Streaming audio data from ADC with I2S

Question

Hello, I´m working on my audio project and I try to stream some audio data (currently a 1 kHz square wave signal with 3.3 V peak) over I2S. The audio signal is sampled by an ADC in the free-running mode. 
 #include "app_error.h"
#include "nrf_drv_ppi.h"
#include "nrf_drv_i2s.h"
#include "nrf_drv_saadc.h"
#include "nrf_drv_timer.h"
#include "nrf_gpio.h"

#include "nrf_log.h"
#include "nrf_log_ctrl.h"
#include "nrf_log_default_backends.h"

#define LED_0 17
#define BUTTON_0 13
#define BUTTON_1 14
#define TRIGGER 31
#define SAMPLES 128

static bool IsStarted = false;
static volatile uint32_t*	I2S_BlockToFill = NULL;
static volatile uint32_t*	ADC_BlockToFill = NULL;

static uint32_t I2S_Buffer[2][SAMPLES];
static nrf_saadc_value_t	ADC_Buffer[2][SAMPLES];
static nrf_ppi_channel_t	PPI_Channel;

static const nrf_drv_timer_t	Timer = NRF_DRV_TIMER_INSTANCE(0);

void on_Timer_Handler(nrf_timer_event_t EventType, void* p_Context)
{
}

void on_SAADC_Handler(nrf_drv_saadc_evt_t const* p_Event)
{
 if(p_Event->type == NRF_DRV_SAADC_EVT_DONE)
 {
 APP_ERROR_CHECK(nrf_drv_saadc_buffer_convert(p_Event->data.done.p_buffer, SAMPLES));

	ADC_BlockToFill = (uint32_t*)p_Event->data.done.p_buffer;
 }
}

static void DataHandler(nrf_drv_i2s_buffers_t const* p_Released, uint32_t Status)
{
 // 'nrf_drv_i2s_next_buffers_set' is called directly from the handler
 // each time next buffers are requested, so data corruption is not
 // expected.
 ASSERT(p_Released);

 // When the handler is called after the transfer has been stopped
 // (no next buffers are needed, only the used buffers are to be
 // released), there is nothing to do.
 if(!(Status & NRFX_I2S_STATUS_NEXT_BUFFERS_NEEDED))
 {
 return;
 }

 // First call of this handler occurs right after the transfer is started.
 // No data has been transferred yet at this point, so there is nothing to
 // check. Only the buffers for the next part of the transfer should be
 // provided.
 if(!p_Released->p_tx_buffer)
 {
 nrf_drv_i2s_buffers_t const NextBuffer = {
 .p_tx_buffer = I2S_Buffer[1],
 .p_rx_buffer = NULL,
 };

 APP_ERROR_CHECK(nrf_drv_i2s_next_buffers_set(&NextBuffer));

	nrf_gpio_pin_clear(TRIGGER);

 I2S_BlockToFill = I2S_Buffer[1];
 }
 else
 {
 // The driver has just finished accessing the buffers pointed by
 // 'p_Released'. They can be used for the next part of the transfer
 // that will be scheduled now.
 APP_ERROR_CHECK(nrf_drv_i2s_next_buffers_set(p_Released));

 // The pointer needs to be typecasted here, so that it is possible to
 // modify the content it is pointing to (it is marked in the structure
 // as pointing to constant data because the driver is not supposed to
 // modify the provided data).
 I2S_BlockToFill = (uint32_t*)p_Released->p_tx_buffer;
 }
}

void SAADC_Init(void)
{
 APP_ERROR_CHECK(nrf_drv_saadc_init(NULL, on_SAADC_Handler));
 APP_ERROR_CHECK(nrf_drv_ppi_init());

 nrf_drv_timer_config_t timer_cfg = NRF_DRV_TIMER_DEFAULT_CONFIG;
 timer_cfg.bit_width = NRF_TIMER_BIT_WIDTH_32;

 APP_ERROR_CHECK(nrf_drv_timer_init(&Timer, &timer_cfg, on_Timer_Handler));
 nrf_drv_timer_extended_compare(&Timer, NRF_TIMER_CC_CHANNEL0, nrf_drv_timer_us_to_ticks(&Timer, 23), NRF_TIMER_SHORT_COMPARE0_CLEAR_MASK, false);
 nrf_drv_timer_enable(&Timer);

 APP_ERROR_CHECK(nrf_drv_ppi_channel_alloc(&PPI_Channel));
 APP_ERROR_CHECK(nrf_drv_ppi_channel_assign(PPI_Channel, nrf_drv_timer_compare_event_address_get(&Timer, NRF_TIMER_CC_CHANNEL0), nrf_drv_saadc_sample_task_get()));

 nrf_saadc_channel_config_t Channel0 = NRF_DRV_SAADC_DEFAULT_CHANNEL_CONFIG_SE(NRF_SAADC_INPUT_AIN1);
 Channel0.acq_time = NRF_SAADC_ACQTIME_10US;

 APP_ERROR_CHECK(nrf_drv_saadc_channel_init(0, &Channel0));
 APP_ERROR_CHECK(nrf_drv_saadc_buffer_convert(ADC_Buffer[0], SAMPLES));
 APP_ERROR_CHECK(nrf_drv_saadc_buffer_convert(ADC_Buffer[1], SAMPLES));
 APP_ERROR_CHECK(nrf_drv_ppi_channel_enable(PPI_Channel));
}

int main(void)
{
 APP_ERROR_CHECK(NRF_LOG_INIT(NULL));
 NRF_LOG_DEFAULT_BACKENDS_INIT();

 SAADC_Init();

 nrf_gpio_cfg_input(BUTTON_0, NRF_GPIO_PIN_PULLUP);
 nrf_gpio_cfg_input(BUTTON_1, NRF_GPIO_PIN_PULLUP);

 nrf_gpio_cfg_output(TRIGGER);
 nrf_gpio_cfg_output(LED_0);
 nrf_gpio_pin_clear(TRIGGER);
 nrf_gpio_pin_set(LED_0);

 nrf_drv_i2s_config_t I2S_Config = NRF_DRV_I2S_DEFAULT_CONFIG;
 I2S_Config.sdin_pin = NRFX_I2S_PIN_NOT_USED;
 I2S_Config.sdout_pin = 27;
 I2S_Config.lrck_pin = 26;
 I2S_Config.sck_pin = 25;
 I2S_Config.mck_pin = 2;
 I2S_Config.mck_setup = NRF_I2S_MCK_32MDIV8;
 I2S_Config.ratio = NRF_I2S_RATIO_96X;
 I2S_Config.channels = NRF_I2S_CHANNELS_STEREO;
 APP_ERROR_CHECK(nrf_drv_i2s_init(&I2S_Config, DataHandler));

 nrf_drv_i2s_buffers_t const InitialBuffer = {
	.p_tx_buffer = I2S_Buffer[0],
 .p_rx_buffer = NULL,
 };

 while(1)
 {
	if(nrf_gpio_pin_read(BUTTON_0) == false)
	{
 if(IsStarted == false)
 {
 APP_ERROR_CHECK(nrf_drv_i2s_start(&InitialBuffer, SAMPLES, 0));
 NRF_LOG_INFO("I2S sender started...");
 IsStarted = true;
 nrf_gpio_pin_set(TRIGGER);
 nrf_gpio_pin_clear(LED_0);
 }
	}
	else if(nrf_gpio_pin_read(BUTTON_1) == false)
	{
 if(IsStarted)
 {
 nrf_drv_i2s_stop();
 NRF_LOG_INFO("I2S sender stopped...");
 IsStarted = false;
 nrf_gpio_pin_clear(TRIGGER);
 nrf_gpio_pin_set(LED_0);
 }
	}

	if(I2S_BlockToFill && ADC_BlockToFill)
 {
 for(uint32_t i = 0x00; i < SAMPLES; i++)
 {
 //int16_t Sample = (int16_t)(32768 * sin(2.0 * 3.14 * (double)i / ((double)SAMPLES)));
 I2S_BlockToFill[i] = (uint32_t)(0x0000 | (ADC_BlockToFill[i] & 0xFFFF));
 }

 I2S_BlockToFill = NULL;
 ADC_BlockToFill = NULL;
 }

	if(NRF_LOG_PROCESS())
	{
 NRF_LOG_FLUSH();
	}
 }
} 
 The code from above will give a pretty poor signal when I use the ADC. 
 
 I have tried to figure out the reason for this and replaced the line 
 I2S_BlockToFill[i] = (uint32_t)(0x0000 | (ADC_BlockToFill[i] & 0xFFFF)); 
 with 
 I2S_BlockToFill[i] = (uint32_t)(0x0000 | (i & 0xFFFF)); 
 to produce data from a ramp signal and the signal looks much better. 
 
 So it looks like the problem is based on the ADC. I have tried to use two buffers to use the same swapping buffer system as in the I2S example. How can I improve this result?

Karl Ylvisaker · Accepted Answer

Great - I look forward to hearing your results! :) In the case that you do not see a noticeable difference, could you have the SAADC sample a sine wave instead, and share its raw digital output with me, along with the sine's frequency and amplitude, so I may take a closer look at what might be going wrong. Best regards, Karl

Karl Ylvisaker · Answer

Hello again, Daniel Thank you for your continued patience with this. I have taken a look at the behavior you described, and concluded that it is likely caused by the 'm_zero_samples_to_ignore' parameter of the unmodified I2S example, which comes into play at the beginning of every transfer. The comment in the check_samples function details it further: 
 // Normally a couple of initial samples sent by the I2S peripheral
 // will have zero values, because it starts to output the clock
 // before the actual data is fetched by EasyDMA. As we are dealing
 // with streaming the initial zero samples can be simply ignored. You could also read more about this in the comments at the beginning of the prepare_tx_data function. I will be going out of office starting today, and I will not be back until after new years. If you require further technical support in my absence, or have any other issues or questions, please create a new ticket for this. Please also note that the forum will be operating with reduced staffing during the upcoming holiday season here in Norway - apologies for any inconveniences this might cause. I hope you have a great holiday season, Daniel! :) Best regards, Karl

Streaming audio data from ADC with I2S

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