The start of a regular column where members of the teaching staff of the Gateway School Of Recording & Music Technology answer readers questions.

CREATIVE GATING

Q: Could you give some examples of how one might use a sidechain feature on a noise gate? I have heard people talking about it.

M Flanagan
Dublin

A: The sidechain of a noise gate is the area of the electronics from which the instructions to open and close the gate are derived. On most of today's noise gates there is a break-jack point in the sidechain from which it is possible to control the noise gate from an external triggering source. This input is often called the trigger input and sometimes the key input.

Imagine that you had a low frequency (60Hz) tone in a channel and took a parallel feed from your bass drum track, which you inserted into the trigger input socket on the gate. The noise gate would now open only on bass drum beats and, in theory, you could use the extra burst of 60Hz tone to mix underneath the track and give the bass drum more 'oomph'. Tricks like this are also possible using white noise (generated by a synthesizer or a transistor radio set between stations) and the snare drum, but may be of limited use depending on your taste and application. It can particularly help on drum machine rhythm tracks.

Sometimes noise gates open too slowly for the particular instrument or voice that you are dealing with, truncating the all-important attack portion of the sound. To overcome this, it is possible on occasion to feed the trigger input with a direct line from the sync head of a tape recorder which will open the gate slightly before the same signal from the replay head. Some engineers like to strap a C-ducer contact microphone to the side of a snare drum and use this to trigger the noise gate on the snare drum, which gives them a measure of safety if the drummer is varying the dynamics of snare drum beats and the gate would otherwise fail to open.

There are many other creative applications of working with noise gates, but I hope that this will start you off on some trains of thought. How about gating a synthesizer that is playing big sustained chords and triggering the noise gate from a cowbell playing 8th and 16th notes?

SYNTHETIC ALGEBRA

Q: In a recent interview, you used the phrases additive and subtractive synthesis. What do they mean?

Alan Garrett
Doncaster

A: The sound of a musical instrument is made up of not one pure tone but many tones added together in what we call a harmonic structure. The nearest thing to a pure tone that most laypeople experience is the high pitched whine (a 1 kHz signal) that accompanies the test-card on the television.

When we synthesize a sound we bring different parts together to make a whole, and early synthesizers had many oscillators - all set to different frequencies - with their outputs added together to produce the required harmonic structure. This method was called additive synthesis.

A different approach is to use oscillators that produce waveforms very rich in harmonics (a square wave, for example) and to filter out those harmonics that are not required. This is called subtractive synthesis. On a modern synthesizer this filtering is done in the VCF (Voltage Controlled Filter) section.

On a synthesizer that has two Voltage Controlled Oscillators (VCOs), we might even blend the outputs of these two oscillators together and thus be using a mixture of additive and subtractive synthesis.

Musicians find subtractive synthesis more appealing as it is a more natural process. To draw an analogy, most people find it easier to say what is wrong with a badly drawn picture and what should be modified to put it right, than to draw a similar picture themselves beginning only with a piece of blank paper.

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