From: utzoo!decvax!decwrl!turtleva!ken
Newsgroups: net.video
Title: Re: Video compatibility problems
Article-I.D.: turtleva.186
Posted: Fri Apr 29 00:45:49 1983
Received: Fri Apr 29 22:03:37 1983
References: uwvax.844

There are a few fundamental differences in the NTSC and PAL color
television encodings, but there are a number of similarities.  SECAM is
radically different from PAL.  The composite video bandwidth is
approximately the same (~4.5 MHz), and the horizontal scanning
frequency is approximately the same (~16 kHz).  The vertical scanning
frequencies are different, though: 60 Hz for NTSC and 50 Hz for PAL.
The subcarriers are not the same, although close, but are incompatible
as far as demodulation goes because they are locked to the line frequency.

PAL stands for Phase Alternating Lines, or something like that.  This
means that the phase of the color subcarrier changes from line to line;
this helps reduce the crawling on edges.  NTSC (sometimes used as an
abbreviation for "Never Twice the Same Color") has no such phase
alternation, and has a relatively simple demodulator.

Both PAL and NTSC use quadrature amplitude modulation to generate the
composite waveform from the components.  In order to go from R, G, and
B to Y, I, and Q (for NTSC) or Y, U, and V (for PAL) one uses a linear
invertible transformation matrix, which concentrates the luminance into
the Y channel, and the chrominance into the other two channels.  These
chroma channels are amplitude modulated onto two sine waves of the same
frequency but 90 degrees out of phase, then added together to the Y.
The I-Q channels represent the same color space as U-V, except that the
two sets of axes are about 20 degrees out of phase with each other.

To demodulate, one passes the composite video through a low pass filter
(or a "comb" filter for fidelity) to regenerate the Y channel, then
subtracts this from the composite waveform to give the quadrature
modulated chroma.  Color burst provides a reference for the phase and
frequency of the subcarrier, which is then used to synchronize an
oscillator used to demodulate the chroma.  When one adjusts the "tint"
or "chroma" knob on the TV receiver, one is adjusting the phase of this
oscillator.  You might think that you could just adjust the tint
control to compensate for the 20 degree phase shift, but the Phase
Alternating Lines of PAL add an extra complication:  the polarity of
one of the signals (V, is it?) gets reversed on every line.  You need
to have a special circuit that switches the polarity for PAL but not
NTSC, unless you don't mind screwy colors or monochrome.

The vertical scanning frequency is a real problem:  only a few
*receivers* can be tweaked 20% up or down, without a capacitor change.

The VTR may not have a very hard time at all recording either PAL or
NTSC, but you will definitely need a different decoder for NTSC and
PAL.  Most RGB *monitors* will handle either NTSC or PAL syncs and
video, so I would suggest getting a component video system with two
decoders.

SECAM, as far as I know, is only used in the USSR and France.  The
chrominance is frequency modulated onto the color subcarrier, and hence
has a higher bandwidth, and is grossly incompatible with NTSC and PAL
VTR's.  The H and V syncs are similar to PAL, though.

		    Ken ("Digital Video") Turkowski
		{decvax,ucbvax}!decwrl!turtlevax!ken