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Combining Composite Video Outputs ?

Discussion in 'Hobby Electronics' started by ALAN GILCHRIST, Apr 15, 2005.

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  1. I have 2 Composite video outputs ( DVD & Set Top Box ) but the TV has only
    one Comp. Video IN.
    Can I combine them in any way ? I dont wish to use a switch if I can get
    away with it complicated for
    other members of household - flicking a switch is fairly high tech !!!

    Only one device would be in use at any one time, but power would still be
    applied to both units.
    Can it be done by just combining the leads ? Can I expect any problems or
    even damage ?
    Can I knock up a small combiner with minimal components ?


  2. Poxy

    Poxy Guest

    Assuming you mean only one unit is operating at a time, it'll almost work,
    except each video output is designed for 75 ohm load, if you hook up 2
    outputs together, as well as the TV, then each output will see 2 x 75 ohm
    loads in parallel, or 37.5 ohms, which will overload the output and result
    in a poor picture, although the TV may be able to compensate somewhat.

    To alleviate this you could use a 33R resistor in series with each of the 2
    signals. Note that if you disconnect one of the devices, the picture will
    also go bad.
  3. Phil Allison

    Phil Allison Guest


    ** This is nonsensical - if the STB is powered all the time then HOW do
    you imagine it will NOT come through on the TV screen all the time ????

    ............ Phil
  4. In this case, the answer is most almost certainly NO.
    If the STB and DVD are powered up then they will be both be generating
    a signal be it a blank screen, menu screen, screen saver etc

    You *might* be able to get away with combining the outputs if you only
    have one device powered on at a time. But that would require you to use
    the power switches accordinly, and that is more complicated than a
    simple source switch. If they switch both on at once then you'll most
    likely get a grabled display, and then if they'll probably freak out
    that they have broken something given that a simple switch is too high
    tech for them.
    Damage, most likely not.

    Get a switch box, it's the best solution. Put big DVD & TV labels on,
    they'll soon get the hang of it. If they can't figure it out they go
    without :->
    Even if you got a new TV with multiple inputs, you'd still have to push
    a button to select different inputs, so there is no real advantage
    there either.

    Dave :)
  5. Mind control?

    I'd suggest that if flicking a switch is too high tech for them then turning
    on the TV in the first place will be well beyond their ability and hence you
    have no problem!

    Otherwise, I fail to see any solution other than a switch box, unless the OP
    wants to ensure that only one device is ever switched on at a time, even the
    'blank' screen will interfere with the picture from the other device......

    Another solution - Run the DVD via the VCR AV in to the RF input of the TV
    and then the STB to the AV inputs - problem solved!
  6. Phil Allison

    Phil Allison Guest

    "Martin, VK2UMJ"
    ** Yeah - I figured out this dude must be into prescient switching.

    ** Arrrrrhhhhhhh !!!

    Running a DVD via a VCRs RF modulator reduces the pic to VHS quality !!

    .............. Phil
  7. Brad Hogan

    Brad Hogan Guest

    You can use the RF output of the set top box . its not a bad output on a
    small screen TV.

    Its only free to air tv after all.
    there has to be a switch somewhere, whether on the tv, a manual $15 switch
    a VCR...
    Nope. thats still a switch "must switch this one on and the other one

    if they are both on, you can see one signal with the other picture
    scrolling it through it.

    And what about sound ? how do you do the same thing with sound ?

    buy a switch.
  8. KLR

    KLR Guest

    you CANNOT do this - the signals wouldnt be syncd and as a result you
    would end up with a total mess.

    The switchbox is about the only practical means that I can see to
    solve this problem, short of changing over the TV for a newer one with
    multiple inputs.
  9. Yes, I agree it isn't desirable, but if you don't want to use an AV switch
    then you are rather limited in your options...

    IMHO the simplest solutions are to either use an AV switch and maintain the
    high quality input, or sacrafice that quality and shunt one via the VCR.
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