vaeffects.lib

A library of virtual analog filter effects. Its official prefix is ve.

References

Moog Filters

(ve.)moog_vcf

Moog "Voltage Controlled Filter" (VCF) in "analog" form. Moog VCF implemented using the same logical block diagram as the classic analog circuit. As such, it neglects the one-sample delay associated with the feedback path around the four one-poles. This extra delay alters the response, especially at high frequencies (see reference [1] for details). See moog_vcf_2b below for a more accurate implementation.

Usage

_ : moog_vcf(res,fr) : _

Where:

  • res: normalized amount of corner-resonance between 0 and 1 (0 is no resonance, 1 is maximum)
  • fr: corner-resonance frequency in Hz (less than SR/6.3 or so)

References

(ve.)moog_vcf_2b[n]

Moog "Voltage Controlled Filter" (VCF) as two biquads. Implementation of the ideal Moog VCF transfer function factored into second-order sections. As a result, it is more accurate than moog_vcf above, but its coefficient formulas are more complex when one or both parameters are varied. Here, res is the fourth root of that in moog_vcf, so, as the sampling rate approaches infinity, moog_vcf(res,fr) becomes equivalent to moog_vcf_2b[n](res^4,fr) (when res and fr are constant). moog_vcf_2b uses two direct-form biquads (tf2). moog_vcf_2bn uses two protected normalized-ladder biquads (tf2np).

Usage

_ : moog_vcf_2b(res,fr) : _
_ : moog_vcf_2bn(res,fr) : _

Where:

  • res: normalized amount of corner-resonance between 0 and 1 (0 is min resonance, 1 is maximum)
  • fr: corner-resonance frequency in Hz

(ve.)moogLadder

Virtual analog model of the 4th-order Moog Ladder, which is arguably the most well-known ladder filter in analog synthesizers. Several 1st-order filters are cascaded in series. Feedback is then used, in part, to control the cut-off frequency and the resonance.

References

[Zavalishin 2012] (revision 2.1.2, February 2020):

This fix is based on Lorenzo Della Cioppa's correction to Pirkle's implementation; see this post: https://www.kvraudio.com/forum/viewtopic.php?f=33&t=571909

Usage

_ : moogLadder(normFreq,Q) : _

Where:

  • normFreq: normalized frequency (0-1)
  • Q: quality factor between .707 (0 feedback coefficient) to 25 (feedback = 4, which is the self-oscillating threshold).

(ve.)moogHalfLadder

Virtual analog model of the 2nd-order Moog Half Ladder (simplified version of (ve.)moogLadder). Several 1st-order filters are cascaded in series. Feedback is then used, in part, to control the cut-off frequency and the resonance.

This filter was implemented in Faust by Eric Tarr during the 2019 Embedded DSP With Faust Workshop.

References

Usage

_ : moogHalfLadder(normFreq,Q) : _

Where:

  • normFreq: normalized frequency (0-1)
  • Q: q

(ve.)diodeLadder

4th order virtual analog diode ladder filter. In addition to the individual states used within each independent 1st-order filter, there are also additional feedback paths found in the block diagram. These feedback paths are labeled as connecting states. Rather than separately storing these connecting states in the Faust implementation, they are simply implicitly calculated by tracing back to the other states (s1,s2,s3,s4) each recursive step.

This filter was implemented in Faust by Eric Tarr during the 2019 Embedded DSP With Faust Workshop.

References

Usage

_ : diodeLadder(normFreq,Q) : _

Where:

  • normFreq: normalized frequency (0-1)
  • Q: q

Korg 35 Filters

The following filters are virtual analog models of the Korg 35 low-pass filter and high-pass filter found in the MS-10 and MS-20 synthesizers. The virtual analog models for the LPF and HPF are different, making these filters more interesting than simply tapping different states of the same circuit.

These filters were implemented in Faust by Eric Tarr during the 2019 Embedded DSP With Faust Workshop.

Filter history:

(ve.)korg35LPF

Virtual analog models of the Korg 35 low-pass filter found in the MS-10 and MS-20 synthesizers.

Usage

_ : korg35LPF(normFreq,Q) : _

Where:

  • normFreq: normalized frequency (0-1)
  • Q: q

(ve.)korg35HPF

Virtual analog models of the Korg 35 high-pass filter found in the MS-10 and MS-20 synthesizers.

Usage

_ : korg35HPF(normFreq,Q) : _

Where:

  • normFreq: normalized frequency (0-1)
  • Q: q

Oberheim Filters

The following filter (4 types) is an implementation of the virtual analog model described in Section 7.2 of the Will Pirkle book, "Designing Software Synthesizer Plug-ins in C++". It is based on the block diagram in Figure 7.5.

The Oberheim filter is a state-variable filter with soft-clipping distortion within the circuit.

In many VA filters, distortion is accomplished using the "tanh" function. For this Faust implementation, that distortion function was replaced with the (ef.)cubicnl function.

(ve.)oberheim

Generic multi-outputs Oberheim filter that produces the BSF, BPF, HPF and LPF outputs (see description above).

Usage

_ : oberheim(normFreq,Q) : _,_,_,_

Where:

  • normFreq: normalized frequency (0-1)
  • Q: q

(ve.)oberheimBSF

Band-Stop Oberheim filter (see description above). Specialize the generic implementation: keep the first BSF output, the compiler will only generate the needed code.

Usage

_ : oberheimBSF(normFreq,Q) : _

Where:

  • normFreq: normalized frequency (0-1)
  • Q: q

(ve.)oberheimBPF

Band-Pass Oberheim filter (see description above). Specialize the generic implementation: keep the second BPF output, the compiler will only generate the needed code.

Usage

_ : oberheimBPF(normFreq,Q) : _

Where:

  • normFreq: normalized frequency (0-1)
  • Q: q

(ve.)oberheimHPF

High-Pass Oberheim filter (see description above). Specialize the generic implementation: keep the third HPF output, the compiler will only generate the needed code.

Usage

_ : oberheimHPF(normFreq,Q) : _

Where:

  • normFreq: normalized frequency (0-1)
  • Q: q

(ve.)oberheimLPF

Low-Pass Oberheim filter (see description above). Specialize the generic implementation: keep the fourth LPF output, the compiler will only generate the needed code.

Usage

_ : oberheimLPF(normFreq,Q) : _

Where:

  • normFreq: normalized frequency (0-1)
  • Q: q

Sallen Key Filters

The following filters were implemented based on VA models of synthesizer filters.

The modeling approach is based on a Topology Preserving Transform (TPT) to resolve the delay-free feedback loop in the corresponding analog filters.

The primary processing block used to build other filters (Moog, Korg, etc.) is based on a 1st-order Sallen-Key filter.

The filters included in this script are 1st-order LPF/HPF and 2nd-order state-variable filters capable of LPF, HPF, and BPF.

Resources:

(ve.)sallenKeyOnePole

Sallen-Key generic One Pole filter that produces the LPF and HPF outputs (see description above).

For the Faust implementation of this filter, recursion (letrec) is used for storing filter "states". The output (e.g. y) is calculated by using the input signal and the previous states of the filter. During the current recursive step, the states of the filter (e.g. s) for the next step are also calculated. Admittedly, this is not an efficient way to implement a filter because it requires independently calculating the output and each state during each recursive step. However, it works as a way to store and use "states" within the constraints of Faust. The simplest example is the 1st-order LPF (shown on the cover of Zavalishin * 2018 and Fig 4.3 of https://www.willpirkle.com/706-2/). Here, the input signal is split in parallel for the calculation of the output signal, y, and the state s. The value of the state is only used for feedback to the next step of recursion. It is blocked (!) from also being routed to the output. A trick used for calculating the state s is to observe that the input to the delay block is the sum of two signal: what appears to be a feedforward path and a feedback path. In reality, the signals being summed are identical (signal*2) plus the value of the current state.

Usage

_ : sallenKeyOnePole(normFreq) : _,_

Where:

  • normFreq: normalized frequency (0-1)

(ve.)sallenKeyOnePoleLPF

Sallen-Key One Pole lowpass filter (see description above). Specialize the generic implementation: keep the first LPF output, the compiler will only generate the needed code.

Usage

_ : sallenKeyOnePoleLPF(normFreq) : _

Where:

  • normFreq: normalized frequency (0-1)

(ve.)sallenKeyOnePoleHPF

Sallen-Key One Pole Highpass filter (see description above). The dry input signal is routed in parallel to the output. The LPF'd signal is subtracted from the input so that the HPF remains. Specialize the generic implementation: keep the second HPF output, the compiler will only generate the needed code.

Usage

_ : sallenKeyOnePoleHPF(normFreq) : _

Where:

  • normFreq: normalized frequency (0-1)

(ve.)sallenKey2ndOrder

Sallen-Key generic 2nd order filter that produces the LPF, BPF and HPF outputs.

This is a 2nd-order Sallen-Key state-variable filter. The idea is that by "tapping" into different points in the circuit, different filters (LPF,BPF,HPF) can be achieved. See Figure 4.6 of * https://www.willpirkle.com/706-2/

This is also a good example of the next step for generalizing the Faust programming approach used for all these VA filters. In this case, there are three things to calculate each recursive step (y,s1,s2). For each thing, the circuit is only calculated up to that point.

Comparing the LPF to BPF, the output signal (y) is calculated similarly. Except, the output of the BPF stops earlier in the circuit. Similarly, the states (s1 and s2) only differ in that s2 includes a couple more terms beyond what is used for s1.

Usage

_ : sallenKey2ndOrder(normFreq,Q) : _,_,_

Where:

  • normFreq: normalized frequency (0-1)
  • Q: q

(ve.)sallenKey2ndOrderLPF

Sallen-Key 2nd order lowpass filter (see description above). Specialize the generic implementation: keep the first LPF output, the compiler will only generate the needed code.

Usage

_ : sallenKey2ndOrderLPF(normFreq,Q) : _

Where:

  • normFreq: normalized frequency (0-1)
  • Q: q

(ve.)sallenKey2ndOrderBPF

Sallen-Key 2nd order bandpass filter (see description above). Specialize the generic implementation: keep the second BPF output, the compiler will only generate the needed code.

Usage

_ : sallenKey2ndOrderBPF(normFreq,Q) : _

Where:

  • normFreq: normalized frequency (0-1)
  • Q: q

(ve.)sallenKey2ndOrderHPF

Sallen-Key 2nd order highpass filter (see description above). Specialize the generic implementation: keep the third HPF output, the compiler will only generate the needed code.

Usage

_ : sallenKey2ndOrderHPF(normFreq,Q) : _

Where:

  • normFreq: normalized frequency (0-1)
  • Q: q

Effects

(ve.)wah4

Wah effect, 4th order. wah4 is a standard Faust function.

Usage

_ : wah4(fr) : _

Where:

  • fr: resonance frequency in Hz

Reference

(ve.)autowah

Auto-wah effect. autowah is a standard Faust function.

Usage

_ : autowah(level) : _

Where:

  • level: amount of effect desired (0 to 1).

(ve.)crybaby

Digitized CryBaby wah pedal. crybaby is a standard Faust function.

Usage

_ : crybaby(wah) : _

Where:

  • wah: "pedal angle" from 0 to 1

Reference

(ve.)vocoder

A very simple vocoder where the spectrum of the modulation signal is analyzed using a filter bank. vocoder is a standard Faust function.

Usage

_ : vocoder(nBands,att,rel,BWRatio,source,excitation) : _

Where:

  • nBands: Number of vocoder bands
  • att: Attack time in seconds
  • rel: Release time in seconds
  • BWRatio: Coefficient to adjust the bandwidth of each band (0.1 - 2)
  • source: Modulation signal
  • excitation: Excitation/Carrier signal