foscil — A basic frequency modulated oscillator.


A basic frequency modulated oscillator.


ares foscil xamp, kcps, xcar, xmod, kndx, ifn [, iphs]


ifn -- function table number. Requires a wrap-around guard point.

iphs (optional, default=0) -- initial phase of waveform in table ifn, expressed as a fraction of a cycle (0 to 1). A negative value will cause phase initialization to be skipped. The default value is 0.


xamp -- the amplitude of the output signal.

kcps -- a common denominator, in cycles per second, for the carrier and modulating frequencies.

xcar -- a factor that, when multiplied by the kcps parameter, gives the carrier frequency.

xmod -- a factor that, when multiplied by the kcps parameter, gives the modulating frequency.

kndx -- the modulation index.

foscil is a composite unit that effectively banks two oscil opcodes in the familiar Chowning FM setup, wherein the audio-rate output of one generator is used to modulate the frequency input of another (the “carrier”). Effective carrier frequency = kcps * xcar, and modulating frequency = kcps * xmod. For integral values of xcar and xmod, the perceived fundamental will be the minimum positive value of kcps * (xcar - n * xmod), n = 0,1,2,... The input kndx is the index of modulation (usually time-varying and ranging 0 to 4 or so) which determines the spread of acoustic energy over the partial positions given by n = 0,1,2,.., etc. ifn should point to a stored sine wave. Previous to version 3.50, xcar and xmod could be k-rate only.

The actual formula used for this implementation of FM synthesis is xamp * cos(2π * t * kcps * xcar + kndx * sin(2π * t * kcps * xmod) - π), assuming that the table is a sine wave.


Here is an example of the foscil opcode. It uses the file foscil.csd.

Example 158. Example of the foscil opcode.

See the sections Real-time Audio and Command Line Flags for more information on using command line flags.

; Select audio/midi flags here according to platform
; Audio out   Audio in    No messages
-odac           -iadc     -d     ;;;RT audio I/O
; For Non-realtime ouput leave only the line below:
; -o foscil.wav -W ;;; for file output any platform

; Initialize the global variables.
sr = 44100
kr = 4410
ksmps = 10
nchnls = 1

; Instrument #1 - a basic FM waveform.
instr 1
  kamp = 10000
  kcps = 440
  kcar = 600
  kmod = 210
  kndx = 2
  ifn = 1

  a1 foscil kamp, kcps, kcar, kmod, kndx, ifn
  out a1


; Table #1, a sine wave.
f 1 0 16384 10 1

; Play Instrument #1 for 2 seconds.
i 1 0 2



Example written by Kevin Conder.