an open-source digital signal processing and sound synthesis language
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AMINST -- amplitude modulation synthesis
in RTcmix/insts/std


quick syntax:
AMINST(outsk, dur, AMP, CARFREQ, MODFREQ[, PAN, MODAMPENV, CARWAVETABLE, MODWAVETABLE])

CAPITALIZED parameters are pfield-enabled for table or dynamic control (see the maketable or makeconnection scorefile commands). Parameters after the [bracket] are optional and default to 0 unless otherwise noted.



   p0 = output start time (seconds)
   p1 = duration (seconds)
   p2 = amplitude (absolute, for 16-bit soundfiles: 0-32768)
   p3 = carrier frequency (Hz)
   p4 = modulation frequency (Hz)
   p5 = pan (0-1 stereo; 0.5 is middle) [optional; default is 0]
   p6 = modulator amplitude [optional; default is 1.0 (full amplitude)]
   p7 = reference to carrier wavetable [optional; defaults to sine wave]
   p8 = reference to modulator wavetable [optional; defaults to sine wave]

   p2 (amplitude), p3 (carrier freq), p4 (modulator freq) p5 (pan) and p6 (modulator amp)
   can receive dynamic updates from a table or real-time control source.

   p7 (carrier wavetable) and p8 (modulator wavetable), if used, should be
   references to pfield table-handles.

   Author Brad Garton; rev for v4, JGG, 7/22/04



AMINST synthesizes a sound using amplitude modulation, a type of modulation where a carrier signal (of frequency C is varied by a modulator signal (of frequency M) such that two sidebands are produced of frequency C + M and frequency C - M. This results from multiplying the amplitude of the carrier signal by the amplitude of the modulator signal. The operation is simple, each instantaneous sample value of the carrier is multiplied by the corresponding sample value of the modulator.

At low modulator frequency rates (< 20 Hz) this is perceived as an amplitude 'tremolo' of the carrier signal. At higher rates the sidebands manifest as audio components in a more complex spectrum.

If non-sinusoidal waveforms are used, each sine-wave component acts as a separate carrier or modulator. Each sine-wave partial in the signal(s) will have distinct C + M and C + M components in the resulting spectrum.

Usage Notes

Depending on the amplitude of modulator, the original carrier frequency component will be present in the resulting sound. Negative frequencies resulting from the AM operation 'wrap' around 0 Hz and appear as positive components 180 degrees out of phase.

Sample Scores

very basic:
   rtsetparams(44100, 1)
   load("AMINST")

   // use defaults to produce a signal with components at
   // 178.0 + 315.0 Hz, 178.0 - 315.0 Hz, and 178.0 Hz)
   AMINST(0, 3.5, 10000, 178, 315)


slightly more advanced:
   rtsetparams(44100, 1)
   load("AMINST")

   ampenv = maketable("line", 1000, 0,0, 1,1, 9,1, 10,0)
   modenv = maketable("line", 1000, 0,0, 1,1, 2,0)
   AMINST(3.9, 3.4, 10000*ampenv, cpspch(8.00), cpspch(8.02), 0, modenv)


fun stuff!
   rtsetparams(44100, 2)
   load("AMINST")

   ampenv = maketable("line", 1000, 0,1, 10,0)
   carfreq = makeLFO("sawup", 0.9, 100, 500)
   modfreq = makeLFO("sawdown", 0.7, 250, 1400)
   carwave = maketable("wave", 1000, "saw3")
   modwave = maketable("wave", 1000, "square")
   panner = makeLFO("sine", 0.5, 0, 1)
   AMINST(0, 7, 20000*ampenv, carfreq, modfreq, panner, 1.0,  carwave, modwave)


See Also

maketable, makeLFO, AM, FMINST, HALFWAVE, MULTIWAVE, SYNC, VWAVE, WAVESHAPE, WAVY, WIGGLE