Skream - the Valis proof of concept

Why this one

skream-plan.md proposed a bespoke plugin: port the DSP from Scream, approximate its GUI in DPF, ship ten presets. That is a reasonable plugin and a poor proof of anything.

The better version is the same sound built as a Valis circuit - a Turtle file the user can open, read, re-patch and hand to an LLM. It is the right proof of concept because the scream filter is small enough to write in forty lines of Turtle and demanding enough to exercise the parts of Valis that are easy to get wrong:

a nonlinearity inside a feedback loop, which is the hard case for a block-level engine and the thing that forces val:UnitDelay to be real rather than decorative;
a filter with several simultaneous outputs, which forces ports to be named rather than assumed;
antiderivative anti-aliasing, which is a better answer than oversampling for a memoryless nonlinearity and which the ontology should therefore be able to express;
a gate driven by a signal from elsewhere in the circuit, which forces control-rate arcs to be first-class.

If Valis can express Scream faithfully, it can express most of what a virtual-analog effect does.

What Scream actually is

Not a synth. Scream is a distortion effect that recreates the filter from Massive that made the 2010-era growl, and it is built on published work rather than guesswork: Zavalishin's The Art of VA Filter Design ch. 6 for the nonlinearities, Andy Simper's SvfLinearTrapOptimised2 for the filter, Jatin Chowdhury's second-order ADAA for the saturator, and Giannoulis/Massberg/Reiss for the compressor.

Its inner loop (src/plugin.c, around line 600) is:

  in ──[in gain]──▶(+)──[tanh ADAA2]──[SVF lowpass]──┬──[dry/wet]──▶ out
                    ▲                                │
                    │                                ▼
                    └──[z⁻¹]──[gate]◀──[tanh ADAA2]──[SVF highpass]──[fb gain]
                                 ▲
                                 └── envelope of the input

The feedback path is what screams: the loop gain, highpass corner and second saturator set the resonant howl, and the gate - driven by an envelope follower on the input, not on the loop - stops it self-oscillating when nothing is playing.

Two details worth keeping. The Resonance control is not the filter's Q alone; in the original it raises the internal distortion drive and the Q together, which is why the sound gets dirtier as it gets more resonant. And Scream deliberately drops the original's keytracking, because an effect cannot know the pitch of its input.

The same thing in Valis

:screamLoop a val:Mixer .

:sat1 a val:Tanh ; val:antialiasing val:ADAA2 .
:lp   a val:StateVariable ; val:cutoff 800.0 ; val:resonance 4.0 .
:hp   a val:StateVariable ; val:cutoff 300.0 ; val:resonance 2.0 .
:sat2 a val:Tanh ; val:antialiasing val:ADAA2 .
:fb   a val:Gain ; val:gain -6.0 .
:z    a val:UnitDelay .
:gate a val:Expander ; val:threshold -120.0 ; val:ratio 2.0 .

# forward path
:a1 a val:Arc ; val:from [ val:node :in    ; val:port "out" ] ;
                val:to   [ val:node :screamLoop ; val:port "in" ] .
:a2 a val:Arc ; val:from [ val:node :screamLoop ; val:port "out" ] ;
                val:to   [ val:node :sat1  ; val:port "in" ] .
:a3 a val:Arc ; val:from [ val:node :sat1  ; val:port "out" ] ;
                val:to   [ val:node :lp    ; val:port "in" ] .

# feedback path, broken by the unit delay
:a7 a val:Arc ; val:from [ val:node :z     ; val:port "out" ] ;
                val:to   [ val:node :screamLoop ; val:port "in" ] .

# the gate is opened by an envelope of the input, not of the loop
:m1 a val:Arc ; val:from [ val:node :env   ; val:port "out" ] ;
                val:to   [ val:node :gate  ; val:port "amount" ] ;
                val:depth 1.0 .

The full circuit is examples/skream.ttl.

Why this beats a bespoke plugin

The commented-out lines become the interface. Scream's inner loop carries five abandoned saturators in comments - sinarctan, sinarctan2, softsine, softsine2, a plain clamp. Choosing between them meant editing C and rebuilding. In Valis it is one word in a Turtle file, live, with the audio running.

Presets can differ in topology, not just in values. A preset system stores parameter values against a fixed graph. A Valis preset is the graph, so one preset can put the saturator before the filter and another after it, or add a second feedback path, or remove the gate. The ten presets the brief asks for become ten .ttl files, and none of them are limited to the knobs the author thought of.

An LLM can design the sound. Every operation is an Op reachable over HTTP MCP, so "make the growl more vocal, keep the low end" is a sequence of graph edits against a described vocabulary rather than a request to nudge opaque sliders. That is the claim in the project's name, and this is the circuit that tests it.

It is inspectable. The graph view shows the feedback loop, the unit delay that breaks it, and the gate's control arc - the actual signal flow, not a faceplate.

What it demands of Valis

Gaps between this circuit and the ontology as it stands:

Control-rate arcs. An arc must be able to end on an lv2:ControlPort, carrying one value per block, with modulation depth as a property of the arc (val:depth) rather than of either endpoint. Without this there is no modulation and Valis is a static chain.
val:antialiasing on val:Transfer, with val:ADAA1, val:ADAA2, val:Oversample2x, val:Oversample4x and val:None. ADAA2 tanh needs double internally - single precision overflows the second antiderivative.
Named multi-output filters. val:StateVariable should expose lp, bp and hp output ports rather than a numeric mode, matching the Cytomic mixing-coefficient form it implements.
val:Expander - hard-knee expander/gate with attack and release.
val:EnvelopeFollower - peak and RMS detection, the control source for the gate.
val:Compressor - Giannoulis/Massberg/Reiss, soft knee, upward and downward, for the OTT-style stage the brief asks for after the filter.
More saturators - val:SinArcTan (x/√(x²+1)) and val:SoftSine (x/(|x|+1)), both straight from Scream and both cheap.
Feedback validation. The compiler must accept a cycle that passes through a val:UnitDelay and reject one that does not, naming the offending arc.

Acceptance

Load examples/skream.ttl in the standalone, feed it a bass line, and hear the growl. Change val:antialiasing from val:ADAA2 to val:None in the Code tab and hear the aliasing appear. Open the graph view and see the feedback loop. Raise the feedback gain over MCP and hear it approach self-oscillation, with the gate holding it silent between notes.

Licensing note

Scream is a reference for technique, not a source of code to copy. The techniques it uses are published and independently implementable: the Cytomic SVF paper, Zavalishin's book, the Giannoulis compressor paper, and Chowdhury's ADAA (BSD-3). Check /home/danny/github/Scream/LICENSE before lifting anything verbatim.