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From: al@cs.strath.ac.uk (Alan Lorimer)
Newsgroups: sci.electronics
Subject: Re: NTSC-PAL (in)compatibility
Message-ID: <235@baird.cs.strath.ac.uk>
Date: 27 Jun 89 16:31:27 GMT
References: <124@nisca.ircc.ohio-state.edu> <566@axis.fr> <367@ctycal.UUCP>
Sender: news@cs.strath.ac.uk
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Organization: Comp. Sci. Dept., Strathclyde Univ., Scotland.
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In article <367@ctycal.UUCP> ingoldsb@ctycal.COM (Terry Ingoldsby) writes:
>In article <566@axis.fr>, philip@axis.fr (Philip Peake) writes:
>....> 
>First of all, unless I'm very mistaken ordinary VCR's do not separate the
>chrominance and luminance info when they store it on the tape.  It is all
>stored together.  Somebody please correct me if I'm wrong about this
Er.....
Sorry, you are wrong. One of these days I'm going to post a complete
and concise description of television, colour systems (PAL SECAM NTSC)
and various types of video recording techniques, so everyone will know
once and for all! (You maight mail me if anyone's interested)

VHS is a colour under system. The reason for this is that the
luminance signal is band limited to around 3.5MHz. This unfortunately
chops off the colour sub-carrier from most vision systems. Hence it is
necessary to separate the colour signal and modulate it seperately
onto the tape. The luminance signal is FM modulated and occupies a
frequency band in the range 5-12MHz. The colour signal is modulated
seperately onto a carrier at some freqency below the luminance
signal. Hence, the colour is under the luminance signal.

>(no flames, just corrections please :^).  As for putting the chroma on a
>parallel track, do you mean a parallel helical track or a linear track?
>If the former, then you are going to complicate the head design considerably,
>if the latter then
>  1) You will have a hard time getting enough bandwidth out of a linear track
>     to store chroma info

Quite right, that's why it is recorded by the helical scanning head.

>  2) It will be a *devil* of a job keeping chroma and luminance synchronized
>     to the necessary accuracy.  (You don't notice if the audio is 5 msec
>     off, but you would sure notice if the chroma was 5 msec out)

No problem, since the luminance and chrominance are recorded by the
same head on the same track, but at different frequencies
synchronisation is not an issue.

To answer another point made by another poster, the colour under
signal is recorded as it is coded in the orginal signal be it PAL or
NTSC (don't really know for sure about SECAM, since there will be lots
of FM sidebands to try to fit in the same space, but it's probably not
two much hassle to convert it to PAL and the back to SECAM - no I'm
not joking, there are lots of reasons for doing this!)

This means that you can't interchange tapes between standards, but
there are a lot of multi-standard TV sets out there which won't mind.

As an VHS tapes can be exchanged between PAL systems having different
sub-carrier freqnencies, since the colour under frequency will be the
same in all cases. Playing back a tape in Germany say, which was
recorded in the UK will work in colour since the native video recorder
will play back the colour at its native subcarrier frequency.

PAL and SECAM tapes can also be interchanged without any hassle, but
you won't get any colour.

PAL and NTSC tapes cannot be interchanged as a rule, because the field
rate of the vision signal is different. Some VCR servos may be willing
to adapt, and you may get a picture (the TV set probably won't care
much since the line frequency is nearly the same), but the comments on
colour standard still apply. Note that if you do get a picture, the
sound will probably be at the wrong speed, since your VCR will try to
adapt to the field rate of the pulses on the control track.

Have I answered any questions, or just provoked them ?

Alan.
____________________________________________________________________________
Alan G. Lorimer, Strathclyde University, 26 Richmond Street, Glasgow G1 1XH.
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