eqfil — Equalizer filter


The opcode eqfil is a 2nd order tunable equalisation filter based on Regalia and Mitra design ("Tunable Digital Frequency Response Equalization Filters", IEEE Trans. on Ac., Sp. and Sig Proc., 35 (1), 1987). It provides a peak/notch filter for building parametric/graphic equalisers.

The amplitude response for this filter will be flat (=1) for kgain=1. With kgain bigger than 1, there will be a peak at the centre frequency, whose width is given by the kbw parameter, but outside this band, the response will tend towards 1. Conversely, if kgain is smaller than 1, a notch will be created around the CF.


asig eqfil ain, kcf, kbw, kgain[, istor]


istor --initial disposition of internal data space. Since filtering incorporates a feedback loop of previous output, the initial status of the storage space used is significant. A zero value will clear the space; a non-zero value will allow previous information to remain. The default value is 0.


asig -- filtered output signal.

ain -- input signal.

kcf -- filter centre frequency.

kbw -- peak/notch bandwidth (Hz).

kgain -- peak/notch gain.


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

Example 270. Example of the eqfil 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     ;;;realtime audio out
;-iadc    ;;;uncomment -iadc if realtime audio input is needed too
; For Non-realtime ouput leave only the line below:
; -o eqfil.wav -W ;;; for file output any platform

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

instr 1

kcf	= p4
kfe	expseg	10, p3*0.9, 1800, p3*0.1, 175
kenv	linen	.03, 0.05, p3, 0.05 ;low amplitude is needed to avoid clipping
asig	buzz	kenv, kfe, sr/(2*kfe), 1
afil	eqfil	asig, kcf, 200, 10
	outs	afil*20, afil*20

; a sine wave.
f 1 0 16384 10 1

i 1 0 10 200	;filter centre freq=200
i 1 + 10 1500	;filter centre freq=1500

See also

Specialized Filters: Parametric EQ


Author: Victor Lazzarini
April 2007

New in version 5.06