Number of active lines - 405 line system.

[Geordie McBoyne]

Yesterday during a conversation with Trevor the subject about the number of active lines came up. From the mid-sixties the number of active lines became 377. However, wasn't the original specification for the 405 line system 385 lines? Also, the video part of a line of was 83microseconds or so. After conversion from 625 lines the video became 80microseconds.

Geordie McBoyne.

[ppppenguin]

In the early days of 405 I don't think the length of vertical interval was fixed very accurately. Some other parameters weren't accurately fixed either.

When converting 625 to 405 you can design the input/output ratio to set the picture height. It's fun to make it variable and do a vertical zoom. Exactly which numbers you should use is another question. In practice the difference ins't very important.

[ppppenguin]

Looking at how the introduction of 625 influenced the 405 parameters slightly it's instructive to look at how the digital CCIR 601 standard affected both 625 and 525 standards.

A critical decision was to use a common pixel clock (13.5MHz) for both 625 and 525. This was convenient at the time but resulted in non-squre pixels which had different aspect ratios on 625 and 405. This made interface with computer generated pictures more difficult. Later HD standards used square pixels but needed 2 different clocks to cope with all standards.

Choosing 720 pixels per active line encompassed all possible 625 and 525 signals with a margin for poor quality blanking and ringing due to sharp cutoff at the edges of the picture. This concept was carried forward to HD and has proven its worth.

When converting 625 to 405 it makes sense to use an output clock of 13.5MHz * (405/625). Hence there is a 1:1 correspondence along a line between input and output pixels. This ultimately sets the horizontal parameters of the 405 signal. There is no fundamental need to use a different clock. Designers could have used an interpolator to change the line length. AFAIK, none did.

[`FRANK.C]

A few pictures below for comparison.
None of the controls on the TV were altered between photos.
The first is 377 active lines video 80uS
The second is 385 active lines video 80uS
The third is 377 lines video 83us

Frank

[ppppenguin]

You can see the differences but only by direct comparison. You can't tell which one is correct. It would have been beyond the capability of all but the very best broadcast grade monitors to tell by looking. A viewer at home would never know.

[Jac Janssen]

David,

I looked in 2 publications:

The Journal of the Television Society published this:
   

The Black Book gave this waveform:
   

So, the J of the TVS mentioned at least 10 suppressed video lines per frame, resulting in 385 actives lines or less.
The BB specifies a video suppression period of 14 video lines per frame, resulting in 377 active lines.
= the numbers you already mentioned.

Jac

[Geordie McBoyne]

I believe the 441 line system from the Paris transmitter had 383 active lines. The field blanking was 29 lines, much longer than the 405 line system.
In terms of vertical definition there would be no visual difference between the two systems.

Geordie McBoyne.

[ppppenguin]

The modern way to describe a system gives the number of active lines rather than the total. The 720p HD standards have fewer active lines than the old French 819. Though progressive scan gives a higher apprarent vertical resolution than interlaced. I doubt that much, if any, new material is being shot at 720p.

The early American 343/60 system used about the same bandwidth as 405/50. One had less flicker, the other more vertical resolution.

[Jac Janssen]

Found in a 1947 French book:
   

So it the French norm is quite flexible...
Number of lines between 440 and 455, 10 to 15 lines sync per frame.

Picture of the only 441 line set I have (Same chassis as the Philips TX390A and TX402A), here working @ 441 lines:
   

Jac

[Jac Janssen]

And the German 441 line standard of 1939:
   

Jac