butterlp — A low-pass Butterworth filter.


Implementation of a second-order low-pass Butterworth filter. This opcode can also be written as butlp.


ares butterlp asig, kfreq [, iskip]


iskip (optional, default=0) -- Skip initialization if present and non-zero.


These filters are Butterworth second-order IIR filters. They are slightly slower than the original filters in Csound, but they offer an almost flat passband and very good precision and stopband attenuation.

asig -- Input signal to be filtered.

kfreq -- Cutoff or center frequency for each of the filters.


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

Example 60. Example of the butterlp 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 butterlp.wav -W ;;; for file output any platform

; Initialize the global variables.
sr = 22050
kr = 2205
ksmps = 10
nchnls = 1

; Instrument #1 - an unfiltered noise waveform.
instr 1
  ; White noise signal
  asig rand 22050

  out asig

; Instrument #2 - a filtered noise waveform.
instr 2
  ; White noise signal
  asig rand 22050

  ; Filter it, cutting frequencies above 1 KHz.
  alp butterlp asig, 1000

  out alp


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


See Also

butterbp, butterbr, butterhp


Author: Paris Smaragdis
MIT, Cambridge

Existed in 3.30