ESP32 Forum

Posted: **Wed Nov 28, 2018 10:04 am**

I am trying to produce sine wave with I2S. There are several strange things I have found I don't know how to overcome.
Here is my code:

Code: [Select all] [Expand/Collapse]

#include <Arduino.h>
#include <driver/i2s.h>
#include <driver/dac.h>
 
/* I2S DAC */
#define  DAC_CHANNEL            DAC_CHANNEL_1                   // GPIO25 channel
 
#define I2S_DAC                 I2S_NUM_0
#define I2S_SAMPLE_RATE         8000
#define I2S_BITS_PER_SAMPLE     I2S_BITS_PER_SAMPLE_16BIT
#define I2S_CHANNELS_NUM        I2S_CHANNEL_MONO
#define I2S_CHANNEL_FORMAT      I2S_CHANNEL_FMT_ONLY_RIGHT
#define I2S_COMM_FORMAT         I2S_COMM_FORMAT_I2S_MSB
#define I2S_DAC_CHANNEL         I2S_DAC_CHANNEL_RIGHT_EN        // corresponds to DAC 1 channel
 
#define SINE_SAMPLES_NUM            256
#define SINE_800HZ_SAMPLES_NUM      10
 
 
 
/*
const uint16_t i2s_sine_wave_800hz[SINE_800HZ_SAMPLES_NUM] = {
    32768, 53830, 65038, 61145, 43975,
    21561,  4391,   498, 11706, 32768
};*/
 
 
const int16_t i2s_sine_wave_800hz[SINE_800HZ_SAMPLES_NUM] = {
        0,       21062,  32269,  28377,     11206,
        -11206, -28377, -32269, -21062,     0
};
 
static bool prepare_i2s_dac();
static bool play_sound();
 
 
void setup()
{
    Serial.begin(921600);
    Serial.println("Hello!");
 
    if(!prepare_i2s_dac())
    {
        Serial.println("Failed to initialize DAC");
        while(true);
    }
 
    play_sound();
}
 
void loop()
{
    //delay(10);
 
}
 
 
static bool play_sound()
{
    static uint8_t sin_samples[SINE_800HZ_SAMPLES_NUM*2];
    for(uint32_t i=0; i<SINE_800HZ_SAMPLES_NUM; i++)
    {
        sin_samples[i*2] = (i2s_sine_wave_800hz[i]) & 0xFF;
        sin_samples[i*2+1] = (i2s_sine_wave_800hz[i] >> 8) & 0xFF;
    }
 
    if(i2s_write_bytes(I2S_DAC, (const char*)sin_samples,
            SINE_800HZ_SAMPLES_NUM*2, portMAX_DELAY) != SINE_800HZ_SAMPLES_NUM*2)
    {
        return false;
    }
 
    return true;
}
 
 
static bool prepare_i2s_dac()
{
    i2s_config_t i2s_config = {
        mode : (i2s_mode_t)(I2S_MODE_MASTER | I2S_MODE_TX | I2S_MODE_DAC_BUILT_IN),
        sample_rate :  I2S_SAMPLE_RATE,
        bits_per_sample : I2S_BITS_PER_SAMPLE,
        channel_format : I2S_CHANNEL_FORMAT,
        communication_format : I2S_COMM_FORMAT,
        intr_alloc_flags : 0,
        dma_buf_count : 2,
        dma_buf_len : 128,
        use_apll : false,
        fixed_mclk : 0
    };
 
    if(i2s_driver_install(I2S_DAC, &i2s_config, 0, NULL) != ESP_OK)
    {
        Serial.println("Failed to initialize i2s driver for DAC");
        return false;
    }
 
    if(i2s_set_dac_mode(I2S_DAC_CHANNEL) != ESP_OK)
    {
        Serial.println("Failed to set i2s DAC channel");
        return false;
    }
 
    i2s_set_sample_rates(I2S_DAC, I2S_SAMPLE_RATE);
 
    return true;
}

GeSHi © Codebox Plus Extension

First of all I am totally missing why signal is produced more than once. Image shows that pattern is repeated each ~30ms. Why is that so?

: Signal repeated; photo_2018-11-28_12-41-17.jpg (100.22 KiB) Viewed 8858 times

Secondly I am not sure how does I2S interpret sent data, should it be signed or unsigned?

However I tried both approaches(you can see two arrays in my code). What for unsinged pattern, it produces the following output:

: Unsigned DAC signal; photo_2018-11-28_12-59-12.jpg (100.89 KiB) Viewed 8858 times

And here is how it should look like:

: Unsigned sequence how it should be; Figure_1.png (24.2 KiB) Viewed 8858 times

Clearly sequences of points are not the same.

Could someone clarify what is wrong here?

Posted: **Wed Nov 28, 2018 11:03 am**

1. The I2S needs 16-bit samples, you're stashing (repeated) 8-bit samples into it.
2. The I2S reads the samples as 32-bit words and outputs the high 16-bit first and the low 16-bit second. This means that if you want to send it 16-bit samples, you'll need to swap the even and odd 16-bit words.
3. The DAC only plays the high byte of the 16-bit sample as an unsigned value (0..255, not -128..127).

This should (=untested) work:

Code: Select all

   static uint16_t sin_samples[SINE_800HZ_SAMPLES_NUM];
    for(uint32_t i=0; i<SINE_800HZ_SAMPLES_NUM; i++)
    {
    	uint16_t samp = i2s_sine_wave_800hz[i] + 32768;
        sin_samples[i^1] = samp; //note i XOR with 1 to swap even and odd samples
    }
 
    if(i2s_write_bytes(I2S_DAC, (const char*)sin_samples,
            SINE_800HZ_SAMPLES_NUM*2, portMAX_DELAY) != SINE_800HZ_SAMPLES_NUM*2)
    {
        return false;
    }

Posted: **Tue Dec 11, 2018 10:56 pm**

ESP_Sprite wrote: ↑
Wed Nov 28, 2018 11:03 am
1. The I2S needs 16-bit samples, you're stashing (repeated) 8-bit samples into it.
2. The I2S reads the samples as 32-bit words and outputs the high 16-bit first and the low 16-bit second. This means that if you want to send it 16-bit samples, you'll need to swap the even and odd 16-bit words.
3. The DAC only plays the high byte of the 16-bit sample as an unsigned value (0..255, not -128..127).

This should (=untested) work:
Code: Select all
   static uint16_t sin_samples[SINE_800HZ_SAMPLES_NUM];
    for(uint32_t i=0; i<SINE_800HZ_SAMPLES_NUM; i++)
    {
    	uint16_t samp = i2s_sine_wave_800hz[i] + 32768;
        sin_samples[i^1] = samp; //note i XOR with 1 to swap even and odd samples
    }
 
    if(i2s_write_bytes(I2S_DAC, (const char*)sin_samples,
            SINE_800HZ_SAMPLES_NUM*2, portMAX_DELAY) != SINE_800HZ_SAMPLES_NUM*2)
    {
        return false;
    }

Sorry for the late reply! Thanks a lot for your answer. I did not test your solution since I have found that ESP has hardware cosine waveform generator which solves the issue easily. Nonetheless thanks!

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