"Jim Lesurf" wrote in message
...

For CRT that would make little sense to me. Perhaps you aren't making it
clear, or are confusing different things. Are you saying every 16:9 CRT
you've used displays 4:3 by scanning the entire 16:9 area, but puts the
4:3
image into the central portion?

Yes.

if so, why does it do this when fed with
something like 4:3 via SCART?

What difference does using SCART make?

Seems a weird option to me. Changing
the amplitude of the horizontal scan level seems a trivially easier
way to deal with the matter, and probably gives better results.

Changing *horizontal* scan amplitudes isn't trivial (things are vastly
easier with vertical scanning). The horizontal scan waveform is created by
connecting a constant voltage across the inductance of the scan coils.
Reducing the scan amplitude requires either increasing the inductance (by
adding an additional high-current inductor in series, a technique prone to
problems with the extra resistance, stray capacitance etc.; or reducing the
voltage.

But reducing the voltage also lowers the EHT thus increasing the deflection
sensitivity of the CRT which in turn largely cancels out the width-reduction
effect of the HT voltage reduction (but it does result in a dimmer,
less sharply focused, picture)

So the EHT has to be brought back to the original value by fiddling with the
third-harmonic tuning of the horizontal output transformer. This means extra
high-voltage polyprop capacitors plus the relays to switch them in or out.
Alternatively you can do the job properly (but more expensively) by having
an entirely separate EHT supply monitor-style. Compared to all that shoving
the video into a FIFO and reading it out at a different clock rate seems
pretty trivial to me.

But so far as I can tell, when my TV CRT sees the input change to 4:3
from 16:9 there is a click as the horizontal scan amplitude alters, and
the
picture falls back into the central portion of the screen. No scanning
outwith that area. Since it is a pretty standard Panasonic CRT set bought
just a few years ago I assume this is a fairly standard option. Seems like
sensible engineering to me. But if what you say is right Panasonic seem to
have shown rather more sense than other makers. Are they unique? I'd
be surprised to find they were given the simplicity of the method.

I'd agree that the levels used for the scans are set 'digitally', though,
as the widths, etc, are all accessible via onscreen menus - once you have
found the hidden service menus. :-) I assume this just means the scaling
voltages are held in something like a non-volatile RAM and then used to
reference the sizes of the analogue scan waveforms.

Indeed.

But perhaps they do
write waveforms into RAM to get shapes right, then read the RAM series
during each line and frame. Not exactly what I'd call 'digital
processing',
though. Just like using look up tables with a cheap sig gen.

That technique has been used to create convergence waveforms in some
seriously high-grade monitors with delta-gun CRTs, but never in domestic TVs
to the best of my knowledge. You couldn't store the horizontal scan waveform
like that anyway, as it's created in the scan coils as explained above.


Depends what you mean. See above. You may be confusing a 'digital box'
(i.e. external tuner) having to do the job with the display coping
with changes in aspect.

No. I'm taking about the digital video processing that sits between the
analogue tuner, or analogue baseband video inputs (SCART etc.) and the video
amps that drive the CRT guns. PAL decoding has been done digitally for
years, it's possible to do the necessary filtering and signal delays that
way better and cheaper than using analogue techniques. Once the video is in
digital form things like picture-in-picture and image size/shape correction
become easy and cheap to do.

David.