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Video SYNC'ing

Discussion in 'General Electronics' started by Quack, Sep 25, 2003.

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  1. Quack

    Quack Guest


    I am thinking about ways to make a video switch that would switch more
    cleanly (than just switching at random points within the signal).

    After searching around, it appears to be quite a complex thing to

    The main method i have seen is to keep some kind of buffer of each
    incoming signal to be used as slack to have each one sync'ed together,
    so that when the output is switched, it doesnt break up the signal.

    In my application, 4 inputs, 1 output - i dont see the need to have
    each stream synch'ed together, why not instead switch 'just before' a
    sync, would that not work ?.

    for example,

    if you had a PIC chip (or other faster processor) receiving commands
    to switch, lets say its asked to switch to input 4.
    It would then set itself up to monitor input 4, and wait for a SYNC,
    once it gets a sync, it would then start counting, and just BEFORE a
    new sync occurs (time based calculation), it would switch input 4 ON
    (and the original input, whatever it WAS, OFF) - this would allow
    input 4's SYNC to be the first thing OUT the output once switched
    (**even though the last frame may not have been complete**).

    Would this 'incomplete frame' of the previous signal cause much
    disturbance in the process of video capture ?.
    It seems logical that it would be less of a problem than getting half
    of one frame, then possibly another half (or more than) another frame
    etc due to random switching times.

    Would the improvement be noticable/worth the effort ?

    Currently the technique used is to switch at the time the command is
    received and understood (whenever that may be in the signal). This
    creates a SYNC problem in the video capture process, and the first few
    frames after a switch are usually white/black/all over the place. As
    you would expect.
    To get around this, we insert a pause after a switch command in the
    encoding process, so that we dont include these bad frames in the
    end-encoded stream.

    Using this 'switch just before sync' technique, would it atleat make
    this pause time less ?

    Perhaps im way off here, any opinions ? :)

    After all that - i think my question boils down to something simpler;
    whats better - a video signal with a frame thats too long (greater
    than 625 scan lines) or a video signal thats too short (less than 625
    scan lines), in between switching.


    PS: this is only in regards to PAL signals.
  2. You're way off but at least giving it serious thought. The destination
    devices fall into 3 basic groups as I see it. The simplest is a CRT
    monitor. Causing a non-synchronous switch with your over or under
    length frames will cause the monitor to roll vertically and
    horizontally. The H rate from disturbing the H PLL in the monitor anf
    the V rate from having vertical start at the wrong time. The next
    group would be the VTR with the rotating drum. The drum is locked to
    the vertical sync and disturbing that timing will take quite a while
    to relock. The last group would be the computer type devices that
    actually count lines. Non-sync switches here will result in PLLs
    relocking, memory addressing out of range, etc. There is a reason for
    genlocking and frame synchronizers to deal with this. Conceptually it
    isn't too difficult but it does require a chunk of hardware to make it
    work. TV is cool, huh? BTW, PAL vs NTSC vs SECAM vs anything else all
    has similar issues.

    Starting and stopping counters looks good on paper but would actually
    be pretty tricky to get everything exactly on the right pixel.
  3. Quack

    Quack Guest

    Starting and stopping counters looks good on paper but would actually
    Oh well, i'll just stick with the pause. its the simplest solution.
    This device is not worth the effort/expense of getting everything in-sync.

    Thanks :)
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