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I have a small device (based on an AVR microprocessor) that outputs a Black & White composite PAL video signal, designed to be plugged into a TV. I also have an old Amstrad CTM644 monitor and want to know if the two can be connected together.

The Amstrad monitor has a six pin DIN input with the following pins

Pin No. Signal Name Description
1           R           Red analogue signal
2           G           Green analogue signal
3           B           Blue analogue signal
4           Sync        Composite Sync
5           GND         Common
6           LUM         Luminance

Since the image produced by the device is only black and white is it possible to build a very simple circuit to separate the luminance and sync, or is it more complex than this?

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  • \$\begingroup\$ You may be able to feed the compsite vide signal into the sync and have it still separate the sync. It depends on whether the sync input is TTL level or intended for a composite voltage levels. If the latter it might work. Try it and see. \$\endgroup\$ – Olin Lathrop Jan 2 '12 at 14:01
  • \$\begingroup\$ I tried that and saw a rather green looking diagonally rolling image. \$\endgroup\$ – Piku Jan 2 '12 at 14:24
  • \$\begingroup\$ OK, that won't work then. I have seen it work on some monitors, so I thought it worth a try. Your sync input possibly wants TTL levels then. \$\endgroup\$ – Olin Lathrop Jan 2 '12 at 14:27
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I remember that there there was (I don't know if they still make it) a cheap and specific IC to do this: LM1881.

http://www.national.com/mpf/LM/LM1881.html#Overview

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    \$\begingroup\$ It's still available at Digi-Key, and in a DIP package too. \$\endgroup\$ – tcrosley Jan 3 '12 at 15:00
  • \$\begingroup\$ I found this circuit online gamesx.com/misctech/lm1881.htm if I built that, I'd just need to use the composite sync output and connect the original composite video signal to the luminance pin, right? \$\endgroup\$ – Piku Jan 3 '12 at 18:09
  • \$\begingroup\$ If the luminance is a composite input, yes. Otherwise you might connect it to the green pin. I don't know at all the specific CTM644 connections. \$\endgroup\$ – Axeman Jan 4 '12 at 16:17
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Some devices that output composite video have capacitively-coupled outputs, but many have resistively-couple outputs; nearly all composite-video inputs are capacitively coupled. If your device has a resistively-coupled output, you're in luck; all you'll have to do to detect sync is to use a comparator to detect whether the signal is above or below a certain DC threshold (sync will probably be at ground potential, and the lowest non-sync signal will be well above 0.1 volts, so 0.1 volts may be a good threshold).

Note that in addition to detecting sync, it may be necessary to also perform DC level restoration; some monitors may perform it automatically. On a resistively-coupled output, black will probably be represented by a DC level of 0.3 volts. Some monitors may expect a black signal to have a DC level of 0.0 volts, or they may expect to sample the signal level shortly after each sync pulse and regard that as black. Depending upon the design of the monitor, you may get acceptable results feeding in your video signal as the signal level, or you may find that black becomes an annoyingly light gray.

One final note: composite video typically uses 75 ohm DC termination. Many devices which output composite video generate a signal whose voltage is twice normal, but then feed it through a 75 ohm resistor, and many devices which input composite video have a 75 ohm resistor to ground. The analog inputs to your monitor may or may not include such termination resistors internally; connecting them may thus affect the signal level.

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