I2S Standard

Welcome to I2S Audio

In this lab, you will learn about Inter-IC Sound (I2S) — the digital bus that lets the Pico play high-quality audio. Say it like "eye-two-ess."

Inter-IC Sound (I2S) — pronounced "eye-two-ess" — is a serial bus that connects digital audio devices together. It sends Pulse-Code Modulation (PCM) audio data between chips. PCM is the same format used on CDs and in WAV files.

Key Idea

I2S and I2C sound almost identical but are completely different buses. I2C is for sensors and slow devices. I2S is specifically for high-quality digital audio — it is much faster and designed to keep audio in perfect time.

Why Use I2S?

A simple piezo buzzer can play tones, but it cannot play high-quality audio files. I2S lets you connect a proper Digital-to-Analog Converter (DAC) or a digital amplifier to the Pico. With I2S, you can play WAV files, synthesized music, and recorded speech at CD quality.

How I2S Works

I2S uses three wires:

Wire	Name	Purpose
SCK	Serial Clock (Bit Clock)	Clocks each audio bit
WS	Word Select (Left/Right Clock)	Selects the left or right stereo channel
SD	Serial Data	The actual audio samples

The clock speed depends on the audio quality you want. For standard 44.1 kHz stereo audio at 16-bit depth:

• SCK runs at 44,100 × 2 channels × 16 bits = 1.41 MHz

The Pico's I2S hardware can handle this easily.

MicroPython I2S Support

Since MicroPython version 1.17, the machine.I2S class lets you send and receive I2S audio. Here is a short example that plays a raw WAV file from the Pico's flash memory using an I2S DAC module:

from machine import I2S, Pin  # import I2S and Pin classes
import os                      # import os to work with files

# I2S pins — change these numbers to match your wiring
SCK_PIN = 10   # serial clock (bit clock)
WS_PIN  = 11   # word select (left/right channel clock)
SD_PIN  = 9    # serial data (audio output)

# Set up an I2S transmitter
audio_out = I2S(
    0,                          # use I2S peripheral number 0
    sck=Pin(SCK_PIN),           # connect the bit clock to SCK_PIN
    ws=Pin(WS_PIN),             # connect the word select to WS_PIN
    sd=Pin(SD_PIN),             # connect the serial data to SD_PIN
    mode=I2S.TX,                # TX means transmit (we are playing audio)
    bits=16,                    # use 16-bit audio samples
    format=I2S.STEREO,          # stereo (left and right channels)
    rate=44100,                 # 44,100 samples per second (CD quality)
    ibuf=20000                  # internal buffer size in bytes
)

# Open a WAV file and play it (skip the 44-byte header first)
with open("audio.wav", "rb") as wav_file:
    wav_file.seek(44)           # jump past the WAV file header (44 bytes)
    buf = bytearray(4096)       # create a 4 KB buffer to read chunks into
    while True:
        num_read = wav_file.readinto(buf)   # read up to 4 KB of audio data
        if num_read == 0:
            break               # stop when we reach the end of the file
        audio_out.write(buf[:num_read])     # send the audio data to the DAC

audio_out.deinit()             # release the I2S hardware when done

What Each Line Does

1. I2S(0, mode=I2S.TX, ...) — creates an I2S transmitter using peripheral 0.
2. bits=16 — each audio sample is 16 bits wide.
3. format=I2S.STEREO — two channels (left and right).
4. rate=44100 — 44,100 samples per second (CD quality).
5. ibuf=20000 — the Pico buffers 20,000 bytes of audio so playback stays smooth.
6. wav_file.seek(44) — skips the WAV file header. The first 44 bytes describe the file format, not the audio.
7. audio_out.write(buf[:num_read]) — sends the chunk of audio data to the DAC over I2S.
8. audio_out.deinit() — frees the I2S hardware so other programs can use it.

Common I2S DAC Modules

Module	Chip	Channels	Cost
MAX98357A	MAX98357A	Mono, 3 W	about $2
PCM5102A	PCM5102A	Stereo	about $3
UDA1334A	UDA1334A	Stereo	about $5

The MAX98357A is the most popular choice for beginners. It includes a speaker amplifier, so you can connect a small speaker directly to it with no extra parts.

Monty's Tip

Convert your audio files to 16-bit, 44.1 kHz WAV format before copying them to the Pico. The Converting Audio Files lab shows you exactly how to do this with free tools.

References

1. Wikipedia — I2S
2. MicroPython I2S Documentation
3. The I2S Bus Lab — detailed wiring and usage examples

Great Work!

You now understand how I2S works and how to use it in MicroPython. Next, you will learn how to convert MP3 files into the WAV format the Pico can play.