Making sounds with analogue electronics - Part 4: Voltage-controlled amplifiers

An envelope follower takes an audio signal or CV, converts it to just positive values, and then low-pass filters it with a filter which has a very low cut-off frequency – a few hertz (Figure 3.3.35). This removes any high frequencies from the input, and leaves just a CV which represents the envelope of the input audio or CV. It is thus almost the opposite of a VCA: a VCA causes a CV to change the envelope of an audio signal, whilst an envelope follower takes an audio signal and produces a CV.

FIGURE 3.3.35 An envelope follower is used to 'extract' the envelope from an audio signal. This can be used to process external signals in a synthesizer. The audio signal is low-pass filtered and then a diode pump circuit is used to provide the final output voltage.

Some envelope followers also produce gate and trigger outputs, which are suitable for controlling EGs – the envelope follower is then a complete module for interfacing an external audio signal with an analogue synthesizer. If the envelope follower is used to process source CVs, then it can be used to 'smooth' rapidly changing waveforms that have sharp transitions, or even produce portamento effects if the keyboard pitch CV is processed.

Externally triggered sample and hold
If a sample and hold circuit has an external sample clock input, then it can be used to sample voltage sources at non-periodic intervals. One suitable sample clock source is the keyboard gate or trigger signals. Using the keyboard to control the sample and hold, an output is produced, which changes only when a new key is pressed on the keyboard. By using an envelope follower to produce gate or trigger signals from an external audio input, the sample and hold can be driven from an external audio signal. In this way, any audio signal can be used as a source of CVs.

Waveshaper
Although rarely implemented on analogue synthesizers, the waveshaper is a nonlinear amplifier, which allows control over the relationship between the input and output signals. Any non-linearity in this relationship changes the shape of the waveform passing through the waveshaper, and this changes the harmonic content of the signal (Figure 3.3.36). Chapter 5 contains more information on the use of waveshaping in digital synthesizers.

FIGURE 3.3.36 A waveshaper uses a non-linear transfer function to change the shape of a waveform. This is often used to convert a triangle waveform into an approximation of a sine wave and is adequate for shaping LFO and VCO outputs.

Another interpretation of an analogue waveshaper is that it adds distortion to the signal and so it is best used for monophonic signals. A more familiar waveshaper is the 'fuzz box' used by guitarists, where the passing of polyphonic audio signals through a clipping circuit produces large amounts of distortion.