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From: dave@onfcanim.UUCP
Newsgroups: rec.video,comp.graphics
Subject: The origins of frequencies in NTSC
Message-ID: <15180@onfcanim.UUCP>
Date: Thu, 8-Jan-87 01:34:49 EST
Article-I.D.: onfcanim.15180
Posted: Thu Jan  8 01:34:49 1987
Date-Received: Fri, 9-Jan-87 05:52:07 EST
References:  <4017@caip.RUTGERS.EDU>
Reply-To: dave@onfcanim.UUCP (Dave Martindale)
Organization: National Film Board / Office national du film, Montreal
Lines: 169
Keywords: NTSC video
Xref: watmath rec.video:245 comp.graphics:129

This article is a reply to a question in rec.video, but it may be of
interest to computer graphics people also, thus the double posting.

The information that follows comes from "Principles of Color Television",
Knox McIlwain and Charles E. Dean, John Wiley and Sons, 1956.  It is
essentially a book about the process that produced the NTSC colour standard.
However, the library containing the book is now 400 miles away, so there
could be an error or two in what follows.


When the NTSC standard was being discussed, there were a number of things
already "cast in stone".  There was already a black&white standard, and
a large number of TV sets in consumer hands.  The new colour standard
was to be compatible with the black&white one, in the sense that B&W
broadcasts would appear in B&W on colour sets, and colour transmissions
would produce good-quality B&W pictures on B&W TV sets.

The B&W standard specified a 60 Hz vertical frequency and 15750 Hz horizontal
frequency, giving 525 lines in two interlaced fields.  The spacing of
TV channels was 6 MHz, and the sound carrier was located 4.5 MHz above
the picture carrier.

I won't discuss the reasoning behind the way that the colour signals
were encoded into a luminance and two colour difference signals of different
bandwidths.  Here, it is enough to know that the two colour difference
signals were to be transmitted as quadrature amplitude modulation of
a colour subcarrier signal that is added to the luminance and sync
signals to produce a composite video signal.

Since the colour information occupies part of the frequency spectrum
that is also taken up by fine detail in the B&W (luminance) signal,
it is necessary to minimize the interference between them.

It is an observed fact that for most TV images containing typical
patterns of objects, the detail in the luminance signal is not
evenly spread across the frequency spectrum, but occurs mostly at
multiples of the horizontal scanning frequency.  In other words,
most of the luminance information is found at frequencies that are
whole integer multiples of the horizontal frequency.
(To make things easier, let's call the horizontal frequency Fh).
For the same reasons, most of the energy in the colour component
of the signal will be found at the subcarrier frequency (Fsc)
plus integral multiples of Fh above and below it.

To separate the luminance and chrominance information as much as
possible, we can make sure that the subcarrier frequency Fsc is
an odd multiple of half Fh.  Thus, for some integer k,
Fsc = (2*k+1) * Fh / 2.

If we do this, most of the energy of the luminance component will be
found at frequencies of (2*N) * Fh/2, and most of the energy of the
chrominance component will be at frequencies of (2*N+1) * Fh/2.
The luminance and chrominance signals are "interleaved" in frequency
space.

In general, this means that fine detail in the luminance signal,
even though it generates frequencies near the colour subcarrier
frequency, is not mistaken for colour information by the TV receiver.
Only unusual images containing fine diagonal stripe patterns, fine
herringbone patterns, and other sorts of fine detail that is neither
horizontal nor vertical, may be misinterpreted.

This is, in fact, why the suits that Johnny Carson is famous for
wearing on occasion cause such bizarre effects - the fine black&white
detail in the suit is being mistaken for colour information by the
receiver.  (Note: it is possible to avoid this problem by using
a notch filter on the luminance signal ahead of the NTSC encoder -
it just throws away the fine detail that might cause problems.)

When a colour signal is displayed on a B&W TV, the colour information
shows up as bogus fine detail in the picture - the higher the saturation
of the colour, the greater the amplitude of the false detail.
By selecting the colour subcarrier to be an odd multiple of half the
line frequency, you guarantee that any visible pattern in the image
inverts polarity from one line to the next.  Thus, highly coloured
areas show up as being covered by an extremely fine "checkerboard"
pattern.  This checkerboard pattern is far less visible than the
vertical stripes you would get if Fsc was an even multiple of Fh/2.
Since Fsc is locked to Fh in phase, the checkerboard pattern is
perfectly stationary, which is far less visible than the drifting
pattern that would result if Fsc was not locked to Fh.
At normal viewing distance, the checkerboard just looks uniform grey.

All of this so far has just explained why Fsc must be some odd multiple
of Fh/2, and must be phase-locked to it (in practice, they are generated
by dividing down the same oscillator).  Now we have to pick Fsc itself.

Tests with viewers had shown that, given the available video bandwidth
of 4.2 MHz (which could not be changed and still remain compatible
with B&W), the ideal Fsc is somewhere around 3.6 MHz.  It needs to
be kept as high as possible to reduce the visiblity of the "checkerboard"
pattern displayed on B&W sets - the higher the frequency, the finer the
pattern, and also the lower its amplitude due to the generally poor
high frequency response of consumer TVs.  However, Fsc must be kept
low enough that there is enough space between it and the 4.2 MHz
cutoff to allow at least one of the colour subcarriers to have a
full upper sideband.  With Fsc at 3.6 MHz, there is 0.6 MHz for
the sideband, which allows the Q colour channel a bandwidth of 0.5 MHz -
still pretty minimal.

So, we need an odd number J such that (J * 15750/2) = 3600000,
approximately.  The nearest odd integer is 457.  However, all of these
frequencies are going to be generated by dividing down a high-frequency
oscillator, and 457 is a prime number.  There was no cheap digital
logic available to implement a modulo-457 divider, so picking 457 would
make life difficult for the engineers, and that's who was designing
this standard.  Now, 455 factors as 5*7*13, and modulo-13 dividers were
possible by available analog techniques.  So 455 was chosen.  At this
point, Fsc is 3.583125 MHz.

However, there is yet another frequency relationship involved.
Many TV sets use some of the same RF/IF circuitry for both sound and
video.  There is going to be some interaction between the sound and
colour subcarriers, producing a beat frequency at low amplitude.  To
minimize the visibility of this spurious signal, its frequency should
also be an odd multiple of Fh/2.  The sound carrier (Fs) should also be
near 4.5 MHz, within the tolerances allowed by the B&W standard.

So, Fs - Fsc = (4.5 - 3.583125)MHz = 916,875 Hz.  This is approximately
equal to (L * 15750/2) for some odd integer L.  The closest value
is L=117.

So, now we have Fsc = 455*Fh/2 and Fs = (455+117)*Fh/2 = 572*Fh/2.

With Fh=15750, this gives an actual Fs of 4.504500 MHz, just 0.1% too
high.  Instead of leaving well enough alone, the NTSC decided to tweak
all of the frequencies downward to put the sound carrier back as close
as possible to its nominal 4.5 MHz value.  So, they calculated Fsc as
455/572 * Fs.  This gives Fsc = 3,579,545.454545...

Then they decided that they didn't like the repeating decimal place,
and so defined Fsc as 3,579,545 Hz *exactly*.  All of the other
frequencies are then defined by their relationship with Fsc:

	Fs = Fsc*572/455 = 4,499,999.43 Hz
	Fh = Fsc*455/2 = 15,734.264 Hz
	Fv = Fh*2/525 = 59.94 Hz

The new standards for Fh and Fv are 0.1% lower than the B&W standards.
However, the tolerances on Fsc are +- 10Hz, or about 3 ppm.  Since Fh and
Fv are obtained from Fsc, they now also have tolerances of 3ppm, much
tighter than the old standard.  The new frequencies plus their tolerances
fit within the tolerances of the old frequencies, so the standard is still
safe.

Also, in the early days of B&W, the station's master oscillator was
sometimes locked to 60Hz power line frequency, so that any hum bars on
receivers would be stationary rather than crawling up or down the
screen.  With colour, this was no longer possible, since all of the
frequencies must be crystal-controlled with 3ppm tolerance.  But
locking to the local power line wasn't possible if you were
broadcasting a network program, so stations generally weren't dependent
on this anyway.


So there's the story of where the NTSC frequencies come from.

All of this brings up my favourite way of calibrating a frequency
counter, or similar time measuring device:  Pick up a commercial
broadcast TV station, and feed the demodulated video to a video sync
generator that will genlock to an external signal.  Measure subcarrier
out from the sync generator.  (Or, just sample the subcarrier
oscillator on your colour TV while receiving a network station).  The
signal you get is 3579545 +-10 Hz, virtually guaranteed.  Check several
networks to be sure.  Adjust to minimize average error if you like.
(Avoid the local cable company's own channels - they may not be as
careful as broadcasters are required to be.)

	Dave Martindale