Add Biquad Filters to audiodelays.Echo and audiofreeverb.Freeverb

[relic-se]

This update expands the capabilities of audiodelays.Echo and audiofreeverb.Freeverb by adding synthio.Biquad filter support internally to their audio processing chain.

Echo

The audiodelays.Echo.filter property filters samples before they are stored in the echo buffer to be recalled later. This can produce a "tape-like" effect when repeated echos are processed through the filter on each iteration.

Freeverb

The audiofreeverb.Freeverb.pre_filter and audiofreeverb.Freeverb.post_filter properties allow the incoming sample to be filtered going into and out of the reverb algorithm. These new "Pre-EQ" and "Post-EQ" allow deeper control and shaping of the reverb effect while maintaining the integrity of the original sample.

Demonstration

# boot.py
import usb_audio
usb_audio.enable(sample_rate=8000, channel_count=1)

# code.py
from audiodelays import Echo
from audiofilters import Filter
from audiofreeverb import Freeverb
import synthio
import time
import ulab.numpy as np
from usb_audio import usb_microphone

FILTER = False

synth = synthio.Synthesizer(
    envelope=synthio.Envelope(
        attack_time=0.05,
        release_time=0.05,
        attack_level=0.7,
        sustain_level=0.7,
    ),
    
    sample_rate=8000,
    channel_count=1,
)

effect_echo = Echo(
    max_delay_ms=200,
    delay_ms=200,
    decay=0.7,
    filter=synthio.Biquad(synthio.FilterMode.LOW_PASS, synthio.LFO(offset=1000, scale=800, rate=0.3), 1.5) if FILTER else None,
    mix=0.5,
    freq_shift=True,
    
    sample_rate=synth.sample_rate,
    channel_count=synth.channel_count,
)

effect_reverb = Freeverb(
    pre_filter=synthio.Biquad(synthio.FilterMode.HIGH_PASS, 200) if FILTER else None,
    post_filter=synthio.Biquad(synthio.FilterMode.LOW_PASS, 1000) if FILTER else None,
    mix=0.5,
    
    sample_rate=synth.sample_rate,
    channel_count=synth.channel_count,
)

# Select effect type
effect = effect_echo

# Audio Chain
effect.play(synth)
usb_microphone.play(effect, loop=True)

# Play through scale
root_note = 60  # middle C
major_scale = [0, 2, 4, 5, 7, 9, 11, 12]
try:
    for note in major_scale:
        synth.press(root_note + note)
        time.sleep(0.1)
        synth.release(root_note + note)
        time.sleep(2)
    time.sleep(2)
except KeyboardInterrupt:
    pass
usb_microphone.stop()

Results

This above program was recorded using Audacity over USB Audio under 4 conditions: dry echo, filtered echo, dry reverb, and filtered reverb. filter-tests.zip

All tests were run by a Waveshare RP2040 Zero. In some tests, audible popping can be heard. This is likely due to buffer underflows caused by the slower processor and the computational requirements of these effects or the experimental nature of usb_audio in its current state.

Notes

• Originally, I was just going to implement either pre or post filtering on audiofreeverb.Freeverb. After using the effect under each circumstance, I determined that it would be useful to have both options. "Pre-EQ" does a good job of "taming" the reverb by reducing the effect of sudden pops which can cause the reverb to act abruptly. "Post-EQ" is more artistic in the way that it shapes the reverb output.
• I decided to share much of the filter object handling within a centralized location (audio-module/audiofilters/__init__.c). This has definitely helped simplify implementation and reduce flash usage, but there is likely room for improvement with performance. I'd appreciate direction on how best to share these methods.

[tannewt]

A couple suggestions. Overall I'm excited for more synth improvements!

[tannewt]

I'd just be stricter here. The docs say it's a tuple or single Biquad. Don't support any sequence to save yourself the allocation.

[relic-se]

Do you mean to get rid of:

// convert object to tuple if it wasn't before
filter_in = MP_OBJ_TYPE_GET_SLOT(&mp_type_tuple, make_new)(
    &mp_type_tuple, 1, 0, &filter_in);
mp_obj_tuple_get(filter_in, &n_items, &items);

And replace it with a strict tuple type check? I could see some users providing a list without thinking about it which is why this code is here.

[tannewt]

I wonder if it's worth having a "filter chain" struct that holds the objs, len and states. That may make it easier to manage and clearer what can be shared.

[relic-se]

I totally thinking of this earlier. That'll definitely clear up those function calls.