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ADV7441A Vertical line distortion with VGA input

Question asked by zmicro on Feb 6, 2017
Latest reply on Feb 7, 2017 by GuenterL

With ADV7441A analog VGA input we  find that, with 1920 x 1200 video input, vertical line disturbances occur about 2.4 cm (corresponding to around 88 pixels or so on the screen, or about 570 ns time on the video raster line) to the right of corresponding video features, and move together with those features (in fact, the very subtle disturbances can be made a bit more readily visible in this way).  This character of disturbance and magnitude of displacement appears indicative of certain digital data transitions, occurring some distance down the video data processing chain, somehow influencing the analog signals being captured and digitized at the front end of that chain.  Upon further investigation, it was found that those measured displacements match quite closely with the measured signal delay through the Analog Devices video decoder chip being used here, from the analog signal input to the neighborhood of the digital signal output.  We also found that, in some conditions, similar artifacting can be made to be observed with other video input resolutions, but with screen pixel displacements modified as expected by the scaling operation (to fill the native 1920 x 1200 pixel screen) which is carried out in the digital processing chip immediately following the decoder – although it seems the scaling operation and/or the adjustment of pixel sampling phase (“Horiz Fine”, as discussed earlier) can often tend to lessen or “mask” the symptom appearance.  This and other related testing gives confidence that the hypothesized “feedback” to the analog front end is in fact originating somewhere in the neighborhood of the decoder’s output.  However, finding the exact path of that supposed feedback / unintended coupling, and determining a way to remove or block it, can be a much more difficult prospect.