From: lowen@lorc.eskimo.com (Lamar Owen)
Newsgroups: alt.folklore.computers
Subject: NTSC and FM techn. (Was: Re: minor typo in my cdrom/ld post)
Date: Mon, 4 Jan 93 09:02:50 -0500
Organization: LORC Enterprises

[Apologies in advance if any lines are > 80 characters.  I am editing this in
 a proportional font.  (I know, not a bright idea... :-)...).

In <1992Dec30.113842.14736@panix.com> dannyb@panix.com (Daniel Burstein) writes:
>meant to say 45 K /sec of audio CD, rather than the number that appeared. 
>I'm not quite sure if this needs to be doubled for stereo, or if they have
>some strange differential only coding in place.  (I've never understood
>how stereo broadcast works. then again, I don't beleive in color tv either)

The CD standard sampling rate is 44.1K samples per second.
And that is per channel.  So, for stereo, you need an aggregate 88.2K
samples/sec.

Stereo broadcast (on FM) works by using the neat phenomenon that allows an FM
signal to occupy less bandwidth than it should--given the Bessel derivation. 
However, by dropping the extra Bessel harmonics on the floor (figuratively),
the s/n ratio is degraded.  Thus, most FM tuners will switch to mono mode on
weak signals.

The signals in the FM pre-modulation audio are as follows:
From 20 to 15Khz: baseband L+R sum signal, mono compatible.
At 19Khz: the stereo "pilot" carrier--the actual carrier is at 38Khz (but
          is suppressed!)
Centered on 38Khz, and not exceeding 15Khz either way: the L-R difference
 signal, double-sideband suppressed carrier modulated with a 38Khz subcarrier.

We won't even discuss the SCA subcarriers....
 
>at first glance color tv should take three times the
>bandwidth of monochrome, but in reality, they were able to fit it (NTSC,
>the North American standard) into the original broadcst width, AND keep it
>compatible with the older monochrome sets.  So there ways ways to "hideA"
>additonal info in the primary signal.

In the case of NTSC color video, several things were done to reduce the chroma
bandwidth.

First, a monochrome video signal is not present in a continuous band 
from 0-4.5Mhz. Rather, it is present as a "comb" sort of signal.  The 
teeth of the comb are spaced by 15.7 Khz (the horizontal sync) and have
teeth inside that spaced at 60Hz.  There are then lots of bandwidth in
between these teeth.  Well, the color phase modulation carrier frequency
was very carefully chosen so as to position the final chroma signal 
interleaved (since the chroma has the same "teeth" as the luma) with 
the luma signal.  Voila! Instant "popcorn-and-milk"-two things occupying
the same space.  This is an obscure technique known as frequency interleave.
It is only just recently that the technology to properly separate the 
luma and chroma has become available--the "comb" filter.

Second, chroma is phase modulated.  This has similar effects as FM 
would have--allowing a lesser aggregate bandwidth for the modulated
signal than the original signal, taking a s/n ratio hit in the process.
(Lossy bandwidth compression, if you will.)

Third, television is transmitted SSB (single sideband supressed carrier) AM.
So, the signal takes a little more than half of the bandwidth that the
straight AM would have taken.

Fourth, since chroma resolution is non-critical, the chroma bandwidth is
chopped at 2Mhz.

Fifth, NTSC chroma is encoded in a biphase arrangement where one of the 
color signals actually is not even transmitted.  It is beyond the scope
of this article (and afc) to discuss those shenanigans (although 
alt.shenanigans might be OK... :-) ).

All five of these create the Never Twice the Same Color problems--
almost always traceable to poor s/n ratios and bad filter roll-off
phase errors.

Now, since NTSC video is a feature of many computers, past and present,
I don't feel guilty about posting this in a.f.c.

>dannyb@panix.com

(BTW, along with being a computer systems consultant, I'm a broadcast
engineer at WGCR and WKJV.)

-- 

Lamar Owen, Systems Consultant          | If there were a tax on syn,
GE Lighting Systems, Hendersonville, NC | we'd all be broke.
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My opinions are not those of GE and do not reflect GE policy in any way.
Newsgroups: alt.folklore.computers