Composite output to modern monitor

Hardware questions and modifications

Composite output to modern monitor

Postby KenG » Tue Jan 03, 2017 2:54 pm

I was given a flat screen TV that has composite (AV) ports, so I thought I would try it as a monitor for the PCjr. When it first came up, it looked like it was going to work, but then it would show a stabile display for a few seconds, the letters would blur and become mulicolored, the screen would blank for a second, and then come back on for a few seconds before ging throught the sequence again. I plan on trying it with another jr, but I was wondering if anyone would know what is causing it. My guess is that it is missing a sync signal sometimes and has to reset. Then it works until it loses sysn again. When working, it seemed to give a fairly sharp image.

Or is there a difference between older composite and AV?
Enhanced PCjr with a jr-IDE (1GB DOM) and a parallel port side car with a compact flash reader and backpack 1.44mb floppy attached. Tandy video mod.
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Re: Composite output to modern monitor

Postby Trixter » Thu Jan 05, 2017 12:11 am

The composite color signal output by a PCjr (and also by CGA cards) is not quite 100% NTSC broadcast spec. For example, the refresh rate is slightly different (59.92Hz vs. NTSC's 59.94 Hz). That, and other very slight differences, can be enough to confuse the NTSC decoders in LCD monitors and TVs. That said, I've connected my PCjr to my 55" 2010 Vizio LCD TV without any issues, and also my HP LP2475w monitor from 2008, so if you have access to any other TVs or monitors, try those to see if they work. (If you can find one, a glass tube CRT/TV will always work fine.)

If multiple monitors and TVs all show the same thing, then it is possible you have one or more capacitors going bad on the PCjr motherboard, which would affect the video signal being output through the composite port. Replacing those may fix that problem.

My gut feeling is that your specific display couldn't handle the PCjr output, and I'll bet a different one will work fine. If all you have is a single display to use with your PCjr, you can try the workaround of turning off the color signal. You can do this by booting the system and then typing "MODE BW40" or "MODE BW80" with a DOS disk in the drive. This will disable color at the DOS prompt and in text modes, which isn't ideal but might stabilize the picture (if the color burst is what is confusing your display device).
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Re: Composite output to modern monitor

Postby KenG » Thu Jan 05, 2017 4:14 am

Thanks for the answer. I tried it on a crt TV last night. It was a little better. It never totally lost the picture, but colors would bloom every few seconds. The funny thing is, the color screen during POST seems fine. It starts when the BASIC screen comes up. I plan to try it with a different Jr. I have my main Jr output going to a VGA LCD. This was just an attempt to use a free LCD TV I was given.
Enhanced PCjr with a jr-IDE (1GB DOM) and a parallel port side car with a compact flash reader and backpack 1.44mb floppy attached. Tandy video mod.
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Re: Composite output to modern monitor

Postby Meknapp » Thu Jan 05, 2017 9:02 pm

Right now I'm playing with the Composite video output to an LCD monitor and I'm seeing very similar results to you. I also find it interesting that the IBM color bar BIOS screen seems fine, but when you boot DOS it gets wonky.

Going MODE BW40 looks pretty good on it.
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Re: Composite output to modern monitor

Postby KenG » Thu Jan 05, 2017 9:19 pm

Ok, glad it isn't just me. I was hoping to get one output to an LCD without the expense of building another RGBI to VGA converter. I know it wouldn't be good for 80 column text, but I thought maybe for games. I have a couple other JRs to try. If one of them works, I may try replacing capacitors in the video circuit. These things are old and capacitors go bad with age. The periodic nature of it may indicate a capacitor leaking at a certain voltage level.
Enhanced PCjr with a jr-IDE (1GB DOM) and a parallel port side car with a compact flash reader and backpack 1.44mb floppy attached. Tandy video mod.
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Re: Composite output to modern monitor

Postby Trixter » Fri Jan 06, 2017 10:33 pm

If the startup screen and 40-column mode works fine, but the 80-column text mode does not, that's because 80-column has a different front/back porch, sync pulse, and other things I'm not explaining very well at the moment. When we were making 8088 MPH, we went through a ton of subtle tweaks to try to get a stable picture, so I'm not too surprised you guys are seeing this. 80-column mode was never meant to be used via composite out.

The good news is, anything you are seeing will go away as soon as you start a 160x200 or 320x200 game, as they used the same basic signals and setup as 40-column mode.
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Re: Composite output to modern monitor

Postby KenG » Sun Jan 08, 2017 1:03 pm

I tried this on my old Commodore 1902a monitor. The good news is that the monitor works after almost 14 years in my shed. In separate mode(so there is no color), it gives a good, stabile, monochrom image. As soon as I switch the monitor to composite, the color blooming starts. I guess this means the problem is with the color burst?
Enhanced PCjr with a jr-IDE (1GB DOM) and a parallel port side car with a compact flash reader and backpack 1.44mb floppy attached. Tandy video mod.
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Re: Composite output to modern monitor

Postby KenG » Sun Jan 08, 2017 3:00 pm

I booted to DOS and did some more testing. Modes bw40 and bw80 look OK. I was surprised how good 80 columns looked. 40 and 80 column color modes have color artifacts, but were stabile. I think cartridge BASIC is using a graphics mode that composite doesn't like. I will try with other machines when I get a chance.
Enhanced PCjr with a jr-IDE (1GB DOM) and a parallel port side car with a compact flash reader and backpack 1.44mb floppy attached. Tandy video mod.
KenG
 
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Re: Composite output to modern monitor

Postby KenG » Thu Jan 12, 2017 8:06 pm

I was messing around with my 1902a monitor tonight. I got RGBI mode to work, so I put a memory side car on to boot a cartridge game and test the sound, which works. Later I wanted to try the games in composite mode, but forgot I had removed the disk. It booted to a perfect cartridge basic screen. No color bleed and cutting out. I removed memory and the problems returned. Reinstall memory and OK again. I can't figure out why doing a memory sidecar would have this affect. Any ideas?

Note: Now there is no picture in composite mode on a modern LCD TV.
Enhanced PCjr with a jr-IDE (1GB DOM) and a parallel port side car with a compact flash reader and backpack 1.44mb floppy attached. Tandy video mod.
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