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Electronic multi-switches

Discussion in 'Electronic Basics' started by SixteenThirtytwo, Feb 23, 2007.

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  1. 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):

    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
    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?
  2. Aly

    Aly Guest


    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

    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,

  3. Guest

    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.

  4. Sure is!
    I didn't expect that! Cool :)

    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.

    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.

    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.

    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.

    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.

    Yeah, sure did :)
    Glad to see other Atari users out there.

  5. 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.

    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.

    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?

  6. jasen

    jasen Guest

    you get a much sharper picture from a monochrome vga display.
    get some coloured glasses.
  7. Aly

    Aly Guest

    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

    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 :)
  8. jasen

    jasen Guest

    the Spectrum's red isn't full intensity. iirc Y=1/4 for the bright red,
    same as the colour bar test pattern.

  9. If it works for you you should be fine :)
    Anything that can be done to improve on this?
    Perhaps it's just an artifact we have to live with :)

    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:

    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

    Next we need to determine if that Atari monitor is a colour or mono
    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

    Any comments on this? Anyone have any practical and concrete
    suggestions as to how I should get this done?

  10. 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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