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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.


---
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