FILTSWEEP -- time-varying biquad (bandpass) filter
in RTcmix/insts/jg

quick syntax:
FILTSWEEP(outsk, insk, dur, AMP, ringdowndur, steepness, ampbalance, inputchan, PAN, BYPASS, CENTERFREQENV, BANDWIDTHENV) 

   p0 = output start time (seconds)
   p1 = input start time (seconds)
   p2 = input duration (seconds)
   p3 = amplitude multiplier (relative multiplier of input signal)
   p4 = ring-down duration (seconds)
   p5 = steepness (1-5, inclusive) 1 is a good starting value)
   p6 = balance output and input signals (0:no, 1:yes) (usually use 1)
   p7 = input channel
   p8 = pan (0-1 stereo; 0.5 is middle)
   p9 = bypass filter (0: bypass off, 1: bypass on) (usually use 0)
   p10 = filter center frequency (Hz)
   p11 = filter bandwidth (Hz if positive; if negative, the '-' sign acts as a
      flag to interpret the bw values as a multipler(0-1) of the current center frequency)

   p3 (amplitude), p8 (pan), p9 (bypass), p10 (freq) and p11 (bandwidth) can
   receive dynamic updates from a table or real-time control source.

   Author: John Gibson, 2/5/00; rev for v4, JGG, 7/24/04


FILTSWEEP filters the input sound using a series of time-varying biquad filters. Center frequency and bandwidth can be dynamically altered.



Usage Notes

The ring-down duration parameter (p4) is to let you control how long the filter will sound after the input has stopped. Too short a time, and the sound may be cut off prematurely, resulting in a click.

p5 ("steepness") is just the number of filters to add in series. Using more than 1 decreases the actual bandwidth of the total filter. This sounds different from decreasing the bandwith of 1 filter using the "BANDWIDTHENV" parameter (p11), described below. (Mainly, it further attenuates sound outside the passband.) If you don't set p6 ("ampbalance") to 1, you'll need to change p3 ("AMP") to adjust for loss of power caused by connecting several filters in series. Guard your ears!

The "ampbalance" parameter (p6) tries to adjust the output of the filter so that it has the same power as the input. This means there's less fiddling around with p3 ("AMP") to get the right amplitude (audible, but not ear-splitting) when the bandwidth is very low and/or sharpness is > 1. However, it has drawbacks: it can introduce a click at the start of the sound, it can cause the sound to pump up and down a bit, and it eats extra CPU time.

The output of FILTSWEEP can be either mono or stereo.



Sample Scores

very basic: 

   rtsetparams(44100, 2)
   load("FILTSWEEP")
   
   rtinput("mysound.aif")
   
   inskip = 0
   dur = DUR() - inskip
   
   amp = 1.0
   balance = 0
   sharpness = 2
   ringdur = .5
   
   lowcf = 0
   highcf = 1600
   startbw = -.01
   stopbw = -1
   
   ampenv = maketable("line", 1000, 0,0, dur-.1,1, dur,0)
   
   cfenv = maketable("line", "nonorm", 1000, 0,lowcf, dur/3,highcf, dur,lowcf)
   bwenv = maketable("line", "nonorm", 1000, 0,startbw, dur,stopbw)
   
   FILTSWEEP(start=0, inskip, dur, amp*ampenv, ringdur, sharpness, balance, 0, 1, 0, cfenv, bwenv)
   FILTSWEEP(start=.1, inskip, dur, amp*ampenv, ringdur, sharpness, balance, 0, 0, 0, cfenv, bwenv)


another one: 
   rtsetparams(44100, 2)
   load("FILTSWEEP")
   
   rtinput("mysound.aif")

   inskip = 0
   dur = DUR() - inskip
   
   bypass = 0
   
   amp = 1.1
   env = maketable("line", 1000, 0,0, dur-.1,1, dur,0)
   
   ringdur = 0.5
   balance = 0
   steepness = 2
   
   lowcf = 90
   highcf = 4000
   narrowbw = -0.02
   widebw = -0.90
   
   cf = maketable("line", "nonorm", 2000, 0,lowcf, dur/3,highcf, dur,lowcf)
   bw = maketable("line", "nonorm", 2000, 0,narrowbw, dur,widebw)
   
   start = 0
   FILTSWEEP(start, inskip, dur, amp * env, ringdur, steepness, balance,
      inchan=0, pan=1, bypass, cf, bw)
   
   highcf = 5000
   cf = maketable("line", "nonorm", 2000, 0,highcf, dur/3,lowcf, dur,highcf)
   bw = maketable("line", "nonorm", 2000, 0,widebw, dur,narrowbw)
   
   start = 0.1
   FILTSWEEP(start, inskip, dur, amp * env, ringdur, steepness, balance,
      inchan=0, pan=0, bypass, cf, bw)


See Also

maketable, BUTTER, ELL, EQ, FIR, FILTERBANK, FOLLOWBUTTER, IIR, JFIR, MOOGVCF, MULTEQ SPECTEQ, SPECTEQ2