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Thread: CMVS asynchronous color burst with AD725

  1. #1
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    CMVS asynchronous color burst with AD725

    This thread is in response to this thread here, but rather than bumping a 6 year old thread I figured it would be better to just start a new one.

    http://www.neo-geo.com/forums/showth...D725-Dot-Crawl

    I've experienced the same issues of dot-crawl as discussed in the linked thread, in addition to dot crawl I had horribly inaccurate colors.

    vlcsnap-2018-07-22-21h22m16s936.png

    This image was taken from a cheapo s-video to hdmi converter. When I hooked it to my framemeister, the colors didn't appear as far off, just over saturated. For the framemeister if I just lowered the saturation on my tv the image would be fine, however this fix was ONLY for the framemeister. The colors were far more inaccurate on everything else i tested it on, and the dotcrawl was persistent via composite, not s-video, even on the framemeister. I'm guessing that these issues are either less severe or do not exist on a CRT tv, however I have not owned a CRT for several years now so I am unable to test this. I've tried changing the resistor values on the outputs as well as the cap values, I've also tried different resistor values on the inputs, but none of it has made any difference. I recently ordered a Dual Frequency Oscillator in order to try and correct the slightly offspec video signal the MVS produces, as detailed here. The DFO hasn't arrived yet but after digging into the linked thread I'm not sure that it will fix anything. I'm convinced at this point that the asynchronous color burst must be the cause. I wonder if there is any way to fix this. I'm not an engineer nor do I have any sort of training in this kind of stuff, but from what I understand the AES is able to correct this using a phase-locked loop, as detailed here. I don't fully understand the concept of a Phase-Locked Loop. I've been trying to look up info about it online to see if it is a viable solution to this issue with the asynchronous color burst between the MVS and AD725, but as I am not an engineer and this is not my field I have trouble understanding it. Is it possible to fix this issue of the asynchronous color burst by creating some sort of phase-locked loop? I have found one instance of someone trying to use an MC44144 with the AD724, which is almost the same as the AD725, in order to correct the color burst issue. Upon looking up the MC44144 it appears to be some sort of PLL used for video, discontinued but still fairly easy to find on ebay. Could this be used for a possible solution?

    Am I completely off here as to what the cause of this is? I've tried building two separate ad725 encoder boards based off the the benheck schematic, and have attempted to use them with two different MV1A's, and in all cases the s-video and composite video had turquoise colored greens, red tinted whites, and purplish blues. I try messing with color settings on my tv but it never displays the correct colors.
    Last edited by someitalian123; 10-06-2019 at 11:27 PM.

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  3. #3
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    Quote Originally Posted by Xian Xi View Post
    Your red levels are either too high or green is too low.
    That is not the case, if I hook it up via rgb scart the colors are balanced, it's only when they are encoded via the ad725 that they aren't. As mentioned above the colors are closer to correct when the ad725's s-video is hooked up to a framemeister, but not when I hook s-video or composite up to anything else.

    I've tried adjusting the colors via pots multiple times, I got rid of the pots when found that it made no difference. I think something might be fundamentally wrong with how the s-video/composite is encoded via the ad725.

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    OK so after discovering this

    https://www.eevblog.com/forum/chat/a...90/#msg2724690

    I tried moving the Oscillator closer to the ad725 chip to make the lead connecting it to pin 3 as short as possible and well...

    vlcsnap-2019-11-09-21h20m44s787.png

    Granted this was tested with an mv1fz board, not a mv1a like I used when I originally created this thread. So I can't say for sure if the issue is 100% resolved. However, I am not expecting any sort of major difference between the mv1a and the mv1fz.

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