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]
ares butterlp asig, afreq [, 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/afreq -- Cutoff or center frequency for each of the filters.


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

Example 114. 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
-odac     ;;;RT audio out
;-iadc    ;;;uncomment -iadc if RT audio input is needed too
; For Non-realtime ouput leave only the line below:
; -o butterlp.wav -W ;;; for file output any platform

sr = 44100
ksmps = 32
nchnls = 2
0dbfs = 1

instr 1	; White noise signal

asig rand 0.5
     outs asig, asig


instr 2	; filtered noise

asig rand 0.7
alp  butterlp asig, 1000	  ;cutting frequencies above 1 KHz
     outs alp, alp



i 1 0 2
i 2 2.5 2


See also

Standard filters: Butterworth filters


Author: Paris Smaragdis
MIT, Cambridge

Existed in 3.30

Audio rate parameters introduced in version 6.02

October 2013.