From: elf@ee.ryerson.ca (luis fernandes)
Subject: The NTSC signal (was: minor typo in my cdrom/ld post)
Summary: (Pretty much) everything you wanted to know about TV signals 
Organization: Ryerson Polytechnical Institute, Toronto
Date: Sat, 2 Jan 1993 19:16:18 GMT

dannyb@panix.com (Daniel Burstein) writes:
>seriously... 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.

I managed to dig out an old lab write-up I did on the NTSC colour
signal.  

Appended below is a condensed version in which I have eliminated
the extremely technical aspects of the process while leaving enough
to make it understandable; if it reads rather choppily it is for
this reason.

I don't claim, by any means, to be an authority on the subject,
nevertheless, the information presented is remarkably close account
of what actually happens.

--
elf@ee.ryerson.ca

    "The pursuit of backward-compatibility will be the 
    downfall of the human species."

    _Origin_of_the_Specious_, Charles Darwin

------------------------------------------------------------------------
Type 'n' now to resume your normally scheduled program...


With the advent of colour television in 1952, new standards had to
be adopted, different from those already established for the
transmission of black-and-white (B&W) television signals. These
standards dictated that all the colour-video and audio information
to be contained in the 6MHz bandwidth already allocated for B&W
transmission of a TV channel. 

Since the majority of the consumers already owned B&W television
sets, the new colour signal would have to be compatible with these
old sets, that is, the signal would have to be designed such that
B&W TV sets were able to receive colour transmissions without
distortion and interference from the colour signal.

The standards in question, adopted by the Federal Communications
Commission (FCC), were proposed by the National Television System
Comitte (NTSC), and the signal is therefore called the NTSC
composite colour signal.

When viewing a television program, it is possible to tell whether
it is day or night by the birghtness (luminance) of the scene being
viewed. The total brightness of the scene is transmitted in the
luminance signal (designated the Y-signal). This signal is composed
of 59% of the total green signal from the camera output, 30% of the
red camera-output signal and 11% of the blue signal. Assuming a
peak output voltage of 1 Volt for each of the R, G & B signals, the
Y-signal becomes:

	Y= .59G + .30R + .11B

These percentages were determined empirically through extensive
experimentation.

The signal that carries the colour information is called the
chrominance signal. This signal has both magnitude and a phase and
may therefore be viewed as a vector. The magnitude of the signal
(electrically) represents the amount of colour saturation
(visually); the phase determines the specific colour. This signal
is generated by subtracting the luminance-portion of the camera
output, i.e. given the output of a colour-camera, if the
brightness-information of a scene is removed what is left is the
colour information:

	R-Y= 1R - Y= 1R-(.30R+.59G+.11B)=  .70R - .59G - .11B
	B-Y= 1B - Y= 1B-(.30R+.59G+.11B)= -.30R - .59G + .89B

Only the R-Y and the B-Y signals are transmitted, since a vector
combination of these two signals allows the R-Y signal to be
reconstructed at the television reciever:

	G-Y= -.51(R-Y) - .19(B-Y)

In simple terms, the chrominance signal is added to the luminance
signal. A B&W set receives both signals but since the colour
information is cleverly (and transparently) hidden it is silently
ignored.

The technical wizards at the NTSC used an inherent limitation of
the human eye that small objects are percieved only as dark or
light shapes (i.e. the eye can only percieve the luminance
information) to squeeze colour information transparently (to B&W
sets) into the B&W video signal.

In technical terms, this allowed any signal containing colour
information, above 1.5 MHz to be omitted. Further experimentation
concluded that video frequencies between 0.5 and 1.5 MHz could be
viewed without colour distortion if the colours were artificially
reproduced. The R-Y and B-Y signals are modified into signals
called I (artificially colours (1.5MHz of the spectrum)) & Q (RGB
colour information (.5MHz of the spectrum). These two signals
modulate the colour-sub-carrier (i.e.  the luminance signal has its
own carrier and the chrominance signal was given its own separate
carrier (3.58MHz) so as not to cause interference in B&W sets).

The sub-carrier frequency (3.58MHz) was chosen to be an odd
multiple of the horizontal scanning frequency; the chrominance
(color information) signal was interleaved in the unused portion of
the luminance (B&W information) signal.

The video-signal takes-up 4.2MHz of the 6MHz spectrum, the
audio-signal make up the rest. The NTSC Composite Signal:
________________________________________________________________________

   ^                 ^   ^
   |YYYYYYYYYYYYYIIII|QQ |AAAAAAAAAAAAA
   |             |   | | |            |
   |             |   | | |            | 
   |             |   | | |            |
   |             |   | | |            |
   |             |   | | |            |
   |             |   | | |            |
   +------------------------------------> MHz
   ^               3.58  ^            6
   |                ^    |
Video Carrier(Y)    |    Audio carrier (A)
                    |
                 Video sub-carrier (colour information I & Q)
________________________________________________________________________
---EOT---
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