Can I use the DEC VT220 composite output to connect it to a TV?

Question

I have a DEC VT220 from 1986, it has a composite output. Works great in all its max-19200Kbps monochrome glory.

The composite output plug is a BNC, and it looks like BNC-to-RCA composite jack cables are available.

There's a few YouTube videos of VT220 output from this connected a capture card.

So ... is the output close enough to black and white NTSC that I could connect it to a TV through something that converts NTSC to RF or a TV that has a composite input?

I'm sure the 132 column mode will be horribly unreadable on an analog TV, but just wondering if it's at all possible before I waste a few bucks on the cable.

I think the effective resolution of this is 640x200 but am having trouble finding this information from old documentation.

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Update:

Yep, it worked great. Doesn't look half bad for the type of TV it is either, even though the image isn't centered.

Answer 1

This information is based on information from the DEC VT220 Technical Manual. I've not actually tried this out myself, but it did work for the OP when he tried it.

TLDR: Yes, it's likely to work, and quite cheap to try using a sub-$1 BNC to RCA adapter if you already have an RCA-RCA video cable.

Section 1.4 says that the "BNC connector for composite video output to an optional slave monitor" uses "RS170-like" output, which is the NTSC black-and-white television standard. Thus, the signal timing and format should be compatible, as far as that goes.

There are three issues that might cause problems, most likely some degradation of the display.

1. There's not general agreement on the voltage levels of RS-170; signals range from 0.7 V peak-to-peak or even less up to the 1.4 V peak-to-peak of the original standard. Given that your system is monochrome or will have only a couple of shades of grey, whatever voltage range VT-200 uses is unlikely likely to be an issue.

2. There's a warning in in that section of the manual: "the use of dc coupling is not in strict agreement with RS170 specifications." I don't think that this would be an issue either since it's only the AC signal that carries the information; input issues might also be fixed with a simple coupling capacitor.

3. As you pointed out, horizontal resolution may be an issue. As far as horizontal lines are concerned this is purely an analogue signal, so no harm can come from the TV not having enough horizontal bandwidth to display well the full horizontal resolution the monitor is using, but even 80-column text may be difficult to read. Since this will vary for each television or monitor, the only way to find out how good or bad it is is to test it with your particular TV.

Rather than buying a whole cable there are BNC to RCA adapters are available for less than a dollar from many sources. If you already own an RCA-RCA video cable, sourcing one of these is likely to be cheaper. (Note that your RCA-RCA cable must be a coaxial cable for video; using a non-coaxial audio RCA-RCA cable is likely to hugely degrade the signal quality.)