WAV file adjustable playback rate

[todbot]

Being able to easily play WAV files with audiocore.WaveFile is amazing,
but there's no way to change the playback speed of the WAV playback.

If WaveFile had a .rate property that could be adjustable in real-time,
it opens up many creative / musical uses:

• "ROMpler" sample player keyboard (single sample repitched across a key range)
• Drum loop BPM matching
• Tuning drum samples for drum machines
• Fun dynamic pitch effects for recorded audio

The WAV repitch algorithm can be pretty simple using the standard phase accumulator
technique. The additional code size to WaveFile should have a fairly small impact
on the smaller platforms.
For CPU efficiency, fixed-point integer math can be used for resampling and the common-case of rate=1.0 could early-out and revert to previous behavior.

In use, the feature would look something like:

wav = audiocore.WaveFile("my_cool_sound.wav") 
mixer.voice[0].play(wav, loop=True)
wav.rate = 0.5  # play at half-speed / an octave down
wav.rate = 2.0  # play at 2x speed / an octave up
wav.rate = 1.0  # play at normal speed

[tannewt]

How does this approach compare to having a separate class that would take in any sample and do this? That'd apply to mp3 then too.

[todbot]

How does this approach compare to having a separate class that would take in any sample and do this? That'd apply to mp3 then too.

I had initially investigated it and thought it may not work correctly for looping WAVs and cause too much increase in code size. I'll look at it again and report back.

[tannewt]

Code size will be easier since we can omit it for small boards.

[todbot]

Okay I made a audiocore.SpeedChanger module that works with any audio source. And looping of WAVs works correctly. Close previous PR and proceed with this one?

Looks like it adds 1.5kB to a RP2040 build and needs ~40 bytes + a 512 byte buffer RAM.
This is compared to 600 bytes and 8 bytes of extra RAM of the WaveFile only impl.

It's a LOT of fun. Doing turntable-like effects on an MP3 is pretty dope.

speedchanger-mp3.mp4

Below is what the API looks like. It wraps an audio source and inherits the properties of the source.

import time, random
import board, analogio, audiopwmio
import audiocore, audiobusio, audiomixer, audiomp3
pot = analogio.AnalogIn(board.GP26)  # ADC0
audio = audiopwmio.PWMAudioOut(board.GP28)  # any pin
mixer = audiomixer.Mixer(sample_rate=22050, channel_count=1, buffer_size=2048)
audio.play(mixer)

mp3 = audiomp3.MP3Decoder("01_Bartlebeats_Carpeter_.mp3")
speedmp3 = audiocore.SpeedChanger(mp3)
mixer.voice[0].play(speedmp3, loop=True)

potval = 0.5
smooth = 0.95
while True:
    potval = pot.value/65535 * (1-smooth) + smooth*potval  # simple EMA filter
    print("potval: %.2f rate: %.2f" % (potval, speedmp3.rate))
    speedmp3.rate = max(2*potval, 0.01)
    time.sleep(0.05)

[tannewt]

That's awesome! I'd just make a new module for it instead of putting it in audiocore. I'd definitely go this direction. Thanks!