LM1881 How to detect the start of a new video frame?


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  1. #1
    Fanatical Contributor Tim's Avatar
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    Default LM1881 How to detect the start of a new video frame?

    Hi


    I'm working on a project that will use the LM1881 sync separator. One of the tasks I need to do is count the horizontal lines to know what line I'm on.
    There is a signal generated for every H scan which is great but the Vsync output is not at the start of the first H line. It's in the middle of a load of other Hsync pulses.
    I'm going to be using a Pic to do some analysis of the video signal so can obviously do some timing. Perhaps after the Vsync count the Hsync pulses to get to the first Hsync of the new frame scan. Or time it for x us then wait for the next Hsync and that will be the start of the first line etc etc.


    It's obviously not rocket science as even a 12mhz 16f84 can generate text overlays in the same place every time.


    Can anyone tell me the best method?


    Many thanks


    Tim
    Tim

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    Junior Member creatv's Avatar
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    Default Re: LM1881 How to detect the start of a new video frame?

    Hi Tim,
    remember that the composite video signal is composed of 2 fields, vertical synchronism, odd lines, vertical synchronism, even lines.
    This is 1 frame
    From pin 7 you know if the lines are odd or even.

    Manuel

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    Prolific Poster See_Mos's Avatar
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    Default Re: LM1881 How to detect the start of a new video frame?

    I haven't looked back at the data sheet but I seem to remember there is an alternate field signal available that flips at the end, or possibly start of each frame so maybe you could use that?
    My RAM is failing

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    Fanatical Contributor Tim's Avatar
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    Default Re: LM1881 How to detect the start of a new video frame?

    Hi

    I connected the board up and looked at the signals being generated Pictures below

    The Vsync generates a signal as per the Lm1881 sheet but it generates a number of them in another place as well
    The Odd even trigger seems to be better. At least it only happens every frame and in the same place

    Now we have a standard I'm back to asking my original question. How to tell where I am to start my analysis of the signal.
    In another test on the scope I set the 2 channels to record the video signal and the Hsync triggered by the Odd Even line. It can be seen from here I probably need to set in start on the rising edge of the Odd/Even line then count the Hsync pulses to get to the start point which is the actual frame where data I need to inspect starts.

    Next question is going to be on 2 level comparators but that is for another post.

    Pictures below

    Vsync and extra pulsing

    Pal wave Vsync.jpg

    Odd/Even signal
    Pal Wave Odd Even.jpg

    Odd/Even and Vsync
    Pal wave Odd even and Vsync.jpg

    Video + Hsync triggered on Odd/Even

    Pal Sync Hsync and Video Odd even trig.jpg
    Tim

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    Junior Member creatv's Avatar
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    Default Re: LM1881 How to detect the start of a new video frame?

    cvbs.jpg
    This can help you

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