Using ADV7184 SCART Overlay with ADV7341, isolated pixels are faint

Hi,

I am using the ADV7184 back to back with an ADV7341, to add an RGB overlay over CVBS. I find that when pixels are isolated, with no adjacent pixels in the same line, they are considerably dimmer. This means that vertical lines appear dark and text is of inconsistent brightness.

I thought this may be the the encoder attempting to filter noise, so I experimented with various DNR gain levels and filters but I did not notice a difference.

Here is an example, note how the vertical and diagonal lines in the image are much darker than the horizontal lines.

Vertical and horizontal lines

Is there some way to avoid or mitigate this effect, and have all overlay pixels of the same brightness?

Thanks!

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  • Hi Poornima,

    Just wondering if you can clarify whereabouts in the image below the chroma shaping filter is applied?

    Fast Blanking Configuration

    Based on my reading of the datasheet I think this may be before the YPrPb subpixel blender. If this is the case then the CSH configuration will only affect the CVBS signal and not the RGB which is being overlaid.

    I believe this may be the case because the diagram below appears to show the CSH is applied after digitization but before conversion to YPrPb.SD Processor

    I'm curious to know whether my reasoning is correct or not, could you please comment, thanks.

    Best regards,

    Peter

  • 0
    •  Analog Employees 
    on Nov 26, 2018 2:01 PM in reply to peter_k

    Hi,

    Yes, the CVBS signal is processed by the SDP and converted to YPbPr. The RGB signals are processed by CP section and it converted to YPbPr. And also CSH may cause the CVBS signal not RGB.

    The CVBS signal is processed by the ADV7184 and converted to YPrPb. The RGB signals are processed by a color space converter (CSC),and samples are converted to YPrPb. Both sets of YPrPb signals are input to the subpixel blender, which can be configured to operate in any of the four modes.
    After digitization of the video signal and luma and chroma signal separated finally those signals are added

    Thanks,

    poornima

  • Hi,

    If this is the case, then changing the CSH configuration wouldn't affect the appearance of the overlaid video, and to resolve the issue with faint isolated pixels I would need to look further down the signal processing chain, unless I missing something?

    If the overlay is being done over YPrPb video I would expect it to be immune to chroma issues, isolated pixels should appear no different to any other pixels, so presumably this issue must be happening either in the ADV7341 part or else the signal processing of my video monitor. I would also expect that if I feed component video from the ADV7341 to my monitor instead of CVBS and see the same issue, then that would rule out the monitor.

    Any comments you have that can help narrow down the problem or shed light would be greatly appreciated.

  • 0
    •  Analog Employees 
    on Nov 28, 2018 12:46 PM in reply to peter_k

    Hi,

     Your question has been forwarded to part specialist and will update you soon.

    Thanks,

    Poornima