Subject: Delay line filtering (Was:Re: [linux-audio-dev] Still I cannot understand why...)
From: Lamar Owen (lamar.owen_AT_wgcr.org)
Date: Thu Dec 20 2001 - 22:50:36 EET
On Wednesday 19 December 2001 08:39 pm, Paul Davis wrote:
> >Just curious, but could somebody explain *how* delay lines can be used
> >implement EQ? I have a strong maths background, but no DSP experience if
> >that helps.
> i'm not a dsp programmer, but its really quite simple. if you
> feedback with a delay of just 1 sample, and attenutate both the
> current and previous sample by 0.5:
> the actual details are extremely hairy though - there is a lot of
> sophisticated math that goes into really good filter design, plus a
> lot of subjective, non-double blind tested "opinion" :)
In short: Z transforms are your friend. Once I grasped what the Z transform
did for you in control systems theory, I immediately realized that filtering
is a natural for Z transform math.
But, in a nutshell:
Delaying a set time and adding back to the original produces a 'comb' filter.
The amount of the delay and the depth of the readdition together produce
various degrees of filtering. Notch filters are easiest with delays -- delay
one half cycle at the notch and add back one hundred percent. You get a
rather tight notch. Along with other neat effects. :-) Like the peak at
twice the notch frequency.... :-) And the secondary notch at thrice the
notch frequency. Even multiples peak, odd multiples notch -- thus a comb
filter.
Comb filters are used rather nicely in chroma/luma separation in Never The
Same Color video.
-- Lamar Owen WGCR Internet Radio 1 Peter 4:11
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