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Passive Component Video Switch Design Q

Discussion in 'Electronic Design' started by fourrings, Apr 4, 2007.

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  1. fourrings

    fourrings Guest

    The combination of passion towards DIY and inability to find a
    comparable quality solution, leads me to this question. I want to add
    additional component inputs to my front projector. Active switches
    introduce noise and are unable to handle the bandwidth. Off-the-shelf
    passive ones lack quality. The slightest artifacts are quite apparent
    on a 124" screen.

    My solution is to take a three-pole rotary switch. Tie RGB ground
    together at one point and use the switch to switch the signal leads
    individually. The passive composite switch I have seems to have some
    grounding through resistors. I'm aiming to achieve the same result as
    if physically swapping cables. I also plan to incorporate an analog
    audio switch in the same enclosure. My concerns are:

    1. Is tying grounds together acceptable?
    2. Does the switch need to be break-before-make?
    3. Do I need to introduce any resistors, etc?
    4. Does 1-3 apply to audio switching as well?
    5. Thoughts on result?

    Any input would be greatly appreciated!
  2. you are standing too close
    Generally all analogue video is 75 ohm impedance, you should use good
    quality 75R coax to and from the switch, you could use coaxial or RF
    relays for switching, they go up to GHz , which would be better than
    any crap rotary switch from RS

    Read up on video signal termination

    I have my doubts about your statement

  3. fourrings

    fourrings Guest

    Thanks for advice. I may look into a solution with relays as you
    suggested. At the moment though, I was looking to build a non-powered
  4. jasen

    jasen Guest

    that'll probably work ok especially if the RGB wires to the projector are
    tightly bundled,
    no need. video equipment can handle low-level signals on its outputs, and/or
    shorts, opens etc...
    probably not.
    a resistor from wiper to ground may redude switching noise.
    put shielding between audio and video parts. banding during loud noise and
    buzz during bright scenes doesn't look good.

  5. joseph2k

    joseph2k Guest

    Professional studios use active switches all the time, neither you nor i am
    seeing (or hearing) artifacts from their systems. DIY is fine, just don't
    expect quality results, from junk parts.
    If they are quite apparent on 124 inches they will also be clrearly visible
    on 60 inches.
    Are you trying to ruin your projector?
    The video grounds maybe, tying the audio grounds to the video grounds is
    pure foolhardiness.
    Do you like your equipment to survive?
    terminations, buffers and associated ancilliary equipment will help prolong
    the life of the other equipment.
    same answer
    As previously stated, good parts / equipment does not have the problems you
    are bothered by. Expect to research, learn, pay, accordingly.
    Check out (google) video matrix switches, both relay and electronic.
    I am adding to and reinforcing Martin's information.
  6. Cheap passive crap comes from China. Absolutely no attempt is made
    to control the mandatory 75 ohm impedance from one end of the product,
    to the other. They use cheap connectors, poor layout, both mechanical
    and electrical. Professional video uses a 75 Ohm BNC connector. There
    is no stated impedance for the RCA (Phono) connectors, because it was
    designed as a cheap and easy way to connect a crystal or other type of
    phono cartridge to a table radio where the impedance was 50K ohms or
    higher. Variations in the impedance had very little effect in that
    application. Any change in impedance shows up as lines on the screen
    where the brightness or color changes.

    Wrong. The coax shield should reach within 1/4" of the terminal to
    maintain impedance. "Tightly bundling" affects the impedance and causes
    impedance bumps. You've NEVER designed any video switching gear. Even
    the engineers who designed the early mechanically operated video
    switches in the '50s knew that. When the Japanese copied them, theirs
    didn't work as well because they were careless in controlling the
    impedance in their designs. Six inches of hookup wire to a connector,
    and buss wire between switches just doesn't cut it.

    Video grounds need to be bonded in one set, audio in another.

    Wrong. Any video switching is break before make. With composite
    sync it takes time to settle to the new sync, unless the sources are ALL
    genlocked. The more crap that hits the sync separator, the longer it
    takes to lock to the new source.

    NO. that will introduce losses. Only use and EXTERNAL Terminator for
    unused inputs, if you build more than you need at the moment.

    Very vague and useless reply. Any switching noise would be from the
    contacts. If there are "thumps" when switching, proper split supply
    buffer amps are needed to keep a constant load on the source while
    removing any DC offset.

    Proper construction reduces the need for additional shielding, and if
    it is required, it is usually in the form of a metal plate separating
    wafers. A cheap three pole three way rotary switch may all be on one
    wafer, and there is nothing that can be done to provide any more
    isolation. Good rotary switches are made ot of Ceramic. Centralab made
    excellent, commercial grade rotary switches. They were bought out by
    Electroswitch, and their switches are still available. Once you price a
    good quality switch, you may change your mind about using a rotary
    switch. <>

    Maxim makes the ICs used for broadcast video switching, with
    performance you won't believe. With proper PC board layout you won't
    even know they are there: <>

    As far as small 75 ohm coax, a cable from a dead SVGA monitor should
    supply enough 75 Ohm coax, and is rated to 350 MHz use, or higher. The
    video coax has Red, Blue, and Green jackets.

    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
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