Looking for Matrox Signals levels for both brightnesses.

[cj7hawk]

Hi All,

I know this is a bit of a weird request, but does anyone happen to have an oscilloscope and a functioning Matrox ALT512 with 4 intensities in 256x192 mode and is able to tell me what the signal levels are?

The manual itself is excellent and contains all the timing and voltage information, except this one detail...

I've rebuilt my video circuit desing around a similar output to the Matroz and am trying to work out how deep the intensity differences were and the voltages are a long way from where I expected them to be when they looked right on the screen ( Black, Dark Grey, Light Grey, White )



This is what I'm getting from my 2-chip raster circuit at the moment and while the syncs and black levels all look OK ( it's about 1v PP, 0.7 on the video from black to white, and I know the Matrix was 1.4v PP but I can adjust that ) originally I thought I'd have to have the levels all fairly evenly spread, and found to get 4 visible graduations of grey, I had to drop all the grey levels almost to the black level, otherwise it's very hard to distinguish the grey from the white. It also might be because I'm pushing the signal into a LCD panel at present.

The other thing that isn't certain from the ALT512 manual is whether or not the composite output includes the setup section ( blanking lower than black ) - This is something else I'm trying to find out.

If anyone knows how Matrox did it, or can take a snap next time they have their scope and S100 system out, it would be very much appreciated -

Thanks
David.

 

[Hugo Holden]

I have produced 4 level images from the ALT512 and produced a picture from the combined A an B planes:

www.worldphaco.com/uploads/MATROX_ALT256.pdf

The 4 level signal looked pretty good on an Amber VDU, with each level easily recognized.


And used those planes to create a Light Pen project:

www.worldphaco.com/uploads/LIGHTPEN.pdf

I don't think I took a scope recording of the mixed levels, but as I recall they were set by the ratios of some resistors on the output of gates that mixed the two planes to acquire the 4 levels, so its possible to work those out from the schematic, also taking into account the properties of the output stage in the gates. I'll have another look at the schematic. I don't think the mixing of these components was elaborate, it was not a virtual earth mixer (as in the Dazzler for example), so there were inevitably some interactions.

Next time I power it in my SOL-20 I'll take a scope recording.

Ideally though, you would simply divide your 0.7V of terminated video into levels above 0 (black level), 0.233(dark grey) about 0.466(light grey) and 0.7(white).

The total signal with sync should be 1Vpp terminated into 75 Ohms and 2Vpp unterminated.

I have noticed though that video generated from computer boards often deviates from standard, for example in the Dazzler the burst pulse was much wider and a few other quirks in the color processing.

One quick option could be to run the specific gates, resistors and video output circuit in a simulator (or a protoboard) and get the answer that way if you wanted to get the exact same "grey scale".

 

[cj7hawk]

Next time I power it in my SOL-20 I'll take a scope recording.

Thank you for that -

Ideally though, you would simply divide your 0.7V of terminated video into levels above 0 (black level), 0.233(dark grey) about 0.466(light grey) and 0.7(white)

That's a little closer to what I would have assumed the voltages should have been, but those numbers appear very specific - I assume they were well-known preset levels?

Also, I've noticed that the LCD I'm looking at changes a *lot* when I stood up, so I think that may be the source of my issues ( viewing angle ). I'll pull out a CRT in the next few days and try that instead.

Thanks again,
David.