Re: Can anybody explain Progressive-PAL please?

In article <4314fa71$0$22903$ed2619ec@xxxxxxxxxxxxxxxxxxxxxxxxxx>, Martin
Underwood wrote:
> > SECAM is decoded to Y/C, as is PAL. That is recorded on tape using the
> > "colour-under" method. This is why the tape can handle either - it doesn't
> > know the difference between PAL and SECAM. The in/out signals are
> > dependent
> > on the machine, not the tape.
>
> So does that mean that the format on VHS tape is independent of the colour
> encoding system used for the baseband video - ie that a PAL/SECAM 625/50
> tape would *in theory* play back on an NTSC 525/60 machine (complete with
> regenerated NTSC subcarrier) if it wasn't for the "little" matter of the
> different line/frame rates? Or, more realistically, that a US 525/60 NTSC
> tape would play back in a Brazilian VCR and generate a true 525/60 PAL
> signal?

No. The colour subcarrier of the standard broadcast input signal (whichever
type it is) is converted to a lower frequency for recording to tape by means of
a local oscillator, as in a superhet receiver. The "colour-under" or heterodyne
signal is like the intermediate frequency signal in such a receiver, and
contains any original modulation.

The I.F. signal in a superhet is destined for demodulation withput any further
changes in frequency, but the one on a VHS tape is heterodyned with another
local oscillator on playback in order to re-create a signal at the original
standard subcarrier frequency, still containing the modulation, of whatever
type it originally was. So if it was PAL, the playback will be PAL, and so on.

The way to achieve what you describe is to decode the input signal, removing
its original modulation system altogether, and applying a completely new one
just on the tape. There are tape systems that do this, e.g. the professional
Betacam systems, but VHS isn't that elaborate.

Rod.

.

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