posted 01 May 2002 04:28            

parametric_filters.kym userprog.zip

The improvement to the parametric EQ is the addition of a "spotting" filter -- a narrowband bandpass filter with user adjustable frequency, Q, and gain. When you toggle the "Spotting" button, it cuts out the parametric EQ filters and shows you what sound makes it through the spotting filter. This can help you find the frequencies needing touch up with the parametric EQ filter bank.

Also included are a bunch of individual 2-pole filters, all with hotvalue settable frequency, Q, and gain. These are low-shelf, high-shelf, peaking, bandpass, and band-reject filters.

The bandpass filter included here is different from the Kyma BPF. The Kyma BPF is capable of very broad passbands with nearly constant gain in the passband. The BPF included here is intended more for very narrow filtering. It is an implementation of a "Reson" filter with zeros added at DC and Nyquist to make it nearly constant gain regardless of the center frequency.

Tuning will become less precise at the high end of the spectrum, but if you stay around half-Nyquist (half of half-sample rate, e.g., 12 KHz) and below, then the frequency requested will be very close to what is provided by the filter.

Also included is its twin brother -- the band-reject filter.

All of these filters are named "BiquadXXX" for historical reasons. In fact they are all direct form filters, which as pointed out by Kurt some time ago, offer better control of the noise floor than "biquad" computation.

I didn't bother to include the 2-pole low-pass and high-pass filters because I believe the Kyma LPF and HPF are far superior for all purposes. They are Butterworth filters of arbitrary specifiable order. The Kyma BPF is superior for cases when you need broad bandpasses. It is composed of a Kyma LPF cascaded with a Kyma HPF.

[Actually... what the heck, I included the 2-pole LPF and HPF as well, because even though they are generally inferior to the Kyma Butterworth filters, they do offer a resonance peak at the cutoff controlled by the filter Q parameter. Butterworth filters do not exhibit this kind of resonance, and so maybe you will find a use for these filters too.

Likewise... the shelving filters provided here can produce the classic peak/dip character on either side of the filter frequency when the Q is higher than about 1. Some of you may have seen this behavior in mixing consoles. Watch out though... the high-shelf filter can become unstable with gain boost.]

To use the parametric EQ filters you need to go to your file menu and select on "Choose Microsounds...". Apparently it can only load microsounds from a file named USERPROG.ASY. So if you already have microsounds in use, be sure to put this file in a separate directory so as not to overwrite your existing microsounds file. When (or if...) it asks you whether or not to Edit or Reload the microsounds from this file, choose Reload. Capy will then reinitialize and "Assemble" the microsounds in this file.

Enjoy!

- DM