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Discussion Starter · #1 ·
Hi,


I have a test scenario where I am validating the TMDS signals through an assortment of cables which are my DUT.


To describe my test the setup looks like:


HDMI Source Device (Laptop) -> DUT (Cable) -> HDMI Test Fixture -> SMA Probe Head -> 6Ghz DSA


The DUT cable however will ONLY be connected to the TMDS line, no HPD, DDC, +5VDC or anything. I am able to modify the cable however but I have a few questions:


1) I can jump the +5VDC and hotplug lines together, will this give me the desired result of the laptop detecting that the cable is connected however no EDID block will be received. The laptop is loaded with Windows and Linux, I figure I should be able to force whatever resolution without an EDID block and still be OK.


2) I have ordered a EDID Ghosting/HDCP box to use as an intermiterary between the source device and the DUT which is supplied by +5VDC and send back a emulated EDID of my choosing, would this accomplish sending the TMDS signals?


3) Any other suggestions on how this could be accomplished? Is there a simpler one of some command/utilities in Linux that can force the HDMI port to send even when it doesn't see it is connected a monitor and than using xrandr to set the resolutions?


The EDID manager I am going to use is: http://www.amazon.com/ConnectPRO-Video-Ghosting-Emulator-TMDS-EDID-1/dp/B007KCSL5C however it is not going to be delievered for a number of days and as always time is of the essence. I can wait however, it will not be a big deal at all, but I am also interested in other options.



--- Lastly a different question


The goal of my test is to observe the performance impact of different cable lengths and disconnect setups. My thought process is that the only lines that really matter most of the high frequency lines, as they would be the first to falter before the lower frequency discretes. Would you agree with this logic?


Thank you!
 

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Discussion Starter · #2 ·
Hi,


To update; using a EDID emulator will work fine. If worst comes to worst I can split the cable and have it go to a monitor with the descretes and the TMDS will continue to go to the DSA.


Thank you
 

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Quote:
Originally Posted by sam0505  /t/1472560/forcing-tmds-signals-to-transmit-w-o-monitor#post_23310424


My thought process is that the only lines that really matter most of the high frequency lines, as they would be the first to falter before the lower frequency discretes. Would you agree with this logic?
I am not sure it follows. First, you are comparing differential lines with single ended lines. And it is known that too much capacitance on the DDC line will cause problems.
 

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Usually the transmitter will not send the video signals unless they are "terminated". This means you need to connect 50 ohm resistors from each signal to either ground or 3.3V (doesn't really matter because Ground and Vcc are the same in AC analysis). Sometimes a 100 ohm resistor between the + and - of each differential signal will be enough to get the transmitter sending video signals.


Also, unless you can turn off HDCP on your laptop, chances are it will not send a video signal without the EDID manager. Most HDMI devices need to go through an HDCP authentication process before sending the encrypted video. The source must make sure the monitor is capable of receiving protected content before it will start transmitting.
 
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