How to use Python on a RPi to develop a custom headless guitar FX box

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# How to use **Python** on a RPi<br>to develop a custom<br>headless guitar FX box

## Matthieu Amiguet
### Les Chemins de Traverse

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Improvised prelude in baroque style, Matthieu Amiguet, harpejji (2024)

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## Atypical choices

- Custom audio processing software
- Only FOSS
- Python

</div>

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# Outline

- FX programming with python
- Going headless
- User interaction

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# FX programming<br>with python<br>and the `pyo` module

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## `pyo`

- Developed by Olivier Bélanger in Montréal
- Sound engine programmed in C
- Processing chain manipulated from python
- Combines C efficiency with Python versatility
- <http://ajaxsoundstudio.com/software/pyo/>

![](pics/pyo.jpg)

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## Olivier Bélanger<br>*Run*<br>(2011)

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But `pyo` is also an

*awesome* tool

for real-time audio processing

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## Sound trough
### (a.k.a. *Hello Audio World*)

```python[|1|3|5-6|8|10]
import pyo

s = pyo.Server(audio='jack').boot()

guitar = pyo.Input()
guitar.out()

s.start()

input('Running. Press <enter> to terminate.')
```

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## Sound trough, two inputs

```python[]
bass = pyo.Input(0)
melody = pyo.Input(1)
harpejji = pyo.Mix([bass,melody])
harpejji.out()
```

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## Adding Reverb

```python[|6|5|7-10]
bass = pyo.Input(0)
melody = pyo.Input(1)
harpejji = pyo.Mix([bass,melody]).out()

reverb = pyo.FreeVerb(
  harpejji, # input
  size=.6,  # length of reverb
  damp=1,   # High frequency attenuation
  bal=1,    # Balance between dry and wet signal
  mul=.6    # reverb volume
).out()
```

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## Auto-wah

```python[|5-10|4,11-12|15-21|]
harpejji = pyo.Input([0,1]).mix(1) # no out - no dry sound!

# Amplitude follower, smoothed out
fol = pyo.SigTo(
    pyo.Follower(
      harpejji, # input
      freq=30, # cutoff frequency
      mul=10000, # multiply by 10000
      add=40 # ..and add 40
    ),
    time=0.01,
)

# Bandpass filter
wah = pyo.Biquad(
    harpejji, # input
    type=2,   # 2 -> bandpass
    freq=fol, # center frequency
    q=5,      # width
    mul=2.5   # amplification
)
```

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## Make it bypass-able

```python[]
selector = pyo.Selector(
  [harpejji, wah],
  voice=0,
).out()

# Activate FX
# selector.setVoice(1)

# De-activate FX
# selector.setVoice(0)
```

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![](pics/Créolité_Suisse-15.jpg)

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# <br>Going headless

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# Going headless
## Hardware

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## Which computer for audio processing?

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## Which computer for audio processing?

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## Model selection

![](pics/rpi3b.webp)

Can we achieve high quality low latency<br>audio processing on a 2016 single board?

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## USB-powered sound interface

![](pics/u24xl.jpg)

Problems:

- high-pitched hiss
  
  (switched mode power supply)
- low buzz

(only in certain configurations - floating ground)

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## Pisound

rpi hat with
- high quality stereo audio in and out
- MIDI
- a button

![](pics/pisound.jpg)

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## Pisound (with case)

![](pics/pisound_box.jpg)

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## To solve the buzz

### Don't try this at home!

### (unless you know what you're doing)

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# Going headless
## Software

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# OS

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# OS

- rpi OS 
  - no rt-kernel, latency to high
- debian 
  - possible with some tweaking (rt-kernel, cpu governor, jack config, ...)
  - see <https://matthieuamiguet.ch/blog/rpi-diy-fx-pedal/>
- Patchbox OS by Blokas: works out of the box 🤩

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# Auto-start

<pre><code data-trim data-noescape data-line-numbers="|14|9-11|1" data-fragment-index="0">
# /etc/systemd/system/harpejji.service

[Unit]
Description=Harpejji
Wants=openstagecontrol.service

[Service]
Type=simple
LimitRTPRIO=95
LimitMEMLOCK=infinity
Environment=JACK_PROMISCUOUS_SERVER=jack
User=patch
Group=patch
ExecStart=/usr/bin/python3 /home/patch/harpejji/harpejji.py

[Install]
WantedBy=multi-user.target
</code></pre>

then run

```
# systemctl enable harpejji.service
```

</div>

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# User interaction

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## Three possibilities

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# User interaction
## MIDI

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## Sound trough, two inputs with MIDI pedal volume

```python[|1|2|9|]
pedal = pyo.Midictl(channel=1, ctlnumber=7)
volume = pyo.SigTo(pedal, .1)

bass = pyo.Input(0)
melody = pyo.Input(1)

harpejji = pyo.Mix(
  [bass,melody],
  mul=volume
)

harpejji.out()
```

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## Sustainer

```python[|5|6|7-12]
class Sustainer:

def __init__(self, input, dur=0.5, mul=1):

self.buffer = pyo.NewTable(dur)
        self.bufrec = pyo.TableRec(input, self.buffer)
        self.sustain = pyo.Looper(
          self.buffer,
          dur=dur,
          xfade=[47, 48, 49, 50],
          mul=mul
        ).stop()
        self.out = pyo.Sig(self.sustain, mul=0)

def on(self):
      ...

def off(self):
      ...

def stop(self):
      ...
```

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## Double-buffered pedal sustainer

```python[|5|7-8]
class DBPSus:

def __init__(self, input, midi_pedal, port=1, dur=0.5):
        self.vol = pyo.SigTo(midi_pedal, time=port)
        self.buffers = [Sustainer(input, dur, mul=self.vol) for _ in range(2)]
        self.control = [
          pyo.TrigFunc(pyo.Thresh(midi_pedal, 0.1, dir=0), self.on),
          pyo.TrigFunc(pyo.Thresh(midi_pedal, 0.1, dir=1), self.off),
        ]
        self.current_buf = 0

def on(self):
        self.buffers[self.current_buf].on()

def off(self):
        self.buffers[self.current_buf].off()
        self.current_buf = 1 - self.current_buf
```

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Matthieu Amiguet, Improvisation "like an organ", harpejji (2024)

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# User interaction
## Open Sound Control (OSC)

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## Auto-wah (OSC version)

```python[|1|7-8,16|20,23]
ui = pyo.OscReceive(9901, address=['/wah', '/wah_add', '/wah_mul'])

fol = pyo.SigTo(
    pyo.Follower(
      harpejji,
      freq=30,
      mul=ui["/wah_mul"],
      add=ui["/wah_add"]),
    time=0.01,
)

wah = pyo.Biquad(harpejji, freq=fol, q=5, type=2, mul=2.5)

selector = pyo.Selector(
  [dry, wah],
  voice=ui["/wah"]
).out()

# Activate FX
# ~$ oscsend localhost 9901 /wah i 1

# De-activate FX
# ~$ oscsend localhost 9901 /wah i 0
```

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## Open Stage Control

![](pics/osc.png)

<https://openstagecontrol.ammd.net/>

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Pierre-Yves Diacon, dance // Matthieu Amiguet, harpejji (2022)

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# User interaction
## The Button

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## The PiSound Button

The PiSound only has one button, but you can assign scripts to

- Button down, button up
- Single, double, triple clic (up to 8)
- Button hold for 1 second (or 3, 5, 7 seconds)

</div>

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## Activate auto-wah on single button clic

```python[|5|2|]
#!/bin/sh
# /usr/local/pisound/scripts/pisound-btn/wah_on.sh

. /usr/local/pisound/scripts/common/common.sh
oscsend localhost 9901 /wah i 1
flash_leds 1
```

then use `pisound-config` to map the action.

**Note** There are some useful default actions like

`shutdown` when the button is hold for 5 seconds

see <https://blokas.io/pisound/docs/the-button/>

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## Goodie: DSP load and *xrun* feedback

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```python[|2-3|14|20-22|26-33]
import time
import jack
from pythonosc import udp_client

client = jack.Client(name="OSC-status")

osc_sender = udp_client.SimpleUDPClient("127.0.0.1", 8080)

xruns = 0
total_delay = 0
last_xrun = time.monotonic()

@client.set_xrun_callback
def xrun(delay):
    global xruns
    global total_delay
    global last_xrun

xruns += 1
    total_delay += delay
    last_xrun = time.monotonic()

with client:
    while True:
        now = time.monotonic()
        secs_since_last = now - last_xrun
        osc_sender.send_message("/cpu_load", int(client.cpu_load()))
        osc_sender.send_message(
          "/xruns", f"xruns: {xruns} ({total_delay}) - {secs_since_last:.2f}s"
        )
        time.sleep(1)
```

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# Conclusion

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## Main Takeaways

- Low-latency audio processing on an older rpi is perfectly possible
- python / `pyo`
- Pisound 
- Patchbox OS 
- Systemd
- MIDI / OSC

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Music first!

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Improvisation inspired by Middle-east music, Matthieu Amiguet, harpejji (2024)
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# Questions?

Full code, slides and context:

<https://inclu.re/ep25>

### Credits

- Harpejji & contrabass flute & rig photos by Nicolas Meyer
- Dual USB-C connector is [xkcd #2493](https://xkcd.com/2493/)
- Drawings by Matthieu Amiguet
- *Run* by Olivier Bélanger
- Other audio & video by [Les Chemins de Traverse](https://www.lescheminsdetraverse.net/)