clocks, but some
+        // cards need a little more settling time on a fresh mount attempt).
+        delay(20);
+
+        return SD.begin(
+            STORAGE_SPI_CS,
+            spi,
+            STORAGE_SPI_FREQ,
+            STORAGE_MOUNT_POINT,
+            STORAGE_MAX_OPEN_FILES,
+            false // format_if_empty: never auto-format
+        );
+    }
+
+    void end() override
+    {
+        SD.end();
+    }
+
+    fs::FS& fs() override
+    {
+        return SD;
+    }
+
+    StorageCardType cardType() override
+    {
+        return mapType(SD.cardType());
+    }
+
+    uint64_t totalBytes() override
+    {
+        return SD.totalBytes();
+    }
+
+    uint64_t usedBytes() override
+    {
+        return SD.usedBytes();
+    }
+
+private:
+    static StorageCardType mapType(sdcard_type_t type)
+    {
+        switch (type)
+        {
+        case CARD_MMC:  return StorageCardType::MMC;
+        case CARD_SD:   return StorageCardType::SD;
+        case CARD_SDHC: return StorageCardType::SDHC;
+        case CARD_NONE: return StorageCardType::None;
+        default:        return StorageCardType::Unknown;
+        }
+    }
+};
+
+class SdmmcBackend : public StorageBackend
+{
+public:
+    bool begin() override
+    {
+        // The plain esp32dev variant does not pre-wire the SDMMC pins, so
+        // they must always be set explicitly. The classic ESP32 routes the
+        // SDMMC peripheral through the GPIO matrix, so the pins defined in
+        // storage_config.h are fully re-routable.
+        if (!SD_MMC.setPins(
+                STORAGE_SDMMC_CLK,
+                STORAGE_SDMMC_CMD,
+                STORAGE_SDMMC_D0,
+                STORAGE_SDMMC_D1,
+                STORAGE_SDMMC_D2,
+                STORAGE_SDMMC_D3))
+        {
+            return false;
+        }
+
+        return SD_MMC.begin(
+            STORAGE_MOUNT_POINT,
+            STORAGE_SDMMC_MODE_1BIT,          // false == 4-bit bus
+            STORAGE_SDMMC_FORMAT_IF_FAILED,   // never auto-format
+            STORAGE_SDMMC_FREQ_HZ,            // 40 MHz == SDMMC_FREQ_HIGHSPEED
+            STORAGE_MAX_OPEN_FILES
+        );
+    }
+
+    void end() override
+    {
+        SD_MMC.end();
+    }
+
+    fs::FS& fs() override
+    {
+        return SD_MMC;
+    }
+
+    StorageCardType cardType() override
+    {
+        return mapType(SD_MMC.cardType());
+    }
+
+    uint64_t totalBytes() override
+    {
+        return SD_MMC.totalBytes();
+    }
+
+    uint64_t usedBytes() override
+    {
+        return SD_MMC.usedBytes();
+    }
+
+private:
+    static StorageCardType mapType(sdcard_type_t type)
+    {
+        switch (type)
+        {
+        case CARD_MMC:  return StorageCardType::MMC;
+        case CARD_SD:   return StorageCardType::SD;
+        case CARD_SDHC: return StorageCardType::SDHC;
+        case CARD_NONE: return StorageCardType::None;
+        default:        return StorageCardType::Unknown;
+        }
+    }
+};
+
+} // namespace
+
+
+SDManager::SDManager()
+:
+backend(nullptr),
+mounted(false)
+{
+}
+
+
+bool SDManager::begin()
+{
+    if (mounted)
+    {
+        return true;
+    }
+
+#if STORAGE_IFACE == STORAGE_IFACE_SDMMC
+    backend = new SdmmcBackend();
+#else
+    backend = new SpiBackend();
+#endif
+
+    if (backend == nullptr)
+    {
+        return false;
+    }
+
+    mounted = backend->begin();
+
+    if (!mounted)
+    {
+        Serial.println("[Storage] SD card mount FAILED");
+        delete backend;
+        backend = nullptr;
+    }
+
+    return mounted;
+}
+
+
+void SDManager::end()
+{
+    if (backend != nullptr)
+    {
+        backend->end();
+        delete backend;
+        backend = nullptr;
+    }
+
+    mounted = false;
+}
+
+
+bool SDManager::isMounted() const
+{
+    return mounted;
+}
+
+
+fs::FS& SDManager::fs()
+{
+    return backend->fs();
+}
+
+
+StorageCardType SDManager::cardType() const
+{
+    if (mounted && backend != nullptr)
+    {
+        return backend->cardType();
+    }
+    return StorageCardType::None;
+}
+
+
+const char* SDManager::cardTypeName() const
+{
+    return cardTypeName(cardType());
+}
+
+
+uint64_t SDManager::totalBytes() const
+{
+    if (mounted && backend != nullptr)
+    {
+        return backend->totalBytes();
+    }
+    return 0;
+}
+
+
+uint64_t SDManager::usedBytes() const
+{
+    if (mounted && backend != nullptr)
+    {
+        return backend->usedBytes();
+    }
+    return 0;
+}
+
+
+uint64_t SDManager::freeBytes() const
+{
+    uint64_t total = totalBytes();
+    uint64_t used  = usedBytes();
+    return (total > used) ? (total - used) : 0;
+}
+
+
+uint32_t SDManager::measureWriteSpeed()
+{
+    if (!mounted || backend == nullptr)
+    {
+        return 0;
+    }
+
+    fs::FS& files = backend->fs();
+
+    const char* scratchPath = STORAGE_SPEED_MEASURE_PATH;
+    const size_t bufferSize = 16 * 1024;
+
+    // Remove any leftover from a previous crashed run.
+    files.remove(scratchPath);
+
+    uint8_t* buffer = (uint8_t*)malloc(bufferSize);
+    if (buffer == nullptr)
+    {
+        return 0;
+    }
+
+    memset(buffer, 0xA5, bufferSize);
+
+    File file = files.open(scratchPath, FILE_WRITE);
+    if (!file)
+    {
+        free(buffer);
+        return 0;
+    }
+
+    uint32_t remaining = STORAGE_SPEED_MEASURE_BYTES;
+    uint32_t startUs = micros();
+
+    while (remaining > 0)
+    {
+        size_t toWrite = (remaining < bufferSize) ? remaining : bufferSize;
+        size_t written = file.write(buffer, toWrite);
+
+        if (written == 0)
+        {
+            break;
+        }
+
+        remaining -= written;
+    }
+
+    file.close();
+
+    uint32_t elapsedUs = micros() - startUs;
+    Serial.print("Elapsed microseconds: ");
+    Serial.println(elapsedUs);
+
+    files.remove(scratchPath);
+    free(buffer);
+
+    if (remaining != 0 || elapsedUs == 0)
+    {
+        return 0;
+    }
+
+    uint64_t writtenBytes = (uint64_t)STORAGE_SPEED_MEASURE_BYTES - remaining;
+    return (uint32_t)((writtenBytes * 1000000ULL) / elapsedUs);
+}
+
+
+const char* SDManager::cardTypeName(StorageCardType type)
+{
+    switch (type)
+    {
+    case StorageCardType::MMC:     return "MMC";
+    case StorageCardType::SD:      return "SD";
+    case StorageCardType::SDHC:    return "SDHC";
+    case StorageCardType::Unknown: return "Unknown";
+    case StorageCardType::None:
+    default:                       return "None";
+    }
+}
+
+
+void SDManager::printCardInfo()
+{
+    if (!mounted)
+    {
+        Serial.println("[Storage] Card info unavailable (not mounted)");
+        return;
+    }
+
+    uint64_t total = backend->totalBytes();
+    uint64_t used  = backend->usedBytes();
+    uint64_t free  = (total > used) ? (total - used) : 0;
+
+    Serial.println("----------------------------------------");
+    Serial.println("SD card information");
+    Serial.println("----------------------------------------");
+    Serial.printf("  Type   : %s\n", cardTypeName(backend->cardType()));
+    Serial.printf("  Total  : %llu bytes\n", (unsigned long long)total);
+    Serial.printf("  Used   : %llu bytes\n", (unsigned long long)used);
+    Serial.printf("  Free   : %llu bytes\n", (unsigned long long)free);
+    Serial.println("----------------------------------------");
+}
+
+
+void SDManager::listFiles()
+{
+    if (!mounted)
+    {
+        Serial.println("[Storage] Cannot list files (not mounted)");
+        return;
+    }
+
+    Serial.println("Files on SD card:");
+    Serial.println("----------------------------------------");
+
+    char path[256];
+    snprintf(path, sizeof(path), "/");
+
+    listFilesRecursive(backend->fs(), path, sizeof(path), 0);
+
+    Serial.println("----------------------------------------");
+    Serial.println("End of listing");
+}
+
+
+void SDManager::listFilesRecursive(fs::FS& files, char* path, size_t pathSize, uint8_t depth)
+{
+    if (depth > STORAGE_LIST_MAX_DEPTH)
+    {
+        Serial.printf("  ... (max depth %u reached)\n", STORAGE_LIST_MAX_DEPTH);
+        return;
+    }
+
+    File dir = files.open(path);
+
+    if (!dir)
+    {
+        Serial.printf("  [error] cannot open: %s\n", path);
+        return;
+    }
+
+    if (!dir.isDirectory())
+    {
+        Serial.printf("  %s (%llu bytes)\n", path, (unsigned long long)dir.size());
+        dir.close();
+        return;
+    }
+
+    File entry;
+    while ((entry = dir.openNextFile()))
+    {
+        size_t base = strlen(path);
+
+        if (entry.isDirectory())
+        {
+            snprintf(path + base, pathSize - base, "/%s", entry.name());
+            Serial.printf("  %s/\n", path);
+            listFilesRecursive(files, path, pathSize, depth + 1);
+            path[base] = '\0';
+        }
+        else
+        {
+            snprintf(path + base, pathSize - base, "/%s", entry.name());
+            Serial.printf("  %s (%llu bytes)\n", path, (unsigned long long)entry.size());
+        }
+
+        entry.close();
+    }
+
+    dir.close();
+}

src/storage/sd_manager.h

@@ -0,0 +1,72 @@
+#pragma once
+
+#include <Arduino.h>
+#include <FS.h>
+
+// StorageCardType decouples the rest of the system from the SD/SD_MMC
+// library enum so nothing outside sd_manager.cpp needs to know which
+// hardware interface is in use.
+enum class StorageCardType
+{
+    None,
+    MMC,
+    SD,
+    SDHC,
+    Unknown
+};
+
+// Hardware abstraction for the SD card transport. The concrete backend
+// (SPI today, SDIO later) is selected at build time in storage_config.h.
+// Everything downstream (StorageTask, future DataLogger) talks only to
+// the `fs::FS` reference, so swapping SPI for SDIO touches no other code.
+class StorageBackend
+{
+public:
+    virtual ~StorageBackend() {}
+
+    virtual bool begin() = 0;
+    virtual void end() = 0;
+
+    virtual fs::FS& fs() = 0;
+
+    virtual StorageCardType cardType() = 0;
+    virtual uint64_t totalBytes() = 0;
+    virtual uint64_t usedBytes() = 0;
+};
+
+// Owns the selected backend, mounts the card, and provides the boot-time
+// info/listing report. The mount and any file access are blocking and are
+// expected to be driven from the dedicated StorageTask on core 0.
+class SDManager
+{
+public:
+    SDManager();
+
+    bool begin();
+    void end();
+
+    bool isMounted() const;
+
+    fs::FS& fs();
+
+    StorageCardType cardType() const;
+    const char* cardTypeName() const;
+    uint64_t totalBytes() const;
+    uint64_t usedBytes() const;
+    uint64_t freeBytes() const;
+
+    // Writes a scratch file of STORAGE_SPEED_MEASURE_BYTES and returns the
+    // achieved write throughput in bytes/second (0 on failure). The scratch
+    // file is deleted before returning.
+    uint32_t measureWriteSpeed();
+
+    void printCardInfo();
+    void listFiles();
+
+private:
+    StorageBackend* backend;
+    bool mounted;
+
+    void listFilesRecursive(fs::FS& files, char* path, size_t pathSize, uint8_t depth);
+    static const char* cardTypeName(StorageCardType type);
+};

src/storage/storage_config.h

@@ -0,0 +1,145 @@
+#pragma once
+
+// ============================================================================
+//  Storage configuration
+// ============================================================================
+//
+//  This header is the single place to configure how the Hub talks to the
+//  SD card. Two hardware interfaces are supported by the Arduino-ESP32
+//  framework, and both are implemented behind the StorageBackend interface
+//  in sd_manager.cpp:
+//
+//    STORAGE_IFACE_SPI   - Uses the `SD` library (SPI protocol). This is the
+//                          bring-up path for the current breakout module,
+//                          which only breaks out the 4 SPI lines.
+//                          Max practical throughput: ~1-2 MB/s. NOT enough
+//                          for the 6-8 MB/s audio logging target.
+//
+//    STORAGE_IFACE_SDMMC - Uses the `SD_MMC` library (SDIO protocol, the
+//                          SDMMC peripheral). This is the production path.
+//                          4-bit mode @ 40 MHz (SDMMC_FREQ_HIGHSPEED) gives
+//                          roughly 8-12 MB/s, which comfortably meets the
+//                          target. REQUIRED for the final design.
+//
+//  TO MIGRATE TO SDIO (the new SDIO-capable board that is being shipped):
+//
+//    1. Change STORAGE_IFACE below from STORAGE_IFACE_SPI to
+//       STORAGE_IFACE_SDMMC.
+//
+//    2. Wire the SD card to the SDIO lines listed in the STORAGE_SDMMC_*
+//       defines below. On the classic ESP32 the SDMMC peripheral is routed
+//       through the GPIO matrix, so these pins can be changed to any free
+//       GPIO simply by editing the defines.
+//
+//    3. Keep STORAGE_SDMMC_MODE_1BIT as `false` (4-bit bus is required for
+//       the write throughput) and keep STORAGE_SDMMC_FORMAT_IF_FAILED as
+//       `false` (a foreign/unformatted card must never be auto-formatted).
+//
+//    4. Everything downstream (StorageTask, SDManager, and the future
+//       DataLogger) talks only through the `fs::FS` interface, so no other
+//       code changes are needed.
+// ============================================================================
+
+// --- Backend selection ------------------------------------------------------
+
+#define STORAGE_IFACE_SPI     1
+#define STORAGE_IFACE_SDMMC   2
+
+// SPI until the SDIO-capable board arrives.
+#ifndef STORAGE_IFACE
+#define STORAGE_IFACE STORAGE_IFACE_SPI
+#endif
+
+// --- Common -----------------------------------------------------------------
+
+// Single canonical mount point so file paths never change when the backend
+// is switched (the SD lib defaults to "/sd", SD_MMC to "/sdcard").
+#define STORAGE_MOUNT_POINT      "/sdcard"
+
+#define STORAGE_MAX_OPEN_FILES   5
+
+// --- SPI (bring-up only) ----------------------------------------------------
+
+// Default VSPI pins on the classic ESP32 DevKitC (variant/pins_arduino.h).
+#define STORAGE_SPI_CS     5
+#define STORAGE_SPI_SCK    18
+#define STORAGE_SPI_MOSI   23
+#define STORAGE_SPI_MISO   19
+
+// 10 MHz is a reliable default for breadboard/jumper-wire bring-up. The SD
+// init handshake always runs at 400 kHz regardless (see sd_diskio.cpp), so a
+// mount failure is NOT a frequency problem - check power, wiring, pull-ups
+// and card seating first. Raise to 20 MHz once the wiring is proven.
+#define STORAGE_SPI_FREQ   10000000UL
+
+// --- SDIO / SD_MMC (production, 4-bit) --------------------------------------
+
+// Default ESP32 SDMMC slot-1 pins (GPIO matrix, freely re-routable).
+#define STORAGE_SDMMC_CLK  6
+#define STORAGE_SDMMC_CMD  11
+#define STORAGE_SDMMC_D0   7
+#define STORAGE_SDMMC_D1   8
+#define STORAGE_SDMMC_D2   9
+#define STORAGE_SDMMC_D3   10
+
+// false = 4-bit wide bus (required for >8 MB/s). Do NOT enable 1-bit mode.
+#define STORAGE_SDMMC_MODE_1BIT          false
+
+// NEVER auto-format: an unformatted/foreign card must never be destroyed.
+#define STORAGE_SDMMC_FORMAT_IF_FAILED   false
+
+// 40 MHz == SDMMC_FREQ_HIGHSPEED. Written as a literal to keep this header
+// free of driver includes.
+#define STORAGE_SDMMC_FREQ_HZ            40000000
+
+// --- Boot-time file listing -------------------------------------------------
+
+// Maximum directory depth printed during the recursive boot listing. Guards
+// the StorageTask stack against pathological directory nesting.
+#define STORAGE_LIST_MAX_DEPTH   10
+
+// --- Write-speed estimation -------------------------------------------------
+
+// Path of the temporary scratch file used to measure write throughput. It is
+// created, written, measured, then deleted, so it never appears in listings.
+#define STORAGE_SPEED_MEASURE_PATH   STORAGE_MOUNT_POINT "/.writespeed.tmp"
+
+// Size of the scratch file written per measurement (bytes).
+#define STORAGE_SPEED_MEASURE_BYTES       (0.5 * 1024 * 1024)
+
+// How often to re-measure write speed (ms). This is a bring-up ESTIMATE only:
+// it writes STORAGE_SPEED_MEASURE_BYTES to the card on every tick. Once real
+// logging exists, throughput should be derived from actual logging writes and
+// this benchmark disabled by setting the interval to 0 (measure once at boot).
+#define STORAGE_SPEED_MEASURE_INTERVAL_MS 0
+
+// --- Audio logging ----------------------------------------------------------
+//
+//  The DataLogger (docs/audio_logging.md) writes one multichannel WAV file
+//  per rotation. The Node collection task feeds it interleaved 40-channel PCM
+//  as chunks; the logger appends them and only the consumer touches the card.
+//
+//  Stream rate: 40 ch x 48 kHz x 2 B = 3,840,000 B/s. The 128 KB pool holds
+//  ~34 ms of audio, so collection rounds must stay <= ~25 ms (<= 96 KB) or
+//  the pool must grow (see docs/audio_logging.md section 7).
+
+// WAV parameters (must match what the Nodes produce).
+#define STORAGE_AUDIO_SAMPLE_RATE_HZ   48000
+#define STORAGE_AUDIO_CHANNELS         40      // 10 nodes x 4 mics
+#define STORAGE_AUDIO_BITS             16
+
+// Chunk pool: STORAGE_LOG_POOL_SIZE chunks of STORAGE_LOG_CHUNK_SIZE bytes,
+// allocated with MALLOC_CAP_DMA. 128 KB total.
+#define STORAGE_LOG_CHUNK_SIZE         (16 * 1024)
+#define STORAGE_LOG_POOL_SIZE          8
+
+// Recording directory and rotation policy.
+#define STORAGE_LOG_DIR                STORAGE_MOUNT_POINT "/audio"
+#define STORAGE_LOG_ROTATE_BYTES       (1024LL * 1024 * 1024)   // 1 GiB / file
+#define STORAGE_LOG_FLUSH_BYTES        (16LL * 1024 * 1024)     // f_sync cadence
+
+// Overflow warning LED: lights while chunks are being dropped because the SD
+// card cannot keep up. GPIO 4 is unused by the SD card lines (SPI 5/18/19/23,
+// SDMMC 6-11). Change both values if a different LED is wired.
+#define STORAGE_WARN_LED_GPIO          4
+#define STORAGE_WARN_LED_ACTIVE_HIGH   true

src/tasks/storage_task.cpp

@@ -0,0 +1,98 @@
+#include "storage_task.h"
+
+#include "../storage/storage_config.h"
+
+
+StorageTask::StorageTask(SDManager& manager, StorageState& state)
+:
+storage(manager),
+storageState(state),
+taskHandle(nullptr)
+{
+}
+
+
+void StorageTask::start()
+{
+    // Dedicated task on core 0 so SD card work never blocks the services
+    // running on core 1 (SystemTask / DiagnosticsTask). The 8 KB stack is
+    // sized for the recursive boot-time file listing; the eventual
+    // producer/consumer logging loop will also live here.
+    xTaskCreatePinnedToCore(
+        taskEntry,
+        "StorageTask",
+        8192,
+        this,
+        3,
+        &taskHandle,
+        0
+    );
+}
+
+
+void StorageTask::taskEntry(void* parameter)
+{
+    StorageTask* task =
+        static_cast<StorageTask*>(parameter);
+
+    task->run();
+}
+
+
+void StorageTask::run()
+{
+    Serial.println("[Storage] Initializing SD card...");
+
+    storageState.begin();
+
+    // While the mount has not succeeded the dashboard must show "SD not found".
+    storageState.setMounted(false);
+
+    // Retry the mount until it succeeds. This keeps the card dead-pin-capable:
+    // reseating the card, fixing a wire, or powering the module will bring it
+    // up without rebooting the Hub. (The SD init handshake runs at 400 kHz, so
+    // a failure here is wiring/power/seating/pull-up related, not speed.)
+    while (!storage.begin())
+    {
+        Serial.println("[Storage] SD card mount FAILED.");
+        Serial.println("[Storage] Check: 3.3V power + common ground, CS/SCK/MOSI/MISO wiring,");
+        Serial.println("[Storage]       card fully seated (click), and module pull-ups.");
+        Serial.println("[Storage] Retrying in 5 s...");
+        vTaskDelay(pdMS_TO_TICKS(5000));
+    }
+
+    storageState.setMounted(true);
+    storageState.setCardType(storage.cardTypeName());
+
+    storage.printCardInfo();
+    storage.listFiles();
+
+    // Initial write-speed estimate for the dashboard.
+    storageState.setWriteSpeedBps(storage.measureWriteSpeed());
+    storageState.setCapacity(storage.totalBytes(), storage.usedBytes());
+
+    TickType_t lastWake =
+        xTaskGetTickCount();
+
+    uint32_t lastSpeedMeasure = millis();
+
+    while (true)
+    {
+        // Periodically re-estimate write speed and refresh capacity so the
+        // dashboard stays current. An interval of 0 disables re-measuring.
+        if (STORAGE_SPEED_MEASURE_INTERVAL_MS != 0 &&
+            millis() - lastSpeedMeasure >= STORAGE_SPEED_MEASURE_INTERVAL_MS)
+        {
+            lastSpeedMeasure = millis();
+            storageState.setWriteSpeedBps(storage.measureWriteSpeed());
+            storageState.setCapacity(storage.totalBytes(), storage.usedBytes());
+        }
+
+        // Future integration point: a producer/consumer queue will feed
+        // audio data here to be flushed to the card.
+        vTaskDelayUntil(
+            &lastWake,
+            pdMS_TO_TICKS(1000)
+        );
+    }
+}

src/tasks/storage_task.h

@@ -0,0 +1,24 @@
+#pragma once
+
+#include <Arduino.h>
+
+#include "../core/storage_state.h"
+#include "../storage/sd_manager.h"
+
+
+class StorageTask
+{
+public:
+    StorageTask(SDManager& manager, StorageState& state);
+
+    void start();
+
+private:
+    static void taskEntry(void* parameter);
+    void run();
+
+    SDManager& storage;
+    StorageState& storageState;
+
+    TaskHandle_t taskHandle = nullptr;
+};