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Create sounds using analogue electronics (Part 4)

Posted: 10 Jan 2012 ?? ?Print Version ?Bookmark and Share

Keywords:voltage-controlled amplifier? oscillators?

Part 1 of this series briefly reviews the differences between analogue and digital synthesis, and discusses ""one of the major innovations in the development of the synthesizer""voltage control. Part 2 begins a look at subtractive synthesis with a discussion of VCOs, waveforms, harmonic content, and filters. Part 3 provides a comprehensive discussion on envelopes, andvoltage-controlled amplifiers.

Most analogue synthesizers have a voltage-controlled amplifier (VCA) as the final stage of the modifier section. The CV is used to change the gain of an amplifier.

The VCA controls the volume of the audio signal and is sometimes connected directly to the output of an EG. An offset voltage can also be used to provide a volume control; so even the output volume of a synthesizer can be voltage controlled.

The following are the two types of input to VCAs:

???Linear inputs are used for tremolo and AM (amplitude modulation). They are also used with exponential curve envelopes.
???Exponential inputs are used for volume changes and linear curve envelopes.
The combination of linear and exponential envelopes with linear and exponential VCAs provides confusion. Using an exponential curve envelope with an exponential VCA results in sudden or abrupt changes rather than steady transitions.

Tremolo is a cyclic variation in the volume of a sound. It is produced by using an LFO CV to alter the gain of a VCA. Tremolo normally uses a sine or triangle waveform at frequencies between 5Hz and 20Hz. Higher frequencies from an LFO or VCO produce AM, where the output of the VCA is a combination of the audio signal and LFO or VCO frequency.

Apart from their normal use as volume-controlling devices, VCAs can also be used to provide ''filtering'' effects. By connecting the keyboard pitch voltage to the CV input of a VCA, the gain is then dependent on the pitch CV from the keyboard. Since the keyboard pitch voltage normally rises alongside thekeyboard note position, the VCA will act much as in a high-pass filter, since low notes will be at a lower volume than higher notes. By inverting the keyboard pitch voltage, a low-pass ''filter'' effect can be produced. This coupling of the VCA to the keyboard pitch voltage is called ''scaling'', since the output of the VCA is scaled according to the pitch (figure 1).

Figure 1: A VCA can be used to produce control of volume which follows the keyboard by routing the keyboard CV to the VCA gain control. This is similar to the tracking of a filter, and produces a coarse high-pass filtering effect, where higher notes are attenuated less than lower notes.

Other modifiers
LFOs: LFOs are used to produce low-frequency CVs. They are in two forms: VCOs and special-purpose oscillators. VCO-based LFOs can have their frequency controlled with an external CV, whilst special-purpose oscillators cannot.

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