VCA - TheTechnobear/SSP GitHub Wiki

Overview

A Quad voltage control amplifier. this is a linear quad vca, modelled on the Doepfer A-135,

Documentation

Parameters

Name Desc
Gain1-4 Gain for audio input for VCA
A1 Att -4 Attenuator signal in for VCA
C1 Att -4 Attenuator control in for VCA

Inputs

Name Desc
A1-4 Signal in for VCA
C1-4 Control in for VCA

Outputs

Name Desc
OUT1-4 Output for VCA
SUM Summed Output of all VCAs

Usage

classic use, put the input audio signal into the Audio input (A), and the control modulator (e.g. ENV) into the control input (C) control inputs are linear, not exponential.

Tips n' Tricks

Unity Gain VCA (aka classic)

Unfortunately, the default for the inputs are all 0.5, it is VERY likely you will not want this!

OUT =  (A * A Att) *  (Gain + (C * C Att) )

This issue here is GAIN being set to 0.5 means you will get output when the control input = 0, not what you would expect for a classic VCA.

for a classic VCA, you will want to set GAIN = 0.0, C Att = 1.0 and A Att to 1.0. this will give you the unity gain VCA most would expect. i.e.

OUT =  A  *  C

Mixer

Why do we have Gain at all, seems a bit redundant! the VCA module is a Quad VCA with a sum output, this means it can be used as a mixer! in this case, we may want to control levels manually... and we can use GAIN for this, when we dont require control input.

Do you even need a VCA?

In classic modular synthesis with hardware, the reason for VCAs is to reduce components on Oscillators, by moving amplification to a separate hardware module. with Digital synthesis, this is not an issue, in its simplest form, as shown above, a VCA is a simple multiplication. for this reason, **many ** digital modules include 'built in vacs' into the oscillators.

and similarly, many SSP sound sources include a GAIN input for an 'internal' vca) so we dont need a VCA at all.