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From: rfg@hound.UUCP (R.GRANTGES)
Newsgroups: net.audio
Subject: Re: A Question on Equalizers
Message-ID: <904@hound.UUCP>
Date: Wed, 13-Feb-85 22:21:46 EST
Article-I.D.: hound.904
Posted: Wed Feb 13 22:21:46 1985
Date-Received: Thu, 14-Feb-85 21:15:47 EST
References: <2677@sdcc3.UUCP>, <1026@hou3c.UUCP>
Organization: AT&T Bell Labs, Holmdel NJ
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In the usual octave band equalizer, the boost or cut curve vs frequency
of each part of the spectrum is fixed, you determine its amplitude by
setting a sliding control. So, for example, one of the sliders may
represent a boost or cut centered on 500 hz. By adjusting the control
you set the response at 500 hz between -12db to + 12 db of flat, or
anywhere inbetween. When you have the control set for , say, +6 db at
500 hz, you can then only change the response by similarly adjusting
controls at other fixed frequencies - usually octave related like 250 hz
and 1000 hz. This gives you a lot of control, but not always just the 
control you want. You may be wanting to notch out a tone at ,say, 750 hz.
You can cut 500 and you can cut 1000 but you have cut a wide band of
frequencies in order to affect 750.

A parametric equalizer gives you (usually) continuous control over three
"parameters" of the tone curve for each correcting unit supplied (usually
at least two independent units are supplied (per channel) but sometimes
4,6 or more). Anyhow you can control the center frequency of the tone
shape, the degree of boost or cut at that frequency, and the shape of the
curve with frequency ("Q" of the filter to those who speak that way) -
whether the control curve is narrow or wide.  Hence, to return to the
previous example, with a parametric equalizer you could tune it to 750 hz
set it for narrow band attenuate and notch out the offending tone
without affecting much more of the spectrum. (Ignore phase effects as I
suggested in a previous article, they just needlessly worry you.)

Which is best is a question much mooted about.  For relatively mild effects
extending over moderate amounts of the spectrum, perhaps an octave band
equalizer (or "eq") is best. For specific control over very limited parts  
of the spectrum, like notching out a resonance or hum, a parametric is
best. Para's are also god for mild actions affecting broad sweeps of
spectrum. With a para, unless you have a lot of units, you can't make up 
a very complex curve. Para's are not found very frequently these days and
tend to be expensive. There are many cheap octave units on the market,
some of which may be ok.

-- 

"It's the thought, if any, that counts!"  Dick Grantges  hound!rfg