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Video synch problem

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  • bolandar
    Hello everyone, I have a question about the ELF IIs video output. I ve posted a link to a photo of the ELF video output that is distorted horizontally. My
    Message 1 of 3 , Oct 1, 2005
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      Hello everyone,

      I have a question about the ELF IIs video output. I've posted a link
      to a photo of the ELF video output that is distorted horizontally. My
      ELF has this problem. The lower 2/3 of the display does synch, but
      the upper part jumps around. It also takes a monitor with alot of
      adjustment to stabilize the image to this point. Does anyone have an
      idea what is causing this, and what the ultimate fix is? Thanks in
      advance.

      Bolandar

      http://i4.ebayimg.com/02/i/05/0e/7c/3d_1.JPG?set_id=7
    • sbirdasn
      ... link ... My ... an ... Two guesses (in random order): A -- Missing a NOP ($C4) instruction in your video interrupt routine? You must have an odd number of
      Message 2 of 3 , Oct 1, 2005
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        --- In cosmacelf@yahoogroups.com, "bolandar" <bolandar@y...> wrote:
        > Hello everyone,
        >
        > I have a question about the ELF IIs video output. I've posted a
        link
        > to a photo of the ELF video output that is distorted horizontally.
        My
        > ELF has this problem. The lower 2/3 of the display does synch, but
        > the upper part jumps around. It also takes a monitor with alot of
        > adjustment to stabilize the image to this point. Does anyone have
        an
        > idea what is causing this, and what the ultimate fix is? Thanks in
        > advance.
        >
        > Bolandar
        >
        > http://i4.ebayimg.com/02/i/05/0e/7c/3d_1.JPG?set_id=7

        Two guesses (in random order):

        A -- Missing a NOP ($C4) instruction in your video interrupt routine?

        You must have an odd number of instruction cycles inside the interrupt
        routine to compensate for the interrupt cycle which transfers control
        to the interrupt routine.

        It also has to happen in the first 29 cycles of the interrupt routine,
        with exactly 6 cycles in between DMA bursts to maintain
        synchronization.

        From the looks of the picture, it looses sync right about the time
        that the DMA starts.

        B -- The other possibility is that your video signal amplitude is too
        low for the monitor to properly sync up. If your monitor is terminated
        into 75 Ohms, then the signal will be quite squashed by the
        termination (you need a buffer amplifier to drive 75 Ohm loads with
        anything close to proper video amplitude). If the signal is borderline
        in amplitude, then the monitor's AC coupling/DC restoration circuitry
        often looses clamping on the signal when a large amplitude jump occurs
        like when the first active pixels of the frame begin (at least half of
        every frame is black level at all times).

        Try un-terminating the monitor if you have the option. Check by
        measuring the DC resistance of the video input and see if it is
        terminated (measure with the power off to avoid doing *bad* things to
        the input amplifiers).

        If termination is the problem, then try this simple emitter-follower
        transistor amplifier to drive the monitor:

        2N2222 or 2N3904, resistor summing node (video) -> base, collector ->
        +5V, emitter has 75 Ohm resistor to ground, video signal coax center
        conductor connected to emitter, shield to ground.

        This is very quick and dirty, so be careful and double check
        everything!


        sbirdasn.
      • bolandar
        ... horizontally. ... but ... of ... have ... in ... routine? ... interrupt ... control ... routine, ... too ... terminated ... borderline ... circuitry ...
        Message 3 of 3 , Oct 2, 2005
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          --- In cosmacelf@yahoogroups.com, "sbirdasn" <sbirdasn@y...> wrote:
          > --- In cosmacelf@yahoogroups.com, "bolandar" <bolandar@y...> wrote:
          > > Hello everyone,
          > >
          > > I have a question about the ELF IIs video output. I've posted a
          > link
          > > to a photo of the ELF video output that is distorted
          horizontally.
          > My
          > > ELF has this problem. The lower 2/3 of the display does synch,
          but
          > > the upper part jumps around. It also takes a monitor with alot
          of
          > > adjustment to stabilize the image to this point. Does anyone
          have
          > an
          > > idea what is causing this, and what the ultimate fix is? Thanks
          in
          > > advance.
          > >
          > > Bolandar
          > >
          > > http://i4.ebayimg.com/02/i/05/0e/7c/3d_1.JPG?set_id=7
          >
          > Two guesses (in random order):
          >
          > A -- Missing a NOP ($C4) instruction in your video interrupt
          routine?
          >
          > You must have an odd number of instruction cycles inside the
          interrupt
          > routine to compensate for the interrupt cycle which transfers
          control
          > to the interrupt routine.
          >
          > It also has to happen in the first 29 cycles of the interrupt
          routine,
          > with exactly 6 cycles in between DMA bursts to maintain
          > synchronization.
          >
          > From the looks of the picture, it looses sync right about the time
          > that the DMA starts.
          >
          > B -- The other possibility is that your video signal amplitude is
          too
          > low for the monitor to properly sync up. If your monitor is
          terminated
          > into 75 Ohms, then the signal will be quite squashed by the
          > termination (you need a buffer amplifier to drive 75 Ohm loads with
          > anything close to proper video amplitude). If the signal is
          borderline
          > in amplitude, then the monitor's AC coupling/DC restoration
          circuitry
          > often looses clamping on the signal when a large amplitude jump
          occurs
          > like when the first active pixels of the frame begin (at least
          half of
          > every frame is black level at all times).
          >
          > Try un-terminating the monitor if you have the option. Check by
          > measuring the DC resistance of the video input and see if it is
          > terminated (measure with the power off to avoid doing *bad* things
          to
          > the input amplifiers).
          >
          > If termination is the problem, then try this simple emitter-
          follower
          > transistor amplifier to drive the monitor:
          >
          > 2N2222 or 2N3904, resistor summing node (video) -> base,
          collector ->
          > +5V, emitter has 75 Ohm resistor to ground, video signal coax
          center
          > conductor connected to emitter, shield to ground.
          >
          > This is very quick and dirty, so be careful and double check
          > everything!
          >
          >
          > sbirdasn.
          Thanks sbirdasn,

          I think I had eliminated #1 already, but I was looking for an answer
          like your second suggestion which I plan to try today. I think you
          are right that it is a signal amplitude problem based on a little
          experiment I tried with a new monitor. I get an even worse result
          on the new monitor, and the image that does show up is very faint.

          I appreciate the help, and I'll let you know how it goes,

          Bolandar
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