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A dedicated machine

Notes for turning a handheld or a small PC into a box that boots straight into the rig. The example is an ASUS ROG Ally with a Zoom F4 as the interface. None of this is needed to use SignalPatch on a desktop.

Linux is the easier choice for this: you decide what runs, and PipeWire or JACK at 64 samples is reachable. Windows works too; notes at the end.

Latency

At 48 kHz and 64 samples the software adds 2.7 ms for the round trip. The interface's converters and USB add roughly 2 to 4 ms more, so expect 5 to 7 ms in total.

Two things tell you whether a patch fits:

  • The header shows DSP x% (pk y%). pk is the worst block. Under about 60% at your buffer size is comfortable.
  • SIGNALPATCH_BENCH_NAM_DIR=~/models ./signalpatch_tests prints average and worst cost per capture.

Measured on a desktop Zen 3 core at 64 samples: LSTM pedal captures cost 5 to 9% each, a "feather" WaveNet about 11% on average and 50% in its worst block. So several LSTM captures in a chain are fine, and a WaveNet is roughly one per patch. Measure again on the machine you will play on.

Linux setup

  1. A minimal Debian or Arch install with no desktop environment.
  2. pipewire pipewire-jack wireplumber cage greetd.
  3. Realtime priority. On Arch, realtime-privileges and the realtime group. Elsewhere, /etc/security/limits.d/audio.conf: 
    @audio - rtprio 95
@audio - memlock unlimited
    and add your user to audio. Avoid rtkit with PipeWire 1.6.8 (see BUILDING.md).
  4. A fixed quantum, in ~/.config/pipewire/pipewire.conf.d/lowlatency.conf: 
    context.properties = {
    default.clock.rate = 48000
    default.clock.quantum = 64
    default.clock.min-quantum = 32
    default.clock.max-quantum = 64
}
  5. The performance CPU governor, and a high TDP profile while plugged in.
  6. USB autosuspend off for the interface. usbcore.autosuspend=-1 on the kernel command line is blunt but reliable.
  7. Disable services the box does not need (bluetooth, cups, avahi).

Booting into the rig

/etc/greetd/config.toml:

[initial_session]
command = "cage -- env PIPEWIRE_QUANTUM=64/48000 pw-jack SignalPatch --kiosk --board --unmute"
user = "USER"

--unmute skips the muted start. That is reasonable on a dedicated box and a bad idea on a desktop with open microphones. With no patch argument the last session comes back.

The Zoom F4

Use its audio-interface mode. All six inputs and four outputs show up as ports. Set gain on the F4's preamps and monitor from its outputs.

Models

Captures go in ~/Documents/SignalPatch/models. The neural modules step through that folder with their arrow buttons, and each shows its own cost as a share of the block.

Windows

Pick "Windows Audio (Exclusive Mode)" and 64 or 128 samples under AUDIO. An ASIO build (-DSIGNALPATCH_ENABLE_ASIO=ON, with the interface's ASIO driver) is usually tighter. Use the High Performance power plan and turn off Game Bar and the vendor's overlay and updater. A Task Scheduler entry can start SignalPatch.exe --kiosk --board --unmute at logon.

Afterwards

Once it works, stop updating it, and keep an image of the disk.

Idea: ship this as a bootable image

Not started. Everything above, baked into an image you flash to a stick or a disk, so any PC becomes a dedicated multi-effect.

  • Boot: kernel, a minimal init, cage, then SignalPatch --kiosk --board --unmute, restarted if it exits. No desktop and no login.
  • Audio: ALSA directly, no PipeWire. Nothing else on the box makes sound.
  • Kernel: PREEMPT_RT (mainline since 6.12), threaded IRQs, performance governor, USB autosuspend off.
  • Disk: read-only root, and a writable partition for patches, models and recordings, so the power can be pulled like on a pedal.
  • Build: an Arch-based image with mkosi or archiso first, because it covers most PC hardware. Buildroot later if small and reproducible matters more.

What the app would need, since there is no OS around it:

  • TONE3000 login without a system browser. The current flow redirects to localhost, which a kiosk cannot do. Either a log-in-on-your-phone flow or a small embedded browser for that one screen.
  • Wi-Fi setup, power off and reboot, and screen brightness in the FILE menu.
  • Import and export of patches and models from a USB stick.
  • An updater that takes a release from the network or a stick.

Limits: NVIDIA is awkward to ship, Intel and AMD are fine. ARM boards are a separate project; a Pi 5 lacks the OpenGL 3.3 the app needs, so that means a GLES renderer first.

First step when this starts: an image that boots a ThinkPad X250 with the Zoom straight to the Board, and measured latency and xruns from it.