Glitch in analog audio output when using I2S module

[cory123]

Hello.

I'm using the NRF52832 with I2S module connected to TLV320DAC from TI. I've configured the DAC and I2S module successfully. I'm getting audio output (simple sine wave read from a table) but there is an impulse glitch in the audio signal. The glitch appears to occur at the end of every I2S buffer send. When I set the I2S TX buffer size to 250 samples, I see a glitch in the audio at the corresponding time interval. For example, 250 buffer size at 41666 sample rate, causes a glitch in the audio every 0.006 seconds. When I increase the I2S buffer size, the glitch period increases correspondingly.

I've checked my sine table interpolation algorithm. It looks good. I believe the problem is with the NRF52832 and not the DAC. The I2S buffer size is the only variable which seems to affect the audio glitch. Oh, I also tried using I2S slave mode where I program the DAC to send the I2S clocks to the NRF chip. That did not change the glitch though.

I recorded the audio output in Audacity. Take a look at the waveform and frequency spectrum...

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[liteyear]

I'm seeing the same symptoms with a nRF9160, measured right at the SDOUT pin. I also found the problem did not go away with changes to the buffer size or its contents. Even with a buffer of all zeros, padded to protect against overruns, a glitch would occur between buffers.

Unlike the OP, I found that double buffering my source data only made the problem half as frequent! That led me to try some variations, eventually resulting in this discovery:

• Whenever the original buffer (the one passed to nrfx_i2s_start) is reused by passing it to nrfx_i2s_next_buffers_set, the glitch occurs!

So instead of double buffering, I had to use a throw-away buffer for the start, and could then happily keep using the same buffer in every update after that.

The relevant code is as follows:

ISR_DIRECT_DECLARE(i2s_isr_handler)
{
  nrfx_i2s_irq_handler();
  ISR_DIRECT_PM(); /* PM done after servicing interrupt for best latency */

  return 1; /* We should check if scheduling decision should be made */
}

void init(void)
{
  IRQ_DIRECT_CONNECT(I2S_IRQn, 0, i2s_isr_handler, 0);

  nrfx_i2s_config_t config = NRFX_I2S_DEFAULT_CONFIG(I2S_SCK_PIN,                       \
                                                     I2S_WS_PIN,                        \
                                                     NRFX_I2S_PIN_NOT_USED, /* MCK */   \
                                                     I2S_SD_PIN,            /* SDOUT */ \
                                                     NRFX_I2S_PIN_NOT_USED  /* SDIN */);
  config.mck_setup = NRF_I2S_MCK_32MDIV23; //32 MHz / 23 = 1.391304 MHz
  config.channels = NRF_I2S_CHANNELS_STEREO;
  
  nrfx_i2s_init(&config, data_handler);
  nrfx_i2s_start(&gI2SBuffers0, I2S_DATA_BLOCK_WORDS, 0);

}

static void data_handler(nrfx_i2s_buffers_t const* pReleased, uint32_t status)
{
  if(status | NRFX_I2S_STATUS_NEXT_BUFFERS_NEEDED)
    nrfx_i2s_next_buffers_set(&gI2SBuffers1);
}