mp3_demo: Fix support for mono mp3s by performing an in-place conversion from mono to stereo to match the format expected by es8311. Closes https://github.com/espressif/esp-box/pull/22

examples/mp3_demo/main/audio.c

@@ -235,6 +235,36 @@ static esp_err_t aplay_mp3(const char *path)
             /* Get MP3 frame info and configure I2S clock */
             MP3GetLastFrameInfo(mp3_decoder, &frame_info);
 
+            // if mono, convert to stereo as es8311 requires stereo input
+            // even though it is mono output
+            if(frame_info.nChans ==  1) {
+                size_t new_output_sample_count = frame_info.outputSamps * 2;
+
+                // convert from back to front to allow conversion in-place
+                int16_t *out = (int16_t*)output + new_output_sample_count;
+                int16_t *in = (int16_t*)output + frame_info.outputSamps;
+                size_t samples = frame_info.outputSamps;
+                while(samples) {
+                    // write right channel
+                    *out = *in;
+                    out--;
+
+                    // write left channel
+                    *out = *in;
+                    out--;
+
+                    // move input buffer back and decrement samples
+                    in--;
+                    samples--;
+                }
+
+                // adjust channels to 2
+                frame_info.nChans = 2;
+
+                // recalculate output sample count as its calculated by (channels * samples per frame)
+                frame_info.outputSamps = new_output_sample_count;
+            }
+
             /* Configure I2S clock if sample rate changed. Always reconfigure at first frame */
             if (sample_rate != frame_info.samprate) {
                 sample_rate = frame_info.samprate;
@@ -289,9 +319,8 @@ static void audio_task(void *pvParam)
     /**
      * @brief Initialize I2S and audio codec
      * 
-     * @note Actually the sampling rate can be reconfigured.
-     *       `MP3GetLastFrameInfo` can fill the `MP3FrameInfo`, which includes `samprate`.
-     *       So theoretically, the sampling rate can be dynamically changed according to the MP3 frame information.
+     * @note `MP3GetLastFrameInfo` is used to fill the `MP3FrameInfo`, which includes `samprate`,
+     *       and the sampling rate is updated during playback using this value.
      */
     bsp_codec_init(AUDIO_HAL_44K_SAMPLES);
     bsp_i2s_init(I2S_NUM_0, 44100);

examples/mp3_demo/partitions.csv

@@ -3,4 +3,4 @@
 nvs,      data, nvs,     0x9000,  24k
 phy_init, data, phy,     0xf000,  4k
 factory,  app,  factory, ,        1536k
-storage,  data, spiffs,  ,        14M,
+storage,  data, spiffs,  ,        7M,