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Lab 05: Test the Speaker

This lab plays a steady 400 Hz tone for two seconds. It talks to the amplifier directly, without the kit's sound library, so the only things that can stop it are the wires and the amplifier.

Welcome to Lab 05

Chrono waving welcome This is the lab where your clock finds its voice. One steady beep today means chimes, alarms, and timer beeps later. Let's make time tick!

What You Will Learn

  • how a computer makes sound out of numbers
  • what samples and sample rate mean
  • how the Pico sends sound to the amplifier over I2S

Run It

Keep the speaker away from your ear, open 05-speaker-test.py, and click Run. You should hear a two-second beep. The shell prints:

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05-speaker-test.py v1.0
I2S started: BCLK GP9, LRC GP10, DIN GP11, gain 9 dB
Playing 400 Hz for 2 seconds at volume 40 -- listen now
Done after 2122 ms.
Heard it? The amp and speaker work. Heard nothing? See the list at
the top of this file.

Sound Is a List of Numbers

A speaker makes sound by pushing its cone out and pulling it back, very fast. A 400 Hz tone pushes and pulls 400 times a second. To describe that movement, the program sends a number for where the cone should be, 16,000 times every second. Each number is a sample, and 16,000 per second is the sample rate.

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RATE = 16000            # samples per second
FREQ = 400              # 16000 / 400 = exactly 40 samples per wave

A wave, one number at a time

Chrono thinking Think of a flip book. Each page is one sample, and flipped fast enough the pages become smooth motion. Here, 40 numbers trace one full wave, and 400 of those waves every second are the beep you hear.

The program works out one chunk of the wave with math.sin() and sends it again and again. The first chunk fades in and the last fades out, so the tone starts and stops without a click.

Sending It: I2S

The samples travel to the amplifier on three wires, called I2S:

Wire Pico pin What it does
BCLK GP9 Ticks once for every bit of every sample
LRC GP10 Marks where each sample starts
DIN GP11 The bits of the samples themselves 
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config.set_gain()
audio = I2S(0, sck=Pin(config.I2S_BCLK_PIN), ws=Pin(config.I2S_LRC_PIN),
            sd=Pin(config.I2S_DIN_PIN), mode=I2S.TX, bits=16,
            format=I2S.MONO, rate=RATE, ibuf=4096)

config.set_gain() sets the amplifier's GAIN pin. On this kit the GAIN wire goes to a Pico pin, so a program can pick 6, 9, or 12 decibels of boost (GAIN_DB in config.py).

Try This

  1. Change FREQ to 800. The pitch goes up an octave. Keep FREQ a number that divides 16,000 exactly, like 200, 500, 800, or 1000.
  2. Change VOLUME in config.py to 10, then 80. Which is comfortable?
  3. Change GAIN_DB in config.py to 12. How much louder is it?

If It Doesn't Work

  • No sound at all. Check the amplifier's VIN and GND first: with no power it is silent. Then check that DIN goes to GP11, and that the speaker's wires are in the amplifier's + and − terminals.
  • An error when I2S() starts. BCLK and LRC are swapped. LRC must be on the pin right after BCLK.
  • GAIN_DB must be 6, 9, or 12. Fix the number in config.py.

Your Clock Has a Voice!

Chrono celebrating You turned a list of numbers into a real sound, and you know what samples, sample rate, and I2S mean. Every chime and alarm in this kit is built the same way.

Next: Lab 06: Check Your WiFi