TV -> D-Sub converter possible?

I am going to try to expalin the various types of video signal... Let's start with monochrome. We all know that in a CRT monitor the electron beam scans the screen one line at a time. The main part of the analogue monochrome signal controls the intenstiy of this be am, and thus the brightness of that point on the screen. We also need to synchronise the scanning of the electron beam in the monitor with the source of the signal, to do this it's conventiaonal to hae a pair of signals, one to mean 'start a new line' (horizontal, or 'line' sync) the other to mean 'start a new screen (vertical or 'frame' sync). If the monitor is near the video source, you van send these 3 signals over 3 seaprate wires. Or you can combine the 2 sync signals into one to make something called 'composite sync' and send that over one wire and the video infortmation over another wire. Or you can put it all into one signal, this is known as 'composite video', Of course a major use of such signals is television. You cna't send a cposite video signal directly by radio, jsut as you can't send the audio output of a microphone driectly. In both cases you have to use that signal to modulate an radio frequency carrier. The receiving set demodulates the signal, gettting back the original composite video or audio signal and then outputs that. Now in the 'home computer' era, monitors were expensive, and most TV sets did not have composite video input sockets, they just had sockets to conenct an aerial. Therefore, many home omputers inculded a device called an 'RF modulator' which is bnasically a very low power TV transmitter, It produces an RF signal and modualtes it wit hthe composite video signal fro mthe computer's video circuitry. The result can be fed to thraerial input of a TV set, as it's similalr electrically to the signal you'd get froma TV aerial. The TV set then demodulates it back to composite video and displays it. Now for colour. It is a quirk of the uman eye that you can get the visual effect of any paritcular colour by seeing the appropriate amounts of red, green, and blue light. So you could have 3 separate signals, one for each or red, green, and blue along with the synchronisation signals. This is the RGB video system (for example VGA and its derivatives). Or you coudl combine the syncs with one of the colour signals to make a composite signal, and have hte pther 2 colours separate (for soem reason, the green signal is nearly alwyas used to make the composite signal -- this is the 'sync on green#' system ).One vey obscure method is to combine one of the syncs iwth one colour and the other sync with a different colour, again ending up withe 3 signals. I think Tektronix ddi this at one point, you are not likely to come across it. However, having RGB separate is not suitable for television. One of the requiremetns for a colour television system is that is has to be compatible -- both ways -- with monochrome TV. What I mean by that is that a colour TV must display a monochrome TV signal (with no colour information in it) as a black and white picture. And a monochrom TV set -- even one designed before the colour system was introduced -- must display a colour TV signal as a good monochrome picture. FOr tht reason, the 3 colour signals have to be processed to form other signals. One of this is the luminance (or 'Y') signal, which is a weifgthed sum of the R,G, and B signals. This, if turned into composite video by adding the syncs will display correctly as a monochrome signal. The colour information can be encoded in various ways. The NTSC system has a 'colour suubcarrier' sgianl which is modulated to carry this information/. At the start of each line, a short burst of the subcarrier is sent to give the scoder circuit a phase reference. Then throughout the line, the phhase of this subcarrier determiens the hue (colour) of that part of the picture, the amplitude of the subcarrier determines hte saturation (who far the colour is from what -- for example red and pink could be the same hue, but red has a higher saturation than pink). PAL is much the same except that the phase of the burst and the subcarriar is fillped on every other line, so that phase errors in the system just cause slight desaturation of the colours rather than a change in hue (which is much more noticeable). Once you have combined the colour and luminance information, you have a composite colour video signal. This can then be modualted onto an RF carrier and transmitted. Or in the case of a home computer it can be fed to the RF modualtor circuit and set to the aerial input of a colour TV set. Of course sure ot the compatibility requirements I've mentioend, a home computer that doesn't output colour information (say a ZX81) will work with a monochorme or colour TV set (and display a black and white picture), a home computer that outputs colour information (say the CoCo) will work with a monochrome TV, you just don't get to ser the colours. S video is a conenctor carrying 2 signals. One is the lumninance information (only), the other is the colour information encoded using NTSC or PAL or whatever. The recent-ish system of 'component video' is 3 signals. I beleive (please correct me), that despite the colours of the plugsand sockets uses, these are not separate RGB signals. They are the luminance signal and 2 'colour difference' signals (I suspect R-Y and B-Y). SO far I've said nothign aobtu the scan rates. Home computes, designed to work with TV sets, wll use the same scan atss as the applicable TV ssytem. The horixzontal scan is around 15kHz, the vertical scan is very close ot the mains frequency. Other machiens, particularly workstations can have just about any scan rate immaginable. Interconmverting between composite video, RF-modulated video, adding/decoding the colour, etc are relatively simple compared to converting the scan rate (which often invovles storing the image in a memory deivce nad reading it out at a different rate). For that reason, if toy have to convert a monitor to use with a strange machine, pick one with the same scan rates and encode/decode the video signal appropriately. In geneeral, it is easier to modifiy a monitor to handle a differnt vertical scan rate htan a differnet horizontal scan rate. The European and US TV hjorizotnal san rates are close enough that most monitors that will work with one will wrok with the other, and often a tweak of the vertical hold control will get the vertical to lock as well. If not, it's a fairly simple modification if you have the schematic. A few practical hints... Most home computes have an RF modualtoe module inside. This is a metal box on the PCB, the cable to the TV aerial socket plugs into it. There are 3, 4 or 5 conenctions to this unit, including the metal case (which is systme ground). The other connections are a poer input (often 5V or 12V), compostie video in, audio in (if the =comptue sends sound otthe TV) and maybe a tuning or channel select input. The compostie signal will often tive a composite monitor directly, sometimes you need a simple transistor buffer circuit. I would recoment bypassign the modualtor (and thus the demodualtor circuit in the TV set) if your display device has composite inputs, you get better quality and there's less to go wrong. If you want to strt makniong your own PAL or NTSC to RGB decoders, I would look at hte Philips chips from the 1990s. Philips published data sheets on them (which is a good start...) and you can use the scheamtics of their TVs as an 'application circuit'. Be warned that settign up a colour decoder is not that easy. -tony