feat(i2s): add turn-on time config for mic recorder example

examples/peripherals/i2s/mic_recorder/README.md

@@ -42,6 +42,7 @@ idf.py menuconfig
 In the `Example Configuration` menu:
 
 * Use `Microphone type` to select between the digital (PDM) and the analog (ES8389) microphone.
+* Use `Startup data to discard (ms)` to drop the first N milliseconds of PCM after the microphone path is running. The default is 20 ms. Set it to 0 to keep every sample. Refer to the microphone datasheet for the startup delay (also called turn-on time).
 * The configuration menu shown below the mic type is updated automatically:
   * `PDM MIC Configuration` — assign the PDM clock and data GPIOs (digital microphone).
   * `ES8389 Codec Configuration` — assign the I2C and I2S GPIOs, plus the mic gain (analog microphone).
@@ -71,7 +72,7 @@ A digital-microphone run looks like:
 ```
 PDM MIC recording example start
 --------------------------------------
-I (...) mic_rec_example: Starting recording for 2 seconds!
+I (...) mic_rec_example: Starting PDM recording for 2 seconds!
 I (...) mic_rec_example: Recording done, sending PCM data over console
 AUDIO_META sample_rate=16000 bits_per_sample=16 channels=2 data_size=128000 encoding=base64
 AUDIO_BASE64_BEGIN
@@ -123,4 +124,9 @@ Open it with [Audacity](https://www.audacityteam.org/) to listen to the recordin
     * Check the I2C wiring (SDA/SCL) and that `ES8389 Codec Configuration` matches your board.
     * Increase `Mic gain (dB)` in the `Example Configuration` menu.
 
+* Recording starts with a pop, click, or a short stretch of invalid audio
+
+    * Increase `Startup data to discard (ms)` in the `Example Configuration` menu. The stored recording length stays the same.
+    * Refer to the microphone datasheet for the startup delay (also called turn-on time).
+
 For any technical queries, please open an [issue](https://github.com/espressif/esp-idf/issues) on GitHub. We will get back to you soon.

examples/peripherals/i2s/mic_recorder/main/Kconfig.projbuild

@@ -24,6 +24,18 @@ menu "Example Configuration"
             select CODEC_ES8389_SUPPORT
     endchoice
 
+    config EXAMPLE_STARTUP_DISCARD_MS
+        int "Startup data to discard (ms)"
+        default 20
+        range 0 500
+        help
+            Number of milliseconds of PCM to read and discard after the
+            microphone path is running, before the recording is stored.
+            Use this to skip the microphone or codec startup transient.
+            The default is 20 ms. Set to 0 to keep every sample.
+            Refer to the microphone datasheet for the startup delay
+            (also called turn-on time).
+
     menu "PDM MIC Configuration"
         depends on EXAMPLE_MIC_TYPE_DMIC
 

examples/peripherals/i2s/mic_recorder/main/mic_recorder_example_main.c

@@ -57,6 +57,27 @@ static void capture_pcm(i2s_chan_handle_t rx_handle, uint8_t *pcm_data, size_t p
     }
 }
 
+static void discard_startup_pcm(i2s_chan_handle_t rx_handle)
+{
+    const size_t bytes_to_discard = (size_t)EXAMPLE_SAMPLE_RATE * EXAMPLE_CHANNEL_COUNT *
+                                    (EXAMPLE_BITS_PER_SAMPLE / 8) * CONFIG_EXAMPLE_STARTUP_DISCARD_MS / 1000;
+    if (bytes_to_discard == 0) {
+        return;
+    }
+
+    uint8_t discard_buf[EXAMPLE_DMA_READ_SIZE];
+    size_t bytes_discarded = 0;
+
+    ESP_LOGI(TAG, "Discarding the first %d ms of startup data", CONFIG_EXAMPLE_STARTUP_DISCARD_MS);
+    while (bytes_discarded < bytes_to_discard) {
+        size_t bytes_to_read = bytes_to_discard - bytes_discarded;
+        bytes_to_read = MIN(bytes_to_read, EXAMPLE_DMA_READ_SIZE);
+        size_t bytes_read = 0;
+        ESP_ERROR_CHECK(i2s_channel_read(rx_handle, discard_buf, bytes_to_read, &bytes_read, 1000));
+        bytes_discarded += bytes_read;
+    }
+}
+
 static void send_pcm_as_base64(const uint8_t *pcm_data, size_t pcm_size)
 {
     unsigned char encoded_chunk[EXAMPLE_BASE64_BUFFER_SIZE];
@@ -100,6 +121,8 @@ static void record_from_pdm_microphone(uint8_t *pcm_data, size_t pcm_size)
     ESP_ERROR_CHECK(i2s_channel_init_pdm_rx_mode(rx_handle, &pdm_rx_cfg));
     ESP_ERROR_CHECK(i2s_channel_enable(rx_handle));
 
+    /* Discard the startup PCM to skip the microphone startup transient. */
+    discard_startup_pcm(rx_handle);
     ESP_LOGI(TAG, "Starting PDM recording for %d seconds!", EXAMPLE_RECORD_TIME_SECONDS);
     capture_pcm(rx_handle, pcm_data, pcm_size);
 
@@ -191,6 +214,8 @@ static void record_from_es8389_microphone(uint8_t *pcm_data, size_t pcm_size)
     ESP_ERROR_CHECK(esp_codec_dev_set_in_gain(codec_handle, CONFIG_EXAMPLE_MIC_GAIN));
     ESP_LOGI(TAG, "ES8389 codec initialized");
 
+    /* Discard the startup PCM to skip the microphone or codec startup transient. */
+    discard_startup_pcm(rx_handle);
     ESP_LOGI(TAG, "Starting ES8389 recording for %d seconds!", EXAMPLE_RECORD_TIME_SECONDS);
     capture_pcm(rx_handle, pcm_data, pcm_size);