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From: ISW@cup.portal.com (Isaac S Wingfield)
Newsgroups: sci.electronics
Subject: Re: video chroma circuit
Message-ID: <38104@cup.portal.com>
Date: 16 Jan 91 16:34:24 GMT
References: <5497@exodus.Eng.Sun.COM> <1863@gold.gvg.tek.com>
  <767@ssc.UUCP> <14501@milton.u.washington.edu>
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Organization: The Portal System (TM)
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In article <767@ssc.UUCP> markz@ssc.UUCP (Mark Zenier) writes:

>
>Does anyone know the reason they picked 3.579545... Mhz for the


Most of the energy in a (B&W) television image lies at frequencies which are
multiples of the horizontal rate (15,734.26 for NTSC color). The chroma signal
carrier (actually a suppressed carrier) is at an odd multiple of half the 
horizontal rate; this causes the chroma sidebands to interdigitate with the
luma information and produce minimal interference with it.

As the color subcarrier frequency goes up to higher multiples, the available
bandwidth increases, but the sound subcarrier is 4.5mHz above the video, so
higher sub frequencies will cause progressively more cross-interference
between chroma and sound. 3.579545 is a compromise. Another part of the
compromise, and for which I can't remember the reason, involved changing
the horizontal and vertical rates from the B&W values (15,750Hz & 60Hz)
to slightly different ones (15,734.26Hz & 59.94Hz). This may also have to
do with minimizing crosstalk with the sound carrier.

Isaac
isw@cup.portal.com