One more time into the lab with the Skyworth to take the cover off...
I had read Bosef's post about the Skyworth's Green output (via the VGA connector) having a DC offset so I wanted to check this out for myself and fix it.
I had suggested that it was caused by "sync on green" and that turns out to be the case. The bad news is that Green rides on a DC offset that's about 300mv higher than blue or red. The good news is that they aren't actually *using* the sync (that input pin to the DAC just stays high), so the Green remains at the elevated DC offset and never dips lower than the 300mv. The result of this is that if your projector/display is AC coupled on the RGB inputs, as my ECP is, and I believe everything else probably is too, this makes no difference whatsoever. Once the RGB gets past the DC blocking capacitor on your display device's input, all colors will have the same min/max amplitude.
I went ahead and disabled the sync anyway just to save a little extra current that the DAC was having to drive. To do this, you have to lift pin 12 of the DAC (ADV7123) on the Sage board and run a little wire from the lifted pin to GND. This put the Green output down with Red and Blue but unless you have the tools and/or courage to perform this mod, it's really unecessary.
Next I decided to increase the output of the DAC from the 660mv peak it was currently set at to a nice and snappy 1.0V peak. (I was checking this level using Avia's vertical gray bars pattern).
Why would I do this? Well, besides the fact that the soldering iron was already hot, it improves the signal to noise ratio of the video signal since any noise that the cable picks up is constant and so any increase in real video signal results in a better signal to noise ratio. Also, if you consider that the cathode voltage on a ECP3100's CRT swings maybe about 50V peak to peak (guess at this point but probably close), that means that my video gain in the projector has to be 71x (700mv RGB x 71). I'd much rather the gain be done by the DAC than the projector. Assuming the same contrast level, with the Skyworth doing 1Vpp on RGB, the projector's gain is only 50.
To increase the output level of the DAC, change the Rset resistor that's next to the ADV7123. Unfortunately I don't remember the reference disignator but I think it was R56. Anyway, it's the only resistor marked "511" near the DAC (510ohms). 351ohms will get you 1Vpp output so I added a 1.2Kohm through-hole resistor in parallel with the 510ohm surface mount.
Here's a picture of my green output with Avia's gray bars:
http://home.hiwaay.net/~jcmccorm/cary.jpg
It's just about 1Vpp and still nice and linear at the top (I was
worried about driving the DAC too hard).
I think the big Bang-for-the-buck mods to the Skyworth will have to come from firmware mods these days but messing with the DAC was fun and I'm happier about the output levels and tamed Green output.
Cary
I had read Bosef's post about the Skyworth's Green output (via the VGA connector) having a DC offset so I wanted to check this out for myself and fix it.
I had suggested that it was caused by "sync on green" and that turns out to be the case. The bad news is that Green rides on a DC offset that's about 300mv higher than blue or red. The good news is that they aren't actually *using* the sync (that input pin to the DAC just stays high), so the Green remains at the elevated DC offset and never dips lower than the 300mv. The result of this is that if your projector/display is AC coupled on the RGB inputs, as my ECP is, and I believe everything else probably is too, this makes no difference whatsoever. Once the RGB gets past the DC blocking capacitor on your display device's input, all colors will have the same min/max amplitude.
I went ahead and disabled the sync anyway just to save a little extra current that the DAC was having to drive. To do this, you have to lift pin 12 of the DAC (ADV7123) on the Sage board and run a little wire from the lifted pin to GND. This put the Green output down with Red and Blue but unless you have the tools and/or courage to perform this mod, it's really unecessary.
Next I decided to increase the output of the DAC from the 660mv peak it was currently set at to a nice and snappy 1.0V peak. (I was checking this level using Avia's vertical gray bars pattern).
Why would I do this? Well, besides the fact that the soldering iron was already hot, it improves the signal to noise ratio of the video signal since any noise that the cable picks up is constant and so any increase in real video signal results in a better signal to noise ratio. Also, if you consider that the cathode voltage on a ECP3100's CRT swings maybe about 50V peak to peak (guess at this point but probably close), that means that my video gain in the projector has to be 71x (700mv RGB x 71). I'd much rather the gain be done by the DAC than the projector. Assuming the same contrast level, with the Skyworth doing 1Vpp on RGB, the projector's gain is only 50.
To increase the output level of the DAC, change the Rset resistor that's next to the ADV7123. Unfortunately I don't remember the reference disignator but I think it was R56. Anyway, it's the only resistor marked "511" near the DAC (510ohms). 351ohms will get you 1Vpp output so I added a 1.2Kohm through-hole resistor in parallel with the 510ohm surface mount.
Here's a picture of my green output with Avia's gray bars:
http://home.hiwaay.net/~jcmccorm/cary.jpg
It's just about 1Vpp and still nice and linear at the top (I was
worried about driving the DAC too hard).
I think the big Bang-for-the-buck mods to the Skyworth will have to come from firmware mods these days but messing with the DAC was fun and I'm happier about the output levels and tamed Green output.
Cary