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Is it possible for DM8168 DVO2 to output non-zero value during video blank period?

Ryan Lin1
Prodigy 100 points


We are using a simple VGA DAC connected to DM8168 DVO2. The digital video path is YCbCr 422 16-bit with CLK/HS/VS/DE. The DAC has YC2RGB conversion matrix, but unfortunately the DE singal is somehow not used by the DAC. This means the DAC will output a none-zero Green level during blanking (YCbCr=0x000000 will produce positive GREEN value. DVO2 outputs 0x0 during blanking). This level is treated as black level for pc monitors so that Green component will be sampled with reduced amplitude. I know this issue does not exist if I use RGB24 for the digital video path. If the DM8168 can output YCbCr with value 0x008080 during blanking, the DAC should also work perfectly.  I don't know if it is possible for DM8168 DVO2 to output YCb/YCr with 0x0080 value during video blank period.

Any help is appreciated.

  • 0
    TI__Guru**** 479825 points

    Hi Ryan,

    But Typically, VGA output is in RGB format, isn't it? Why do you need YUV format?

    Also This blanking should be removed by the final display device, isn't it?

    Rgds,

    Brijesh

  • 0 in reply to Brijesh Jadav
    Prodigy 100 points

    Hi Brijesh

            We use YCbCr for 2 reasons. 1) YCbCr 422 (16-bit) has less signals, compared to RGB24 (24-bit). 16-bit is eaiser for PCB routing. 2) That DAC chip claims supporting BT1120 embedded timing, but finally we find BT1120 does not work. Our plan is to use BT1120, and YCbCr 422 with discrete syncs as a backup, for we have not used that DAC before, and the manufacturer are not willing to disclose all the register detail.

            You are right, the blanking content will be removed by display devices. The issue is, if blanking level is not zero, the active line voltage level will be sampled with reduced level, because of offseting by the blanking level. Let's say I have a GREEN level of 0.7V during the active video period, if the blank is 0.3V, then GREEN is sampled as 0.4V only.  RED/BLUE blanking level is 0V, 0.7V RED/BLUE signal will be sampled as 0.7V correctly.  That is why a white picture is displayed as light pink.

    ===================

    I have find a solution that can fix this issue.  In HD_VENC_D_cfg0 register,  set DVO_FMT to 001 (Two channel 10-bit CCIR656 video streams) instead of 100 (Two channel 10-bit video streams with dedicatedHS/VS/FID/ACTVID sync signals) even I need dedicatedHS/VS/FID/ACTVID sync signals. The 2 ch 10-bit CCIR656 video streams will output 0x000/0x200 (10-bit BLACK pixel ) during blanking except SAV/EAV code position. DedicatedHS/VS/FID/ACTVID sync signals are still valid in CCIR656 format and their timing can be programed.

  • 0 in reply to Ryan Lin1
    Prodigy 100 points
    I have find a solution that can fix this issue. In HD_VENC_D_cfg0 register, set DVO_FMT to 001 (Two channel 10-bit CCIR656 video streams) instead of 100 (Two channel 10-bit video streams with dedicatedHS/VS/FID/ACTVID sync signals) even I need dedicatedHS/VS/FID/ACTVID sync signals. The 2 ch 10-bit CCIR656 video streams will output 0x000/0x200 (10-bit BLACK pixel ) during blanking except SAV/EAV code position. DedicatedHS/VS/FID/ACTVID sync signals are still valid in CCIR656 format and their timing can be programed.
  • 0 in reply to Ryan Lin1
    TI__Guru**** 479825 points

    Hello Ryan,

    Good to know that it outputs 00/80 in two channel embedded sync mode. 

    Typically blanking will be 00/80 for luma/chroma respectively in embedded sync by default, but not in discrete sync mode. In Discrete sync mode, we need to transfer blanking data through blanking input channel but this blanking input channel is not supported.

    Even in case of embedded sync mode, VENC will continue to output sync signals, so you could program the VENC embedded sync mode and get still the sync signals in the format you want.

    Regards,

    Brijesh