ADV 7513 DVI DC Balancing Problem


We have an issue with ADV 7513 transmitting DVI (TMDS) signals using non-standard long lines and connectors (38999 type).

The picture size is 1024x768 50-60 Hz so is not a high frequency and it works perfect with cheap standard DVI cables of 15 meters long.

The problem is that black color (0x000000) is being received as a bright green noise. But when we change all black colors (0x000000) to the darkest grey (0x010101) the problem is gone and the picture looks okay.

Is there TMDS DC balancing mode implemented in ADV 7513? And is the specific configuration required to turn this mode on?

[edited by: ekondratiev at 10:38 AM (GMT -5) on 29 Jan 2021]
  • +1
    •  Analog Employees 
    •  Super User 
    on Jan 29, 2021 12:53 PM

    By design DVI encoding is DC-balanced.

  • So, it could be a receiver problem? It is not a standard, let's say, DVI receiver and we don't know how it is organized.

    And is it possible than if TMDS receiver doesn't recornize 10th bit of DC balancing it will generate 00000->111111->00000 data strem instead of the normal data flow?

  • +1
    •  Analog Employees 
    •  Super User 
    on Jan 29, 2021 5:59 PM in reply to ekondratiev

    Any receiver should be able to handle the 8b10 encoding, otherwise it would not be able to decode anything.  Also black is not transmitter as '0000000' but rather what looks like 10 random bit with equal number of '1's and '0's.  If your are really interested in this encoding check out the HDMI1.4 spec.  (same encoding as DVI), it has a good description of the encoding algorithm

    I do not know what your receive is so it is hard to guess at what the problem might be.

    The fact that you can transmit good images over the 15m cable is a sign that it is not a transmitter or cable problem.

    Note DVI spec. does not specify min or max cable length, only the sink side differential voltage amplitude when driven by the transmitter.  Cable impedance is very important here.