Modulation and automation
Move a sound's volume, position, filter or pitch while it plays with curves and LFOs, and automate a track on a timeline.
A curve is a value that changes with time. audioLfo makes one that swings up and down on its own, audioCurve makes one from points you choose. audioModulate points a curve at one parameter of a sound, and audioAutomate points one at a track of a timeline.
A curve is read at the time of the sound it drives: the value at 2 seconds is what the sound does 2 seconds in. audioCurveAt( curve, seconds ) tells you what a curve says at a time, which is useful for checking a curve before you use it.
Curves
An LFO has a shape (sine, triangle, saw, square or random), a rate in Hz, a depth and a center. It starts at the center and swings by the depth in each direction. fadeIn brings the swing in over some seconds, phase starts it elsewhere in its cycle, and seed makes random repeatable.
A curve from points holds the first value before the first point and the last value after the last, and joins the points with linear, smooth or step.
wobble = audioLfo( shape="sine", rate=2, depth=3, center=10 );
println( audioCurveAt( curve=wobble, seconds=0 ) );
println( audioCurveAt( curve=wobble, seconds=0.125 ) );
rise = audioCurve( points=[ { at:0, value:200 }, { at:4, value:8000 } ], interpolation="smooth" );
println( audioCurveAt( curve=rise, seconds=2 ) );
What a curve can drive
param | The curve is | Typical use |
|---|---|---|
gainDb | the gain in dB | Tremolo, fade, swell |
pan | the position, -1 left to 1 right (a mono sound becomes stereo) | Auto pan |
cutoff | the cutoff of a filter in Hz | Filter sweep, wah |
pitch | the pitch in semitones, like a tape | Vibrato, a pitch drop, a tape stop |
cutoff takes a filter (lowpass, highpass, bandpass or notch) and a resonance q. A pitch curve that stays above 0 makes the sound shorter and below 0 longer, because it plays like a record. Around 0 (a vibrato) the length stays.
Tremolo
tone = audioOsc( "saw", 220, 4 );
tremolo = audioModulate( audio=tone, param="gainDb", curve=audioLfo( shape="sine", rate=4, depth=8, center=-8 ) );
audioSave( tremolo, "tremolo.wav" );
The volume swings 8 dB either side of -8 dB, four times a second:
Filter sweep
sweep = audioModulate( audio=audioNoise( seconds=4 ), param="cutoff",
curve=audioCurve( points=[ { at:0, value:200 }, { at:4, value:8000 } ], interpolation="smooth" ) );
audioSave( sweep, "sweep.wav" );
White noise through a low pass that opens from 200 Hz to 8 kHz. The picture shows the bright part of the spectrum climbing:
Vibrato
tone = audioOsc( "saw", 220, 4 );
vibrato = audioModulate( audio=tone, param="pitch", curve=audioLfo( shape="sine", rate=5.5, depth=0.4, fadeIn=1.5 ) );
audioSave( vibrato, "vibrato.wav" );
The pitch swings 0.4 of a semitone, 5.5 times a second, and the swing grows over 1.5 seconds. Every harmonic wobbles, the higher ones more:
Auto pan
tone = audioOsc( "saw", 220, 4 );
autoPan = audioModulate( audio=tone, param="pan", curve=audioLfo( shape="sine", rate=0.5, depth=1 ) );
audioSave( autoPan, "autopan.wav" );
A mono sound moves from the center to the right, across to the left and back, every two seconds. The two lanes are the two channels. Wear headphones for this clip:
Automating a track
audioAutomate does the same for a track of a timeline, while the timeline renders. It takes gainDb and pan. Calling it again for the same track and parameter replaces the curve. A pan curve needs a stereo timeline and balances the track (it is not a mono pan).
t = audioTimeline( bpm=120, beatsPerBar=4, sampleRate=44100, channels=2 );
audioAdd( timeline=t, sound=audioOsc( "sine", 440, 4 ), at=0, track="lead" );
audioAutomate( timeline=t, track="lead", param="gainDb", curve=audioCurve( points=[ { at:0, value:-40 }, { at:2, value:0 }, { at:4, value:-40 } ] ) );
audioAutomate( timeline=t, track="lead", param="pan", curve=audioLfo( shape="triangle", rate=0.5, depth=0.8 ) );
audioSave( audioRender( timeline=t ), "swell.wav" );
Errors
An unknown param, shape or interpolation, an empty or repeated-time points list, or a curve that is not from audioLfo or audioCurve is bxaudio.invalidArgument. A pitch curve that would make a sound longer than 10 minutes is bxaudio.tooLarge.