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Re: NTSC Legalacy Formats

At 06:47 PM 17/02/98 EST, Christopher Bacon wrote:

>In NTSC and PAL, color information is carried by quadrature-modulating the
>phase of subcarrier.  Regardless of image content, the frequency is always
>3,579,545 +/- 10 Hz, give or take some phase difference (in broadcast).  All
>colors of the rainbow (or at least of the TV system) are represented in one
>360-degree phase swing of I and Q.

As you phase modulate something you generate sidebands as the modulation
index changes - just like in FM..

Although a vectorscope shows the phase relationship at 3.579545 there is
still other components of the signal that you can't see. We can't forget
about the response of the sidebands. The I and Q signals are being
transmitted as the sidebands of the double-sideband suppressed carrier and
then placed on top of the regular Y information. The sample of the
suppressed carrier (required for demodulation) is then reinserted into Y and
we all call it color burst.

The I signal has a bandwidth of 1.3 Mhz, so the sideband information can be
contained anywhere down to 2.3 Mhz. The Q signal with it's 600 khz bandwidth
can then therefore extend up to 4.2 Mhz.

Since when we generate color bars it is a square wave in each channel R, G,
B - we only see a specific phase of the carrier when looking at the
waveform. Try looking at multiburst from R,G,B and it is a mess of
frequencies in NTSC.

See what happens when a RF Engineer enters the world of video? For even more
fun, connect a RF spectrum analyzer and look at NTSC - you can see the real
"or active" bandwidth of the 3.579545 and all the other frequencies involved.

Rob Chambers, AScT.
Senior Engineer,
Vancouver Television.

I believe the above to be correct, although there could be some residual RF
still effecting my body from the transmitter days 5 years ago.

Thanks to Blake Jones for support of the TIG in 1998.
No product marketing allowed on the main TIG.  Contact rob at alegria.com
922 subscribers in 36 countries on Tue Feb 17 21:32:46 PST 1998 
complete information on the TIG website http://www.alegria.com/tig3/