Potential feature: external audio passthrough + Surge FX processing

[MitchSchwartz]

Summary

Explore line-in audio passthrough through Surge XT’s FX chain on the MPE Sound Module — potentially concurrent with MPE synth playback and existing footswitch MIDI → FX modulation.

This is not implemented today. The reference build is a one-way instrument appliance: MPE controller → Surge synth → audio out. Effects on the synth path are already handled via programmed footswitches sending MIDI CC into Surge; this issue covers adding external audio into that same FX ecosystem.

Status: potential / parked — not required for current validation or core positioning (dedicated MPE instrument vs multi-engine FX workstation).

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Current behavior

• surge-xt-cli runs headless with MPE enabled at boot (--mpe-enable, --all-midi-inputs, etc.).
• Signal flow: Roli (or other MPE controller) → Surge voices → USB audio out (Sound Blaster Play! 3).
• Footswitches (see docs/FOOT_PEDAL.md, config/pedal-config.json, pedal scripts) send MIDI CC to modulate Surge FX parameters — already working on the synth path.
• Startup scripts configure audio output only (--audio-interface); line-in is not routed.
• Default patches and boot flow assume synth-only operation.

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Why this might be useful

• Process an external source (mic, guitar, second instrument) through Surge’s built-in reverb/delay/filter FX using the same footswitch mappings.
• Layer MPE synth + processed external audio in one box without a laptop.
• Demonstrates Surge XT headless flexibility on ARM Linux beyond pure synthesis.

Out of scope for this issue: hosting third-party VSTs (e.g. Valhalla) — no native Linux builds; Wine/yabridge is impractical on Pi.

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Surge XT capabilities (engine support)

Surge can do this in a single instance:

Mechanism	Role
Scene A	MPE synth (current use)
Scene B	Audio Input oscillator → filters → scene insert FX
Master FX	Process combined mix; footswitch CC targets apply here if mapped
Dual / split scene modes	Layer synth + external path
Sidechain inputs	Stereo line-in (same as Zynthian’s Surge sidechain routing)
Surge FX plugin	FX-only path (separate binary; alternative architecture)

Known Surge quirk: Audio Input in poly/mono modes may require a gate (MIDI note, latch mode, or purpose-built patch) for continuous passthrough — not true always-on hardware-pedal behavior without patch design.

References:

• Surge manual — Audio Input oscillator
• Using Surge as an effect and vocoder (wiki)
• Zynthian Discourse: Surge XT sidechain input routing on Pi

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Proposed behavior (if built)

1. Hardware: route line-in on the USB audio interface (Play! 3 has 3.5 mm input) or a stereo in/out interface.
2. Software: configure Surge to capture input on the same ALSA device as output.
3. Patch(es): dual-path layout — MPE synth on one scene, external audio through FX on the other (or master FX on mix).
4. Footswitches: existing CC mappings modulate FX affecting passthrough, synth, or both (patch-dependent).
5. Optional: dry/wet or bypass via footswitch CC or dedicated patch macros.

Concurrent operation target:

External line-in ──→ Surge (Scene B / master FX) ──┐
                                                    ├──→ audio out
MPE controller ──→ Surge (Scene A / MPE voices) ──┘
Footswitches ──→ MIDI CC ──→ Surge FX params

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Implementation sketch

Phase 0 — Feasibility spike (manual, not headless)

• On reference Pi, test Surge XT standalone (GUI or VNC) with Play! 3 line-in → Audio Input patch → FX → out.
• Confirm always-on passthrough patch strategy (latch / held note / master routing).
• Measure CPU % and xruns at 512 and 256 buffer with MPE + passthrough simultaneously.

Phase 1 — Headless routing

• Extend detect-audio-device.sh / start-surge-cli.sh to select input + output on the same ALSA device (verify surge-xt-cli --list-devices input flags).
• Ship one or more template patches (e.g. Templates/Audio In style) tuned for dual MPE + line-in.
• Document footswitch CC mapping for passthrough FX in docs/FOOT_PEDAL.md.

Phase 2 — Product UX (optional)

• Boot mode or patch category for “FX processor” vs “MPE synth only.”
• OLED/encoder indication when passthrough path is active (if applicable).

Architecture note: current stack uses direct ALSA, no JACK. Prefer single Surge instance over Carla/JACK pipeline unless ALSA routing proves insufficient.

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Hardware / performance notes

Concern	Guidance
Pi 4 vs Pi 5*	Current stack (MPE + footswitch FX) needs no upgrade if stable today. Passthrough adds DSP load — Pi 5 = headroom, Pi 4 = try conservative patches first.
RAM	4 GB sufficient — Surge + Lite OS + patch browser are well under 4 GB; 8 GB not expected to help unless swapping (bad for audio).
Latency (MPE path)	~512-sample buffer ≈ ~11 ms buffer term (~13–18 ms touch-to-sound total at 44.1 kHz). Unchanged by footswitches.
Latency (passthrough path)	Adds input buffer — often ~2× buffer term for external signal; fine for time-based FX, poor for tight dry monitoring.
CPU	Footswitch MIDI = negligible. Passthrough + FX = moderate; bloated 3rd-party synth patches + heavy reverb on both paths = xrun risk.

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Acceptance criteria (if pursued)

• External stereo line-in audible through Surge FX chain to module audio out
• MPE synth playable simultaneously without dropouts on reference Pi (patch TBD)
• Existing footswitch CC mappings modulate at least one FX parameter on the passthrough path
• Documented signal flow, patch setup, and latency expectations in repo docs
• No Wine / third-party VST dependency

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Priority

Low / parked — core product is a dedicated MPE instrument appliance. Pursue only if validation signal or personal performance needs justify the scope (different job than Zynthian-style FX workstation, but narrower than full DAWless mixer).

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Related

• Footswitch FX modulation: docs/FOOT_PEDAL.md, pedal scripts, config/pedal-config.json
• Positioning: single-purpose instrument (Roli → Surge → out) — see README / product narrative
• Community context: users asking about Linux FX alternatives (Valhalla etc.) — native Surge/LV2 path only on Pi

Opened: 2026-07-24 (America/Toronto)