TV horizontal scanning - why flyback?

[russell_w_b]

Originally Posted by Argus25

I have attached an interesting oscillogram (photo of CRT face) of how a raster scan is constructed...can anyone guess how this image was created ?

Very slow pan of the camera, set to 'B' over the CRT with the CRT contrast and brightness set very low?

[Argus25]

Well done Russell !

Hugo.

[russell_w_b]

Originally Posted by Argus25

Well done Russell !

I've been photographing CRTs for years and, more recently, OLED and LCD displays on various bits of test kit - took some photos of spectral analysis today, in fact - although never with the shutter left open long enough for a full raster to appear. For tellies it had to be open more than 1/25 second shutter speed or you'd get part of the image blanked out.

It's an interesting picture, that.

[G6Tanuki]

This has been a really fascinating discussion.

Evil Me ponders if anyone ever proposed a radial raster (like the classic PPI RADAR displays)

While the traditional way to produce these involved mechanically-rotating the scan coils round the neck of the CRT (synchronised using Selsyns etc. to the movement of the scanning-antenna) I will admit that a few years back for a computer-games business I 'reverse-engineered' a Radar-type PPI-display presentation for one of their products.

After a few false-starts I realised that rather than gobbling realtime compute-cycles for each-and-every sine/cosine point of each possible 360-degree rotation (they were expecting at least 30Hz refresh), it was easier and display-faster to malloc() half-a-Megabyte of memory, do a single-pass vector-points computation (which took a second or so, easily lost in loading-time) and thereafter treat the whole thing as a simple lookup-table for X and Y values.

PPI-style TV could have been 'interesting'...

[russell_w_b]

Originally Posted by G6Tanuki

While the traditional way to produce these involved mechanically-rotating the scan coils round the neck of the CRT (synchronised using Selsyns etc. to the movement of the scanning-antenna) I will admit that a few years back for a computer-games business I 'reverse-engineered' a Radar-type PPI-display presentation for one of their products.

It can be done the other way round. Some RADAR equipment gives a regular computer graphic display, but can be configured into simulated PPI for that 'classic' RADAR display.

[PaulM]

There was a proposal in the 1980s for Peano Scanning. We thought about it for use in flight sim where the necessary wide band linear amplifier drive of the scans was commonplace. The basic 'curve' is here:

https://en.wikipedia.org/wiki/Peano_curve

It's a never ending pattern and whilst very difficult to do on a CRT (even with wideband linear amplifier drive), it could be done today on a fixed matrix - but why would you want to do it?

It was always good for a half hour's discussion!

Best regards,

Paul M

[peter_scott]

With limited persistence any scheme that doesn't rescan from scratch will accentuate flicker.

Peter

[PaulM]

Flicker perception depends on many factors, but the primary ones are image brightness (square law), field of view (linear relationship), persistence (complicated response law) and update rate (another complicated response law. . .). In the case of Peano scanning, there is a point where the pattern starts again - the spot comes to the same point - and the time taken to do that is the effective update rate. By the time you get to about 240 Hz 'frame' rate, human perception has as good as lost any sense of flicker. In flight sim, we didn't need to worry about transmission bandwidths, so you could let rip with line and field rates governed only by IG (Image Generator) speed limitations and video bandwidths (often in the hundreds of MHz).

We were never keen on the idea of Peano scanning, but it wasn't much more bizarre than common-place raster calligraphic systems which combined conventional raster scan for about half the frame time with a free address calligraphic (vector scan) overlay. This was used to 'paint' high intensity vector addressed light points (runway lights, cultural lights and other features) over the raster display with all its associated aliasing. Calligraphic lights were precise, bright and moved properly across the display space. Once you have the high power, wide bandwidth deflection amplifiers to do raster-calligraphic, you're half way to be able to do Peano scanning. We never actually tried it, but did think about it.

Cheers,

Paul M

[Argus25]

Originally Posted by russell_w_b

For tellies it had to be open more than 1/25 second shutter speed or you'd get part of the image blanked out.

It's an interesting picture, that.

Back in the early '80's I was tasked with getting a film camera (which had a DC motor) to film a CRT/monitor picture without a rolling bar.

Firstly I found I could reduce the width of the bar by making the shutter exactly 180 degrees. But it was still just visible. (bigger or smaller in angle the bar was grey or white and wider due to under or over exposure effects) . I then drilled a hole in one of the rotating shafts in the camera and fitted an optical sensor. Also I added a current sensing resistor in sensor in series with the motor.

I designed a servo, similar to those in early VCR's, used the motor current to give a little + feedback, to control the speed of the motor for drag and friction, and used the optical sensor output to phase lock the shaft angle to the vertical sync pulse of the video signal driving the monitor with a sample hold arrangement. This phase locked the "rolling bar" and I could get it into the vertical interval where it could not be seen.

(ps: that raster image is from Manfred Von Ardenne's pre war TV book 1936)

[Brigham]

Originally Posted by G6Tanuki

This has been a really fascinating discussion.

Evil Me ponders if anyone ever proposed a radial raster (like the classic PPI RADAR displays)

I seem to remember Karen O demonstrating an American-style (Felix) image using a radial raster.
It was interesting, but had the disadvantage of being not very good!