pal (04/22/83)
I am thinking of buying a video recorder, but one thing bothers me. I am from India, where the TV format is different from the one used in the United States, it is one of the European/British formats, PAL/SECAM, I am not sure which. My folks back home sent me some video cassettes (VHS) which I tried to play on a rented recorder here, but got garbage. If I do buy a recorder, it *must* be able to record/play interchangeably with my folks's machine. Is the incompatibility in the TV or the recorder? i.e. would purchase of a US fromat TV enable them to view my tapes? Can I get a video recorder/TV that would work both places? (Incidentally, VHS format is a must) Any leads appreciated, Anil Pal U. of Wisconsin-Madison
ken (04/29/83)
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!kenken (04/30/83)
OOPS! I made some embarassing errors in my last rather verbose article
about whether NTSC video recorders are compatible with PAL or SECAM.
THEY ARE NOT! I have had some experience with video, but not
necessarily with video tape recorders. After consultation with a
friend in the video production business, I was chastised for even
thinking that the only relevant thing about a video signal's ability to
be recorded is its bandwidth. It turns out that the rotating motion of
the VCR's head is synchronized with the field rate (50 vs. 60 Hz), and
requires substantial modifications to switch from NTSC to PAL. One
field is recorded with each diagonal swipe of the head across the
tape. Some professional VTR's such as Ampex's line can be changed from
one video standard to another simply by changing a few boards, but home
units are designed more for low cost than for flexible generality.
There was also a typo: SECAM is radically different from NTSC, not
PAL, as asserted in my original article. PAL and SECAM are similar in
one way, and PAL and NTSC are similar in others, but NTSC and SECAM
have twice the dissimilarities.
Ken Turkowski
{ucbvax,decvax}!decwrl!turtlevax!ken