Video "quantizing" effect through modulator
 

Sorry for the strange title but it seemed like a good way to describe the problem I have.
I'm running a Bush TV22 from a PC video source, mixed down to monochrome and fed into an ex-Rediffusion modulator. It looks fine on a test card but actual videos (or images) have annoying highlights. It's particularly noticeable on cloudy sky scenes and faces (see attached). It's as if a slowly rising video level results in a sudden jump in luminance at the CRT (presumably not visible on test card C because only discrete levels are used). I suspect the problem is in the signal generation process rather than the TV. It's not the interference limiter as that is fully backed off and produces a rather different effect anyway. Anyone got any ideas?

Re: Video "quantizing" effect through modulator
 

Basically what you are seeing is called a "paint by numbers effect" with sudden obvious jumps in the luminance level. Mostly, this is a common effect seen in the low lights of digital TV, its less commonly seen on the high lights but it is the same problem.

It is a digital effect, meaning the difference between the white level on the face and the grey level next to it, represents a change of one count or one bit, suggesting the resolution (number of bits) feeding the D to A converter is far too low. When you say the signal is from a PC source, is it a composite signal from a connector on your video card ? Can you adjust the resolution of the video card in the computer's settings ?

Re: Video "quantizing" effect through modulator
 

It's a sign that either:

You do not have enough bits of resolution in the signal voltage.

Or you have some bits not working (bad connection, bad DAC)

Or you have some bits swapped around.

David

Re: Video "quantizing" effect through modulator
 

Hi Keith,

What video card are you using?

Peter

Re: Video "quantizing" effect through modulator
 

Agreed, there are not enough bits in the original digital video signal. You should be able to configure the bit depth of the video signal. For example on a Windows PC if you right-click on the desktop you get various options including one to configure the desktop. One of the options is to set the colour depth.

Re: Video "quantizing" effect through modulator
 

Doh! The colour depth was set to 8 bits...


Thank you gentlemen. I had it fixed in my mind that it was some obscure analogue problem in the modulator.


Happy bunny now!