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Electronics Forum / Basics / February 2007Electronic multi-switches
I'm trying to design a video output board for my Mega STe computer which has two different video modes (low/medium resolution colour & high resolution monochrome) designed for Atari's own colour or monochrome monitors. My circuit will allow for TVs and standard VGA monitors as well. Here's my schematic (drawn with Eagle): http://img170.imageshack.us/img170/5139/vgaextraswitchesnn2.gif The normal way the computer determined its display mode is by "sensing" which monitor is connected to it via its 13 pin DIN connector. An Atari colour monitor, a TV (it has an RF output) or nothing connected at all sets it to colour mode. If an Atari monochrome monitor is inserted into the 13 pin connector it senses this and sets the computer to mono mode. It does this because Atari mono monitors have pins 4 (mono detect) and 13 (ground) connected together internally. So in order to bypass this I've created a circuit where I change modes manually with a switch (6 pole double throw(6P2T) , though a 4P2T would do if I didn't want the LEDs). By utilizing specific connectors for the specific modes I'm also ensuring that none of the monitors or TVs will go up in smoke because they're receiving sync rates they can't handle. The challenge comes to the part concerning use of Atari monitors. A mono monitor will set the computer to mono mode by itself (because of the mentioned pins 4+13 connection) which is fine, but it won't prevent any TV from being fried if already connected to the SCART (Euro-connector) or composite out. An Atari colour monitor however is worse off, because the computer has no way of sensing this, and if the switch is set to "mono" I could very well fry it. So I have the following idea: I need to have the circuit separate the Atari monitors and VGA/TV/ composite outputs. Since I haven't found any 13-pin DIN connectors with a built-in switch someone suggested that I modify a connector by adding a micro-switch to it. That way, by just inserting a 13 pin DIN plug into the PCB socket the switch status would change and could control some other circuitry. The rest of the circuitry (which I haven't figured out yet and need help with) would: a) bypass the manual switch (i.e. don't care what mode it is set on, but set the computer to whatever mode the Atari monitor in question demands) b) turn off or direct the sync rates appropriate for the rest of the outputs (VGA monitor, TV and composite output) depending on which mode the computer is set to. Any suggestions to a suitable circuit for this? > I'm trying to design a video output board for my Mega STe computer > > Any suggestions to a suitable circuit for this? Hello, Small world init' :-) I'm doing my ST right now so have everything fresh in my mind from only a few minutes ago. :-) With modern televisions and monitors you should be ok since if the sync signal is out of range then the monitor simply shuts down. In the olden days when almost everything was done in an analogue fashion this clearly was a problem. But almost every signal today is processed digitally. As long as it's not over voltage then it's not a problem. Possibly the only change to your circuit would be to make the 150R RGB resistors presets. Only reason I say that is that I'm finding here right now that 150R is a bit too much, 100R seems more appropriate. I'm making up a low/med/high for a Samsung 19" LCD television which has a VGA input (LE19R71BX). Literally all I have is a DPDT switch which connects ST-pin-4 and GND, and in the other direction supplies 12v to the SCART-pin-8 function pin. It's working fine. I just flick the switch from 31.5Khz to 15.25Khz input and the Samsung does the rest. It's just like having a multisync monitor. Only thing I have noticed is that with the VSYNC connected to both the SCART and the VGA input at the same time, it does just slightly alter the VGA picture. Maybe the other thing I've found is that an LCD doesn't quite process the ST's 15.25Khz low/med as well as what an old Philips CM8833 *analogie electronics* CRT does. I get what I'd describe as digital artifacts if you know what I mean. My suspicion is that these LCD televisions with VGA inputs are nothing more than VGA monitors with a scan doubler bolted on for the television. VGA signals seem pure and precise. Yet the 15.25Khz SCART images seem a little bit processed. Hope you liked that 16/32 :-) You came into the wild west wilderness of Usenet and found someone doing something equally retro. All the best, Aly > > I'm trying to design a video output board for my Mega STe computer > [quoted text clipped - 43 lines] > > Aly Analog video is my world and you're preparing to commit a lot of sins here. 'Y' cord connections are bad. Mis-terminated (other than 75 ohm) transmission lines (cables) are bad. Series connected level controls are bad. Wrong line terminations will cause SWR (ringing) problems along with changing signal levels. Changing R vs G vs B will shift the color balance - a _lot_. 'Y' connections introduce unintended tuned stubs into the signals which will form a trap filter. The cable lengths determines the tuning. Series level controls manage to simultaneously foul up the termination (SWR) and levels. Any wires longer than a few inches need to be 75 ohm transmission line and all transmission lines need to be properly terminated. 'Shielded' cable won't cut it. So Mr Downer, what's the answer? Use distribution amplifers with a gain of 2 to correct for the incoming and outgoing terminations. If you truly require separate RGB levels (why is beyond me), use a separate amplifier for the variable gain stage. Maxim, ADI and many others make triple video amplifiers for just this purpose. I strongly recommend dual balanced power supplies and DC coupling to avoid lots of coupling capacitors. Video is not difficult if you avoid the pitfalls I mentioned. If you think your solution is 'good enough', it won't be. GG On Feb 24, 6:24 am, stratu...@yahoo.com wrote: > Analog video is my world and you're preparing to commit a lot of sins > here. 'Y' cord connections are bad. Mis-terminated (other than 75 ohm) > transmission lines (cables) are bad. Series connected level controls > are bad. I must admit that I'm pretty ignorant when it comes to electronics beyond the practical side of it, and have almost no idea how to design a circuit myself. The schematic I've made is an adaption of several other circuits I've found, so I may very well have made some grave mistakes along the way. > If > you truly require separate RGB levels (why is beyond me), This is so that with a VGA colour monitor I can choose if I want the standard paper-white display in monochrome mode or adjust it to green, orange, red, yellow or whatever I may want. Still monochrome, but not just the plain black/white. I just got an idea that perhaps I should add a "bypass" switch so that if I want to revert back to black/white I don't need to adjust the trimmers each time. > Video is not difficult if you avoid the > pitfalls I mentioned. If you think your solution is 'good enough', it > won't be. I want the quality to be as good as possible. After all it can be seen as an investment in your eyesight if you use the computer for long periods of time. If I can achieve a sharp, vivid image I want to aim for that. Video may not be difficult for those who know what they're doing, but for me I would have no clue as how to improve on the circuit with the things you just said. If you use Eagle I can upload the schematic file so you can edit the circuit with those improvements. As for my initial question about an electronic switch which bypasses the physical 6PDT switch -do you have any thoughts on that? Hallvard > This is so that with a VGA colour monitor I can choose if I want the > standard paper-white display in monochrome mode or adjust it to green, > orange, red, yellow or whatever I may want. Still monochrome, but not > just the plain black/white. > I want the quality to be as good as possible. After all it can be seen > as an investment in your eyesight if you use the computer for long > periods of time. > If I can achieve a sharp, vivid image I want to aim for that. you get a much sharper picture from a monochrome vga display. get some coloured glasses.  SignatureBye. Jasen > > I'm trying to design a video output board for my Mega STe computer > Small world init' :-) I'm doing my ST right now so have everything fresh in > my mind from only a few minutes ago. :-) Sure is! I didn't expect that! Cool :-) > With modern televisions and monitors you should be ok since if the sync > signal is out of range then the monitor simply shuts down. > > In the olden days when almost everything was done in an analogue fashion > this clearly was a problem. But almost every signal today is processed > digitally. As long as it's not over voltage then it's not a problem. True. I want my circuit to be as "idiot proof" as possible, meaning that whatever combination of monitor/TV and switch setting I choose I can't blow anything up. The Atari colour and mono monitors can probably be regarded as "old" and care has to be taken with the sync signals, so this is why I'm designing it so that I'll be on the safe side. > Possibly the only change to your circuit would be to make the 150R RGB > resistors presets. Only reason I say that is that I'm finding here right > now that 150R is a bit too much, 100R seems more appropriate. You mean for the SCART connector? Yes, those values aren't definite, so you may very well be right. I might even have to experiment a little. > I'm making up a low/med/high for a Samsung 19" LCD television which has a > VGA input (LE19R71BX). Literally all I have is a DPDT switch which connects > ST-pin-4 and GND, and in the other direction supplies 12v to the SCART-pin-8 > function pin. It's working fine. I just flick the switch from 31.5Khz to > 15.25Khz input and the Samsung does the rest. It's just like having a > multisync monitor. That sounds like a great solution for your setup. I used to have an NEC Multisync II which I used with my ST. Now I have an SM-144 for mono mode however and it's a LOT better. I really use mono mode the most, but want to make the computer setup as flexible as possible. > Maybe the other thing I've found is that an LCD doesn't quite process the > ST's 15.25Khz low/med as well as what an old Philips CM8833 *analogie > electronics* CRT does. I get what I'd describe as digital artifacts if you > know what I mean. Probably the same sort of compromise my Multisync II gave me. The opposite was the fact there (although I've never had a dedicated Atari/ Atari compatible colour monitor to compare with) where colour was great but mono was fuzzy and unclear. Not good on the eyes. It was a great relief to exchange it for an Atari SM-125 mono monitor, then later the SM-144. > My suspicion is that these LCD televisions with VGA inputs are nothing more > than VGA monitors with a scan doubler bolted on for the television. VGA > signals seem pure and precise. Yet the 15.25Khz SCART images seem a little > bit processed. That may be true. I don't know what I'll be ending up with myself when it comes to display options, but most likely it'll be a VGA monitor of some sort, connected to the VGA (mono) output. The Mega STe will be recased (inside a 19" rack enclosure) and modified in many ways and will be more or less a dedicated MIDI computer for music purposes. Still, I might play the odd game now and then which is why I don't want to restrict it just for mono mode. > Hope you liked that 16/32 :-) You came into the wild west wilderness of > Usenet and found someone doing something equally retro. Yeah, sure did :-) Glad to see other Atari users out there. Hallvard > True. I want my circuit to be as "idiot proof" as possible, meaning > that whatever combination of monitor/TV and switch setting I choose I > can't blow anything up. > The Atari colour and mono monitors can probably be regarded as "old" > and care has to be taken with the sync signals, so this is why I'm > designing it so that I'll be on the safe side. Hi Halvard :-) Okies, progress this end.. I'm boxing mine up this weekend and using a QPDT switch to control the 8-FUNCTION, 4-MONO DETECT, VSYNC and HSYNC lines. That'll be about it really. Aside from anything that should or shouldn't be done, I'm not going into mainstream production with it or attempting to win a coveted electronics design award so that's how it'll stay :-) The ST definitely outputs weird slightly off standard signals though. I've had the Playstation running via both video and rgb and it doesn't mess up the reds, same with a Sinclair Spectrum 128k+ too. The ST's red just seems to be a bit over saturated and the composite sync is just slightly out. The lead (almost identical to yours) has now been tested on three setups; Samsung LCD 19" TV, Panasonic 32" LCD TV, and a separate Commodore 1084/VGA setup. Ahhh, the NEC multisync II. :-) Mine ended up with UV attacking the plastic so it looks horrible now. That and it's a big like looking at a fishbowl. :-) As for an electronic switch. There are the 4016 variations although they're not really suitable for video signals. There are a few other video multiplexers and other things, but by the time you've messed about getting them, paying for them etc. etc. time has passed. I wonder if one of these small little electronic KWM switches might be any good as a smart little out of the box solution... Aly :-) > The ST definitely outputs weird slightly off standard signals though. I've > had the Playstation running via both video and rgb and it doesn't mess up > the reds, same with a Sinclair Spectrum 128k+ too. The ST's red just seems > to be a bit over saturated and the composite sync is just slightly out. the Spectrum's red isn't full intensity. iirc Y=1/4 for the bright red, same as the colour bar test pattern. Bye. Jasen > "SixteenThirtytwo" <sixteen_thirty...@yahoo.com> wrote in message > Aside from anything that should or shouldn't be done, I'm not going into > mainstream production with it or attempting to win a coveted electronics > design award so that's how it'll stay :-) If it works for you you should be fine :-) > The ST definitely outputs weird slightly off standard signals though. I've > had the Playstation running via both video and rgb and it doesn't mess up > the reds, same with a Sinclair Spectrum 128k+ too. The ST's red just seems > to be a bit over saturated and the composite sync is just slightly out. Anything that can be done to improve on this? Perhaps it's just an artifact we have to live with :-) > As for an electronic switch. There are the 4016 variations although they're > not really suitable for video signals. There are a few other video > multiplexers and other things, but by the time you've messed about getting > them, paying for them etc. etc. time has passed. So these "electronic switches" are called multiplexers? I guess not all "switches" are suitable for video and you need specific ones that handle those frequencies. I've worked hard with my schematic lately and come up with some new ideas and a possible solution to the problem with the Atari monitors where the circuit wouldn't know if an Atari mono or colour monitor had been inserted as well as shutting off any accidental mono/colour switching which could fry the monitors. As part of my schematic is theoretical I could need some help finding which parts are to be used and how to connect them. When that's done I would like to improve on the actual signal and hope Stratus could help with that. OK, first, here's my latest schematic: http://img204.imageshack.us/img204/9301/vgalogicpo4.png This is how it should work: An electronic switch (U1) of some sort (a "multiplexer" IC?) works as a multiple 3 way switch. Those 3 positions are: A: when an Atari mono monitor (SM-124 etc.) is present B: when no Atari monitor is present at all C: when an Atari colour monitor (SC-1224 etc.) is present In positions A and C the manual colour/mono switch (S1) is bypassed so changing the modes will have no effect whatsoever (and not allow anyone to accidently fry the monitor(s) in use). In other words, whenever an Atari monitor is used it will work just like with an unmodified Atari ST. In position B however the manual switch (S1) will be in control. Control of the electronic switch (U1) for positions A and C is dependant on some logic (AND/OR gate ICs?), and to determine this a micro-switch is to be placed inside the 13 pin DIN (female) connector. This switch will sense whenever an Atari monitor's (male) connector is inserted. So now we have a system which "knows" when an Atari monitor (mono or colour) is present or not. Whenever the switch is off (e.g. no Atari monitor is present at all), position B is selected meaning that the manual switch (S1) is in control. Next we need to determine if that Atari monitor is a colour or mono monitor. A mono monitor as we know has pin 4 (mono detect) connected to GND (pin 13) internally, which the Atari colour monitor doesn't. I assume this will generate a logic high/low (correct me if I'm wrong) and can further be used to control the U1 logic switch circuit and set it in position A or C. Positions A and C will as mentioned above shut off the manual switch, and also give sync signals only to those connectors (TV or VGA) which correspond to the mode which the computer is currently in. Finally, there's the LEDs. If the pin 4/GND issue above works I would think that yet another logic circuit can be used, only this time for controlling two LEDs. One LED indication "colour mode" and another LED for indicating "mono mode". Any comments on this? Anyone have any practical and concrete suggestions as to how I should get this done? Hallvard Are there any better suited newsgroups, web-forums or mailing lists for video electronics related issues? I'm really digging into unknown territory and need help in figuring out my circuit. |
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