TV system line numbers

[peter_scott]

Originally Posted by ppppenguin

The other way of generating syncs in the very early days was mechanical. Rather like a tone wheel Hammond organ. I wonder if anyone has any fragments of a mechanical SPG.

.

I think the first 405 line transmissions were purely electronic but the Scophony company had problems due to the mechanical inertia of their receivers and the BBC responded by generating the syncs in this way.

Peter.

http://www.earlytelevision.org/scophony.html

[oldeurope]

Hi, I read the text and the 441 system has equalising pulses.
But they take place only after the V sync.
Look at the "Nipkow- Scheibe":

"Schlitze für Bildwechselimpulse mit Nachsignal."
means: Slits for V- sync with equalising signal.

Darius

[David_Robinson]

Originally Posted by oldeurope

In theory it is possible to get interlace with an even amount of lines ... The main problem is that the V linearity changes a bit if a frame is longer or shorter. So you don't get interlace over the whole picture.

Ah, good point. I hadn't thought of that!

On the subject of adding EQ pulses to a 405 signal, the problem is how to make room for the first set of EQs. The field timebase of the receiver will still trigger at the same point (very nearly), so the first train of EQs are effectively in the active picture. To accommodate them, the active picture must be shrunk, cropped or shifted upwards. How very un-authentic!

Just think, if the 625-line system had been designed without EQs, we could have had 8 lines more teletext per frame (for the same flyback ratio)!

[oldeurope]

Originally Posted by David_Robinson

On the subject of adding EQ pulses to a 405 signal, the problem is how to make room for the first set of EQs. The field timebase of the receiver will still trigger at the same point (very nearly), so the first train of EQs are effectively in the active picture. To accommodate them, the active picture must be shrunk, cropped or shifted upwards. How very un-authentic!

Just think, if the 625-line system had been designed without EQs, we could have had 8 lines more teletext per frame (for the same flyback ratio)!

Hi David,
in the 441 line system the H sync is 10% of a line and the V- sync pulses
are 35% of a line. The EQ signal takes place after the V pulse that is
between two lines. So every second V pulse has an EQ pulse at the end.

Interesting solution, you don't loose as much lines as in the american standard.
I am sorry I don't have more exact specifications about the 441 line system.

BTW I just made something so blank teletext, because it is visible in the
vertical fyback of most vintage 625 TV sets.

Darius

[tubesrule]

Hi Darius,
What I was refering to with the Sanabria system was it was actually a triple interlaced system meaning the lines were scanned in the order 1,4,7... then 2,5,8... then 3,7,10...
Although the actual frame rate was only 15 fps, the effective field rate was 45fps, so the flicker was greatly reduced. Of course this had the effect of increasing interlace effects in motion.
Thanks for the info on the German mechanical generator. That was quite some device!

Hi David,
You are correct about adding eq pulses to 405 that something has to give. In my converter I was forced to move the active frame position to make room for them. It is a compromise which is why I have an option switch if you want the original standard or one with eq pulses. This way if you if you have a finicky set, you can always turn them on, otherwise just keep the syncs as original.

Darryl