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Re: genlocking digital camcorders and software genlock summery

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  • Re: genlocking digital camcorders and software genlock summery

    Dear Kevin and all:

    I posted an inquiry sometime ago asking software genlock references.
    Tomislav Pribanic sent me a JB article by Pourcelot et al.:

    Pourcelot et al. (2000). A method to synchronise cameras using the
    direct linear transformation technique. J Biomechanics 33, 1751-1754.

    This was what I was looking for. Thank you, Tomislav!

    The software genlock technique based on the DLT method is using the
    error involved in the point reconstruction. DLT method is no different
    from the regular linear multiple regression and one can compute the MSE
    involved in the least square estimation. Imagine two lines in the space
    not parallel but not crossing each other either. One can draw a third
    line crossing both lines and perpendicular to both which will show the
    shortest distance between the two lines. The least square solution of
    the reconstructed point is around the mid point of the third line. (It
    is not exactly the mid point of the line because the minimization is
    done based on the image plane coordinates, not the object space
    coordinates.) Anyway this is the situation when the cameras are not
    genlocked. The lines of sight from the cameras are not crossing each
    other due to the time error. By systematically introducing time offsets
    among the cameras, one can shorten the third line. Finding the time
    offsets that minimizes MSE is what the software genlock is all about.

    I recently applied this approach to a golf swing study and compared the
    optimized time offsets with the measured time offsets (4 Panasonic
    Proline digital camcorders). The time error was about 1/100 to 2/100 of
    the field interval (1/60 s) or 0.1 - 0.3 ms. The MSE was reduced
    substantially in this process.

    Now back to Kevin's question, I received another email from Thomas
    Seeholzer of SIMI. It sounds like the SIMI system solves this problem at
    the time of recording. SIMI computes the timeoffsets at the time of
    recording. I am not quite sure whether they used the software genlock
    method described above or something else.

    It appears to me that the software genlock method is the only solution
    for the digital camcorders at the moment. In terms of programming, all
    you need to do is to identify the markers to be used in the software
    genlock and determine the frame range. The bottom line is you have to
    use fast-moving points and phase for this purpose because fast-moving
    points will easily magnify the MSE due to time error. Compute the mean
    MSE of the markers and frames. As you introduce time offsets
    systematically, identify the minimum mean MSE conditions. That is the
    optimized time offset.

    I hope it helped.

    Young-Hoo Kwon
    - Young-Hoo Kwon, Ph.D.
    - Biomechanics Lab, Texas Woman's University

    -----Original Message-----
    From: Biomechanics and Movement Science listserver
    [mailto:BIOMCH-L@NIC.SURFNET.NL] On Behalf Of Kevin Arnold
    Sent: Monday, November 10, 2003 8:24 AM
    Subject: [BIOMCH-L] genlocking digital camcorders

    Does anyone know of a way in which to genlock digital camcorders? We're
    urgently needing to synchronise our digital camcorders (Sony
    TRV900E) but haven't been able to find any cable capable of doing so.
    Does anyone have any suggestions towards what makes and models of
    camcorders to use, and whereabouts to find the cables for connecting the


    Kevin Arnold
    Biomechanics Technician
    Department of Sport & Exercise Science
    University of Portsmouth
    St Michaels Building
    White Swan Road
    Portsmouth, PO1 2DT
    Tel: 023 9284 2028

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