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From: fish@ihu1g.UUCP (Bob Fishell)
Newsgroups: net.audio
Subject: Re: EQ question
Message-ID: <240@ihu1g.UUCP>
Date: Wed, 7-Mar-84 14:48:25 EST
Article-I.D.: ihu1g.240
Posted: Wed Mar  7 14:48:25 1984
Date-Received: Thu, 8-Mar-84 08:37:35 EST
References: <1534@burdvax.UUCP>
Organization: AT&T Bell Labs, Naperville, IL
Lines: 37

(oo)
AC theory has been a while, but here goes:

The 120 uS and 70 uS EQ figures you refer to are the time
constants of simple RC highpass/lowpass filters used for
preemphasis and deemphasis of FM broadcasts, and in tape
decks.  A highpass filter looks like this:

                O-----|(---*---O
                      C    |
                           >
                        R  >
                           >
                           |
                O----------*---O

And a lowpass looks like this:

                O--VVVVV---*---O
                     R     |
                          _|_
                        C '|`
                           |
                O----------*---O

I don't remember the equations, but they're pretty simple to look
up.  The important thing to remember is that the greater the
time constant (R*C), the greater the degree of preemphasis/deemphasis
present in the system.  

For FM systems, the corner frequency of the preemphasis curve is
about 5Khz.  I don't know offhand what it is for Dolby.  Standard
FM uses 120 uS preemphasis; Dolby uses 70uS.
-- 

                               Bob Fishell
                               ihnp4!ihu1g!fish