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    #21
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    Quote Originally Posted by g.l View Post
    What kind of switches would you recommend crunchy?
    Certainly electronic ones. Classical (e.g. SPDT) switches can cause some randomness. Anyway, I will probably make electronic switches by myself.


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    #22
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    Quote Originally Posted by crunchy View Post
    My idea is the same, except that I would try to avoid plain switches.
    Agreed. There is a risk of mechanical bounce but as the power switch is single throw, it is unlikely to affect sync.

    What would you recommend ? An SSR might induce noise.


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    #23
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    Quote Originally Posted by David Cole View Post
    Agreed. There is a risk of mechanical bounce but as the power switch is single throw, it is unlikely to affect sync.

    What would you recommend ? An SSR might induce noise.
    Mechanical bounce can, by itself, introduce some missynch due to slight differences in inner electronics, although it should not be large. On the other side, have you measured (on oscilloscope) the difference between the power signals on both cameras (when you power them up?)? I'd also be interested in the measurements.

    What level of noise would be induced by the SSR? Have you tried them yet?


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    #24
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    Quote Originally Posted by crunchy View Post
    ...How do you define your success rate?
    We are using the free Camera Sync Tester application from 3DTV.at (http://www.3dtv.at/Knowhow/Synctest_en.aspx).

    We simply run the application on a PC with a CRT display and point the camera rig at it. To analyze, we bring each file up on a separate display in Quicktime (which offers a frame accurate counter) and look at the same frame, in the left and right camera files. The following conditions are observed:

    1 - perfect sync - the scan line of the CRT illuminates the same position on the recorded chart with both cameras.



    2 - frame slip - the cameras are in sync, as per above, but the record began later in one camera or the other. This is not a problem, you can easily fix in post.

    3 - bad sync - the scan line is in very different positions on the recorded charts.



    By the way, to lay down a sync check in the field, you can use a diffused photo strobe set for fast exposure. Just fire the strobe at the cameras (make sure that the flash is diffused or if could damage the image sensor). You can count the scanlines in post to see if the flash was recorded at exactly the same position in both cameras.


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    #25
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    Quote Originally Posted by crunchy View Post
    ...have you measured (on oscilloscope) the difference between the power signals on both cameras...
    Yes. We've measured the voltage increase that occurs on the remote line (pins 3&4) when the cameras power-up. (Presumably, the GH1s are reading the remote resistance values with this)

    This DSO image shows the power-up event from both cameras (at 2ms / DIV)



    The events seem to occur simultaneously every time (at least as far as this 150MHz DSO can resolve).

    We've moved on to measuring the pull-up of the data line on the GH1's USB output. I'll post those results next.


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    #26
    Senior Member g.l's Avatar
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    Quote Originally Posted by David Cole View Post
    Yes. We've measured the voltage increase that occurs on the remote line (pins 3&4) when the cameras power-up. (Presumably, the GH1s are reading the remote resistance values with this)
    Did you try without memory card installed?


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    #27
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    Quote Originally Posted by g.l View Post
    Did you try without memory card installed?
    I'm not sure how we would measure success or failure w/o the cards installed to record the sync test. We haven't found any other reliable measure of sync (yet). We're game to test anything, however. How would you suggest we conduct the test?


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    #28
    Senior Member g.l's Avatar
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    The idea is to boot up first, then insert the cards, in case the card access during boot time is causing the unpredictable sync shift.

    It's conceivable that a global frame capture timer is initialised only after the cards are accessed for the 1st time, so this access might cause the random delay. Worth trying.


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    #29
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    OK... We'll give it a whirl. Perhaps we'll open a testing WIKI at beampath3d.com to track tests and results. There are a LOT of variables to test against sync and it would be good to organize the test results.

    Also, narrowing in on an electronic measure that correlates with sync (e.g. some wiggle of the USB data line) will probably take some heavy testing as well.


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    #30
    Senior Member g.l's Avatar
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    It would also be interesting if you could compare the sync in live-view before the take, with the sync of the take itself. That would tell us if the capture timer is global, or reset for every take.

    Does shooting the CRT give you a steady-ish bar that you can compare visually on both cams prior to the take?


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