Interlaced vs Non-Interlaced display

[whitebird]

Hello,

In interlaced modes, there are up to 625 lines (some serve for sync, but basically it is 625*64µs) represented as two 312,5 lines fields which interlace

In non-interlaced computer display like Amiga, every field is identical and has only 312 lines, which results in 50fps of 312 lines. As the whole screen's height is covered in 1/50 of a second, it means:

1/ Each line has twice the height of interlaced lines or
2/ There is only one and always the same field displayed every 1/50 of a second with a blank interline between scanlines.

I think 1/ is right, but I haven't seen it writen anywhere. Can someone tell about it?

If 1/ is right this means the electron beam is made twice as wide in non interlaced mode than in interlaced mode.

[alexh]

The answer for old CRT was 2

[whitebird]

OK, what you mean by "old CRT". A PAL/SECAM TV from 2004 is an "old CRT"?

So on an old CRT, switching from interlaced to non-interlaced mode should result in loss of brightness because 50% of the display is made of black lines...

[Damion]

Originally Posted by whitebird

OK, what you mean by "old CRT". A PAL/SECAM TV from 2004 is an "old CRT"?

Exactly, any old 15kHz capable CRT.

So on an old CRT, switching from interlaced to non-interlaced mode should result in loss of brightness because 50% of the display is made of black lines...

Yes.

[Toni Wilen]

There is no noticeable brightness change.

Non-black lines are now refreshed 2 times faster (50/second instead of normal 25/second), which makes brighter display because phosphor's brightness has less time to decay.

btw, normal PAL non-laced has 313 lines. ("long field" mode).

[mc6809e]

Phosphors decay too quickly for there to be much difference in brightness (though non-interlace has always looked brighter to me for some reason -- perhaps it has something to due with flicker fusion).

[Toni Wilen]

Ah, of course, it has to decay quickly or it would create motion blur and CRTs don't have it.

Better explanation: both modes have same number of lines drawn/field = at least average brightness stays the same.

[pandy71]

Originally Posted by Toni Wilen

Ah, of course, it has to decay quickly or it would create motion blur and CRTs don't have it.

Well, it depends...
Older CRT's are with different type of phosphor where afterglow is significantly longer than in newer CRT's dedicated to progressive display usage (mostly IT display CRT).
So older CRT's in TV's may have longer after glow and as such motion blur (and this is easy to check).
Modern CRT's IT type are bad for interlace display (flickering is significantly more noticeable than on TV CRT's)

Originally Posted by Toni Wilen

Better explanation: both modes have same number of lines drawn/field = at least average brightness stays the same.

Side to this lines partially overlap (interline distance is controlled trough analog current and there is always fluctuations of distance - average and local luminance, components quality, adjustment etc)and trough electron scatter also some electrons refresh luminance of neighborhood lines (so difference between progressive and interlace is not so big).

https://books.google.com/books?id=lW...creens&f=false

[whitebird]

Thank you for those technical details,

some electrons refresh luminance of neighborhood lines

When watching an Amiga picture in non interlaced mode, black interlines are not clearly visible when the picture is bright probably because "active lines" outshine the black ones, but when a dark gray background is displayed, the black interlines are more apparent.

Modern CRT's IT type are bad for interlace display (flickering is significantly more noticeable than on TV CRT's)

This is one big paradox of interlaced display: its aim being the reduction of flickering, it in fact creates some flickering because of interline twitter.

[pandy71]

Well... interlace have one important benefit - data reduction by half matched with Human Vision System characteristic.