marco@mcgill-vision.UUCP (Marco Petroni) (08/04/89)
I have asked someone to post this for me, so take note of my return address... We want to control the contrast of the GREEN signal on a ps/2 VGA analog RGB monitor (model 8512). So far our attempts have not worked properly. What we have now is an external box (no I didn't design it) connected to the parallel port. An 8 bit value on the parallel port controls relays which switch a resistor network thus providing a digitally controlled attenuator. There is an op-amp before and after this attenuator. Anyway, the peak-to-peak voltage of the signal is reduced (as expected) and depending on the offset adjustment we can select any base value. As an example, assume we display a sin wave accross the monitor. Looking at a scope, you will see a sin wave on part of the video signal. As you decrease the contrast, the sin wave decreases in amplitude. The zero value of the sin wave can be adjusted by selecting the proper colours and offset on the box. This looks fine on the scope, but it just doesn't work. What happens on the monitor is the intensity drops as the peak-to-peak value drops. It is important for us to keep the intensity constant. It seems that our assumption that the intensity is related to the absolute (DC) value of the signal isn't entirely correct. If anyone out there could find some information on analog RGB monitors (VGA) that would help me greatly. What I'm interested in knowing is: - What type of signal does the monitor expect? - Is there a maximum voltage range? - What exactly controls the intensity and are certain parts of the signal used as a reference and thus should not be attenuated with the rest. - Are there any digital attenuator ICs around that would be suitable for this application, thus removing the relatively bulky relays. - Any other information you think might be usefull or other sources of information where I might find out more. Reply via mail or post to net, I can read but can't post. Thanks, Michael Moscovitch reply here: michaelm@scotty.ee.mcgill.ca or here if above down: mcgill-vision!marley!mike