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!

  • 0
    •  Analog Employees 
    on Nov 12, 2018 1:26 PM

    Hi,

    ADV7184 fast blanking could be used to overlay with dynamic switching using the FB pin.
    For that to work both the RGB and the NTSC/PAL must have identical timings.
    It may be the reason by without setting the FB pin to a logic "1" for every pixel you want blended from RGB onto the CVBS.
    And also refer Pages 16-18 for more information the ADV7184 data sheet.

    Thanks,

    Poornima

  • Hi Poornima,

    Thanks for your reply. I am setting the FB pin to logic 1 for active pixels, and I am deriving the RGB timings from the CVBS sync. Pixels are appearing as expected, except for dimness issue. Just wondering if you can point me in the direction of any settings for the encoder or decoder that could help to reduce this?

    Thanks,

    Peter

  • 0
    •  Analog Employees 
    on Nov 13, 2018 2:01 PM in reply to peter_k

    Hi,

    For overlay applications, text can be more readable if the contrast of the video directly behind the text is reduced.
    Due to the higher bandwidth, the signal transition of the luma component is usually much sharper than that of the chroma component.
    The color edge is not sharp and can be blurred, in the worst case, over several pixels.  
    Chroma transient improvements are needed primarily for signals that have severe chroma bandwidth limitations.
    For these types of signals, it is strongly recommended to enable the CTI block via CTI_EN.
    For more information Please refer Chroma transient improvement and contrast reduction section in the datasheet.

    Thanks,

    Poornima

  • Thanks Poornima, I will experiment with the CTI block and see if that improves things. 
    Reducing the contrast behind the video text does make the text more readable, in fact I have it enabled in the image above. 

  • 0
    •  Analog Employees 
    on Nov 15, 2018 7:37 AM in reply to peter_k

    Hi,

    Please crosscheck the encoder and decoder clocks.To Selecting the clock frequency one over the other may be desirable in systems in which board cross talk between different components leads to undesirable interference between video signals. It is recommended to use a crystal of frequency 28.63636 MHz to clock the ADV7184. Is this issue reproducing from encoder or decoder side?

     

    Thanks,

    Poornima