Using ADV7184 SCART Overlay with ADV7341, isolated pixels are faint


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?


  • Hi,

    Here is what I have found so far.

    Enabling/disabling CTI in the decoder makes no appreciable difference.

    Enabling/disabling DNR and changing the gain in the encoder makes no appreciable difference.

    Enabling FB Edge Enhancement makes the effect slightly worse (although barely noticeable), and the effect becomes worse as edge enhancement level is increased. I would have expected the opposite.

    Slowing down the pixel clock for the RGB overlay does improve things because the overlay pixels become wider and chroma has more time to stabilise, but the result is decreased horizontal resolution and rectangular pixels.

    I am clocking the decoder with a 28.63636 crystal and I'm not encountering crosstalk.

    I can't tell at this stage whether the effect is occurring in the encoder or decoder because I don't have a way to capture the digitized video stream from the decoder and display it. However the effect does not go away if I change the output of the ADV7341 from CVBS to YPbPr, which suggests that it is occurring within the decoder/encoder circuit rather than limitation in chroma bandwidth when decoded by the video monitor.

    It is also definitely a chroma effect, I don't notice a problem with white or grey overlay text.

    Do you have any other suggestions?

  • 0
    •  Analog Employees 
    on Nov 19, 2018 1:57 PM in reply to peter_k


    Please try to adjust the chroma filter register to set different bandwidth in both encoder and decoder part.
    Refer ADV734x datasheet, here chroma filters listed in table 43 and the ADV7340/ADV7341 contain an SSAF filter that is specifically designed for the color difference component outputs, Pr and Pb. This filter has a cutoff frequency of ~2.7MHz and a gain of –40 dB at 3.8 MHz (see Figure 62). This filter can be controlled with Subaddress 0x82, Bit 0.Please refer chroma filter section in the ADV7184 datasheet .   

    Note : Chroma Shaping Filters (CSH):The shaping filter block (CSH) can be programmed to perform a variety of low pass responses. It can be used to reduce selectively the bandwidth of the chroma signal for scaling or compression.      



  • Thanks Poornima, I will give that a go and post the results here.

  • Hi Poornima,

    Unfortunately the Chroma Shaping Filters don't appear to make any difference to the SCART output no matter which one I try. Also, the SSAF filter makes no difference. It looks like I am going down the wrong track. Just wondering if there is anyone who has been involved with this type of video application who can comment?



  • 0
    •  Analog Employees 
    on Nov 23, 2018 1:16 PM in reply to peter_k


    This is related to Chroma filter configuration,So we suggested that one.

    And also please cross check Vertical Blank Control section for chroma text setting NVBIOCCM [1:0],BL_C_VBI, Blank Chroma During VBI, Address 0x04 [2]

    Note: NSFSEL [1:0] /PSFSEL [1:0] selects how much of the overall signal bandwidth is fed to the combs. A narrow bandwidth split filter results in better performance on diagonal lines, but more dot crawl in the final output image. The opposite is true for a wide bandwidth split filter.