Re: Why no "comb fringes" on VCR still frame?

Roderick Stewart wrote:

On Wed, 23 May 2007 15:44:26 +0300, "Jukka Aho" <jukka.aho@xxxxxx>
wrote:

This would involve some complication but wouldn't achieve anything
except a bit of flicker on the very top or bottom line of the frame,
depending on where you decided to put the extra line. It wouldn't
result in interlacing

But that was exactly the point - to make a 50 fps tv-compatible video
signal that is _not_ interlaced. But still 50 refreshes per second,
and 15625 lines per second.

You could do it simply by using an even number of lines, the nearest
of course being 624 or 626. As you say, this would make either the
field or line frequency (or both) slightly different from the neat
numbers thay are now.

Yes. I was only considering the scenario where line frequency and field frequency would stay unchanged. Retaining the neat numbers has some advantages when considering e.g. displaying animation and keeping the (digital) sound in sync with the picture.

Without knowing the details of the sync circuitry in every TV set
and recording device in use, it would be impossible to say what
the effects would be.

Still, most of them - at least the CRT-based sets - manage to sync to all kinds of non-interlaced signals produced by the home computers and game consoles of the 1980s (not to mention domestic VCRs, which are surely even more difficult to deal with) so I guess they would cope with fields that have an alternating number of lines, too.

I thought at first you were suggesting altering the blanking on
alternate fields, but if your idea was for alternate long and
short fields with either 624 or 626 actual line syncs between
field syncs to make the field frequency "average out",

My idea was creating a sequence of a 311-line field followed by a 312-line field followed by a 311-line field followed by a 312-line field, and so on, and placing the vertical sync in the beginning of a line, rather than in the middle. (So no half-lines there.)

My guess is than a great many TV sets would exhibit quite severe
25Hz vertical jitter, but that would depend on details of
indivitual circuits

The idea was that, after dealing with the vertical sync, the active picture content would start on the same line in both fields, and be drawn in the same location on the screen (no half-line offset, but no change in the [average of] vertical refresh rate or the line frequency, either.)

and possibly reception conditions too.

Some earlier poster to this thread was concerned about broadcasting these non-interlaced, slightly non-standard signals, but I'm primarily interested in the signal itself, not the delivery method.

For me, the question was primarily about whether things have _already_ been done this way in some of the 1980s home computers (i.e., with fields that have an alternating count of lines, giving [on average] an exact 50 Hz refresh rate), or whether they're all using these simpler schemes, where all fields are made to be 311 lines or 312 lines. (Which leads to a non-standard vertical refresh rate that is slightly lower or higher than 50 Hz.)

But why do it anyway? Movement portrayal would be exactly the same,
the refresh rate or flicker would be exactly the same.

If you observe the pictures generated by an Amiga computer set into a non-interlaced mode, you'll find out that the non-interlaced 50 Hz picture is much more stable and easier on the eyes than the interlaced (line-doubled) equivalent, which has the 25 Hz half-line jitter.

Granted, the (perceived) vertical resolution is only half of that of the corresponding interlaced picture, but it's still enough for some pretty usable user interfaces.

Mock-up images showing how the non-interlaced Amiga-generated video signal looks like on an ordinary (CRT-based) tv screen:

<http://lipas.uwasa.fi/~f76998/video/modes/_images/shell.gif>
<http://lipas.uwasa.fi/~f76998/video/modes/_images/term.gif>
<http://lipas.uwasa.fi/~f76998/video/modes/_images/protracker.png>

The only thing that would differ would be the line structure,
which would be twice as coarse!

That's true, but in return, you get rid of the 25 Hz vertical jitter, which is a big win when using ordinary tv sets (or the equivalent 15 kHz video monitors) as computer monitors. (And which is what a great number of people did, back in the day, staring their 50 Hz / 15 kHz screens all day long.)

What would be the point of that?

These days, the point is no longer there (except perhaps for some niche applications), with the CRT-based SD resolution TV sets being rapidly replaced by the LCD-based "HD Ready" sets (which are capable of displaying non-interlaced HD resolution pictures straight away.) Still, it's interesting to ponder what kind of tricks and tweaks were once applied to the video signal get an ordinary tv set behave more like a computer monitor.

--
znark

.

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