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feat: change the compressor unit to apply post-gain instead of pregain.
Pregaining ran into trouble: could not bring the signal level back to near 0dB. For example, with infinite ratio in the pre-gain system, the signal level was capped at threshold, which in turn ran into trouble with stereo signals.
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@ -353,18 +353,16 @@ su_op_delay_loop:
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; Stereo: push g g on stack, where g is calculated using l^2 + r^2
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;-------------------------------------------------------------------------------
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{{.Func "su_op_compressor" "Opcode"}}
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fdiv dword [{{.Input "compressor" "invgain"}}]; l/g, we'll call this pre inverse gained signal x from now on
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fld st0 ; x x
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fmul st0, st0 ; x^2 x
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{{- if .StereoAndMono "compressor"}}
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jnc su_op_compressor_mono
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{{- end}}
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{{- if .Stereo "compressor"}}
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fld st2 ; r x^2 l/g r
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fdiv dword [{{.Input "compressor" "invgain"}}]; r/g, we'll call this pre inverse gained signal y from now on
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fst st3 ; y x^2 l/g r/g
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fmul st0, st0 ; y^2 x^2 l/g r/g
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faddp st1, st0 ; y^2+x^2 l/g r/g
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fld st2 ; r x^2 l r
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fst st3 ; y x^2 l r
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fmul st0, st0 ; y^2 x^2 l r
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faddp st1, st0 ; y^2+x^2 l r
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call su_op_compressor_mono ; So, for stereo, we square both left & right and add them up
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fld st0 ; and return the computed gain two times, ready for MULP STEREO
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ret
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@ -389,9 +387,11 @@ su_op_compressor_mono:
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fmul dword [{{.Float 0.5 | .Use}}] ; p=r/2 t*t/l' x
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fxch ; t*t/l' p x
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fyl2x ; p*log2(t*t/l') x
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{{.TailCall "su_power"}} ; 2^(p*log2(t*t/l')) x
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; tail call ; Equal to:
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{{.Call "su_power"}} ; 2^(p*log2(t*t/l')) x
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; Equal to:
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; (t*t/l')^p x
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; if ratio is at minimum => p=0 => 1 x
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; if ratio is at maximum => p=0.5 => t/x => t/x*x=t
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fdiv dword [{{.Input "compressor" "invgain"}}]; this used to be pregain but that ran into problems with getting back up to 0 dB so postgain should be better at that
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ret
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{{- end}}
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@ -409,10 +409,6 @@
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;; Stereo: push g g on stack, where g is calculated using l^2 + r^2
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;;-------------------------------------------------------------------------------
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(func $su_op_compressor (param $stereo i32) (local $x2 f32) (local $level f32) (local $t2 f32)
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(call $push (f32.div ;; the inverse gain is applied on this signal, even if the gain is side-chained somewhere else
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(call $pop)
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(call $input (i32.const {{.InputNumber "compressor" "invgain"}}))
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))
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{{- if .Stereo "compressor"}}
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(local.set $x2 (f32.mul
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(call $peek)
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@ -420,10 +416,6 @@
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))
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(if (local.get $stereo)(then
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(call $pop)
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(call $push (f32.div
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(call $pop)
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(call $input (i32.const {{.InputNumber "compressor" "invgain"}}))
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))
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(local.set $x2 (f32.add
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(local.get $x2)
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(f32.mul
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@ -472,6 +464,10 @@
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)(else
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(call $push (f32.const 1)) ;; unity gain if we are below threshold
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))
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(call $push (f32.div ;; apply post-gain ("make up gain")
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(call $pop)
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(call $input (i32.const {{.InputNumber "compressor" "invgain"}}))
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))
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{{- if .Stereo "compressor"}}
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(if (local.get $stereo)(then
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(call $push (call $peek))
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@ -49,7 +49,7 @@ patch:
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parameters: {stereo: 1}
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id: 1
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- type: compressor
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parameters: {attack: 32, invgain: 32, ratio: 96, release: 64, stereo: 0, threshold: 64}
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parameters: {attack: 16, invgain: 64, ratio: 96, release: 64, stereo: 0, threshold: 51}
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- type: mulp
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parameters: {stereo: 0}
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- type: out
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@ -49,7 +49,7 @@ patch:
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parameters: {stereo: 1}
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id: 1
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- type: compressor
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parameters: {attack: 32, invgain: 32, ratio: 96, release: 64, stereo: 1, threshold: 64}
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parameters: {attack: 16, invgain: 64, ratio: 96, release: 64, stereo: 1, threshold: 71}
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- type: mulp
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parameters: {stereo: 1}
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- type: out
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@ -505,10 +505,8 @@ func (s *Interpreter) Render(buffer []float32, syncBuf []float32, maxtime int) (
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}
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unit.ports[4] = 0
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case opCompressor:
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stack[l-1] /= params[2] // apply inverse gain
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signalLevel := stack[l-1] * stack[l-1] // square the signal to get power
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if stereo {
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stack[l-2] /= params[2] // apply inverse gain
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signalLevel += stack[l-2] * stack[l-2]
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}
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currentLevel := unit.state[0]
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@ -523,6 +521,7 @@ func (s *Interpreter) Render(buffer []float32, syncBuf []float32, maxtime int) (
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if threshold2 := params[3] * params[3]; currentLevel > threshold2 {
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gain = float32(math.Pow(float64(threshold2/currentLevel), float64(params[4]/2)))
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}
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gain /= params[2] // apply inverse gain
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stack = append(stack, gain)
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if stereo {
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stack = append(stack, gain)
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