I2S and sound

Introduction

In order to play audio files we need:

• the audio file itself, which we expect to be in uncompressed wav format
• a file system from which to read the file. This is available in MicroPython either in its internal flash or on an SD card
• a fast 16 bit DAC to convert the audio data into an analogue form that can be consumed by a loadspeaker
• an amplifier to get enough signal power needed for the loadspeaker
• a software driver implementing the I2S protocol

The digital data must be transferred to the DAC, which is done through the I2S protocol invented by Philips in the end 1980 ies.

There are two cheap I2S boards converting digital audio data to an analogue audio signal and communicating through the I2S protocol:

• the ! MAX98357A, a mono converter board with integrated amplifier to which we can directly connect a loadspeaker
• the PCM5102a, a stereo converter board with a connector for head phones. If we want to connect a loudspeaker to this board we have to pass through an audio amplifier.

Connecting the MAX98357A

Here is a table showing how I connected the MAX98357A to the ESP32 board:

MAX98357A pin	pin on ESP32 CPU card	signification
Gnd	Gnd
Vcc	3.3V
SD	n.c.
Gain	Gnd	important: don't leave this pin floating!
Din	D2: GPIO 21	audio data
BCLK	D0: GPIO 26	bit clock
LRC (LRCLK)	D1: GPIO 22	left/right clock

Connecting the PCM5102:

PCM5102 pin	pin on ESP32 CPU card	signification
Vcc	5V
Gnd	Gnd
BCK	D0: GPIO 26	bit clock
Din	D2: GPIO 21	audio data
LRCLK	D1: GPIO 22	left/right clock
FMT	Gnd
XSMT short-circuit with A3V3		3.3 V output from regulator LDO

The SD card module, which may be used to store the music files, uses D5,D6,D7,D8 for its interface pins. These pins should therefore be avoided when choosing the pins for the PCM5102 module. D3 and D4 are used for PSRAM in case of the WROVER-B module.

I2S microphones

Two different I2S microphone breakout boards are available:

• the INMP441 MEMS microphone
• the SPH0645 MEMS microphone

Both microphones are supported by the MicroPython I2S driver.

INMP441 front view	INMP441 back view
SPH0645 front view	SPH0645 back view

I2S examples

Mike Teachman, the author of the I2S MicroPython driver has supplied a series of test programs for his driver. These examples need some slight modifications due to different connections of the I2S pins with respect to his hardware:

• SCK_PIN: corresponds to the hardware pin BCK and must be set to GPIO 26
• WS_PIN: corresponds to hardware pin LRCK and must be set to GPIO 22
• SD_PIN: corresponds to hardware pin Din and must be set to GPIO 21

With these modifications, play-tone.py should work. In order to make play-wav-from-flash-blocking.py work modified the WAV_FILE to wav/side-to-side-8k-16bits-stereo.wav. Then I created the folder /wav on the ESP32 with ampy and copied side-to-side-8k-16bits-stereo.wav to the folder.

-- Uli Raich - 2021-01-24

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