Stuck RGB select signal

[stu-e]

Hi
I am using Dongle 0.1.1 and client 0.1.
I find that when I put my MediaMVP into standby, my TV screen goes black.
I am then unable to watch any TV. This is because the RGB select signal of
the SCART connector is still asserted by the MediaMVP.

Maybe the dongle should select composite mode before going to standby?

[Chris]

I am surprised to hear this. Many versions ago vomp turned off the "active" line on the scart cable to indicate there was no video signal. This proved to be incompatible with some older TVs (I forget which) so then I switched to newer Hauppauge provided drivers which slightly changed the voltage on the active line for when there was no signal. This has worked for everyone as far as I know. I just metered the RGB select pin (which isn't supposed to be anything to do with the active pin) on my system and it changes depending on whether the MVP video is on or off. Neither state matches a spec I found on the net, so I'm at a loss on this one. Besides which, the software can't switch between RGB and composite, in the MVP driver level it is a choice between RGB+composite and S-Video. If you set your MVP to S-Video and then go to standby does it correctly signal your TV?

[stu-e]

I tried setting the MVP to s-video then set it to standby. This unfortunately made no difference.
I measured the voltage on the scart pins of the MVP while it was in standby and unplugged from the TV.
Every pin was 0.0v except the blanking/RGB select pin which was 0.9v. Scanning the internet I think this pin should be less than 0.4v when not in RGB mode.
So I guess this is a problem with hardware. An unfortunate combination of out of spec inputs and outputs.
I will have a play around with this pin. Maybe a series diode or a pull-down resistor might help.

BTW Great work you are doing. I'm looking forward to being able to program recordings via the MVP. I can then maybe drop my somewhat flakey dxr3.

[davep]

I have a very old TV (1988) and found it necessary to disconnect the blanking signal (pin 18) in the SCART lead to make switching work properly.

[Chris]

Well I think the only thing I can do in software to pin 18 is switch between RGB+composite/S-video, so if switching that yourself and then going to stand-by doesn't work, I think we're out of options. Sorry. Good look with the real world electronics investigation though!

I also remember someone who had to have a SCART cable modified to make switching work and that was an old TV. It sounds like there are a couple of competing versions of this scart "standard" 

[davep]

I also remember someone who had to have a SCART cable modified to make switching work and that was an old TV. It sounds like there are a couple of competing versions of this scart "standard"

I think that was me 

According to wikipedia and others, pin 16 (did I say 18? oops...) was originally designated as a 'fast blanking' output. It is a high-speed signal which switches the TV from composite to RGB input, and was intended for teletext and on-screen subtitles - the RGB text signals would be produced by the set-top box and would overlay parts of the picture. The signal is 0 to 0.4v for 'normal' and 1 - 3v for 'RGB', however the TV is supposed to terminate the line in 50 ohms so measuring open circuit is not representative. Nowadays vomp and other set-top boxen just leave the line high or low to select which input mode to use.

[stu-e]

After some investigation pin 16 of my MVP resembles a voltage source that toggles between 3.3v and 0.95v with a source resistance of 33ohms. Even with the 75ohm load of the tv set this will never go below 0.4v which is causing my problem. I don't know if my MVP is broken or has a design fault. If anybody is interested it is a rev D3A.

I have got round the problem by modifying my scart lead at the MVP end adding a couple of resistors to form a potential divider. I can now happily get a good clear RGB signal when the MVP is on and normal tv operation when in standby.

If anyone is interested in the mod, here are the details:
I have disconnected the cable on pin 16 and in its place connected a 62ohm resistor. I then soldered a 620ohm resistor to the corresponding ground pin 18. I then soldered together the cable and the free ends of the two resistors together. When connected to the tv set pin16 at the tv end now toggles between 0.37v and 1.30v.

[davep]

With the TV connected mine ( v D3E) toggles between 2.13v and 0.79v. Looks like a design fault.