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From: herrickd@iccgcc.decnet.ab.com
Newsgroups: comp.arch
Subject: Re: why is 33 MHz a popular number?
Message-ID: <2231.275a1715@iccgcc.decnet.ab.com>
Date: 3 Dec 90 14:12:53 GMT
References: <TOM.90Nov27065615@hcx2.ssd.csd.harris.com> <1990Nov28.030500.16103@mozart.amd.com> <11214@pt.cs.cmu.edu> <1554@ftc.framentec.fr>
Lines: 26

In article <1554@ftc.framentec.fr>, ndoduc@framentec.fr (Nhuan Doduc) writes:
> ok, nothing magic about 33 (or really, 33.33) which is 1/30 or 25 = 1/40 or
> 20 = 1/50 ... but it seems that there is a reason for 4.77MHz on the good old
> PC : the "usual" frequency for a certain communication port is 15.31MHz, thus
> some other magic values: 7.16 and 9.54MHz that you may found on 8088 machines
> (but, *et nous sommes encore dans les valeurs magiques*, most marketing sheets
> and even many tech spec doc still refer to 8MHz (instead of 7.16) and 10 
> (instead of 9.54) 
> 
> --nh
> Nhuan DODUC, 
> Framentec-Cognitech, Paris, France, ndoduc@framentec.fr or ndoduc@cognitech.fr,
> Association Francaise des Utilisateurs d'Unix, France, doduc@afuu.fr

You are too generous with the 4.77 MHz.  It gets used, at least on the
original (1981) IBM PC Motherboard (should we say Great GrandMotherboard?)
for a crucial frequency in producing the imitation NTSC signal to go
to an RF Modulator and be transmitted to your home television receiver.

The color was messed up on mine (I can't remember the symptom) and when
I phoned and complained, they said "bring it in".  While I waited, they
tweaked the main processor clock oscillator to match the picture signal
to my TV receiver's front end.  I was flabbergasted.

dan herrick
herrickd@astro.pc.ab.com


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