Introduction to Computer Music: Volume One

5. Signal Processing

A more extensive discussion of signal processing is coming soon, but for now, here are some basic types of effects found in most studios.

Wet/Dry Mix: Most signal processing units or software have a 'wet/dry mix' to determine the proportion of mix between the original (dry) and 'effected' (wet) signals. For effects like a single echo, you will definitely want to hear both the dry and wet sound. For an effect like pitch shift, a totally wet sound will output only the new pitch and not the original. Many composers chose to control their wet/dry mix from the mixing console and not the signal processing unit, leaving it set to 100% wet. They do so by carfully adjusting the 'effects send' levels on their mixing console and chosing between pre- and post-fader (see Mixing Console Basics)

Controls: reverb time, pre-delay (time before onset of reverb), highpass filter (hpf), lowpass filter (lpf), stereo difference, diffusion (refers to higher frequencies spreading and dying out more quickly than lower frequencies, something we use as an aural cue to the size of a space. A football field will have a higher degree of diffusion than a small room.

Controls: lfo, depth

Echo (delay, multi-tap delay)
Controls: echo time (often different for stereo locations (left, center, right)), feedback (for multiple echoes which die away or build up), pre-delay. Software multi-tap echoes may allow sync'ing 'taps' to rhythmic grid.

Flange, phaser
Controls: rate, depth. By minutely delaying (1-20 ms) a copy of the input signal and reminxing, constructive and destructive interference reinforces varying sets of frequencies. As delay time is shifted up and down by an lfo, a sweeping effect is created. Called flange because original used a copy of a tape slowed down by an engineer lightly touching a reel flange to put it out of phase.

Pitch shift
1-3 pitches. sometimes with location

Gate (reverse gate)
Allows signal through only after it reaches a certain amplitude threshold. Good for sounds with low-level noise (like tape hiss, if you remember what that was). A reverse gate with certain attack and decay settings may make an input signal sound reversed because of the way it shapes it dynamically. Controls: threshold, attack, decay, smoothing

Compressor (limiter, expander, 'compander')
Scales down (or squeezes) dynamic range of input by ratio (for example 5:1). May alter dynamic nature of input signal with high ratios--used to create more "punchy" sounds when applied to input with sharp transients, like a kick drum. Can also be tried on noisy files. A limiter usually does not compress (some do), but stops an input signal from exceeding a certain amplitude without scaling the input below that ceiling. An expander does the opposite of a compressor, in that it expands the dynamic range of a sound (and also any noise associated with it). Controls: % or ratio of compression or expansion, attack, decay, threshold.

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