On Nov 8, 18:07, Richard Erlacher wrote:
> If I remember correctly, the format difference between PAL and NTSC lies
not
> only in the difference in the vertical refresh rate but also in that the
> NTSC format is based on 525 horizontal scans per frame, while PAL uses
625.
> I'm not at all sure about whether PAL interlaces in the same way as NTSC
> does it, and, in fact, I'm not sure that they do that at all.
One is a consequence of the other. The most obvious difference between PAL
and NTSC is the use of different vertical scan rates (50Hz vs 60Hz) but
they use the same horizontal scan rate (15.something kHz), so if you do the
arithmetic, you'll see why they use different numbers of scan lines (625 vs
525). They both use interleave; so the number of full frames is 25 per
second or 30 per second. They also differ in the way they encode colour
information. The basic scheme is similar but the subcarrier frequencies
differ, and in the case of PAL, there is a "colour burst" of pure
subcarrier at the start of every line. This allows the circuitry to
synchronise better. Also, the colour signal is 180deg different in phase
on every second line (hence the name, Phased Alternate Line) which makes it
a little more stable.
In the context of broadcast TV, they also use diffent audio subcarriers (in
fact, there are several PAL variants, used in different countries. They're
called PAL/G, PAL/K, PAL/I, etc).
The net result is that you can often use a PAL display instead of an NTSC
one or vice-versa, but you would expect to lose the colour and sound. You
might possibly have to tweak the vertical hold to get a steady picture.
Since the sync pulses are encoded in the same way, any monitor or TV
should lock on to the horizontal signal perfectly, but the vertical might
be beyond the range of the sync circuit (it's usually OK in practice,
though).
I bet Tony knows more about this than I do.
--
Pete Peter Turnbull
Dept. of Computer Science
University of York
Received on Thu Nov 09 2000 - 03:43:03 GMT