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I have two brands of broken HDMI cables that I need to join to make them longer.

Can't join them by color, because they are completely different. Plus, one of the cables has all the wires, the other floats some of the pins.

So,

What are the most fundamental pins to transmit audio and video between PC and TV ?

Does the shield/shell have to be connected or is it optional ?

Wikipedia treats the pins as "data", but that doesn't tell me anything, even though I know digital image and video are data.

I would connect all the available wires, but it's quite difficult in my case here. If one is not important, I intend to leave it.

Edit:

Let me make an additional question, if there's still time:

How can the HDMI I have here work normally (at least before it lost one of the endings) having only 14 wires ?

It is missing wires 2, 5, 8, 11 and 13

There are 5 extra/individual shields, but the multimeter is not picking them on those missing pins.

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    \$\begingroup\$ You have the pin numbers wrong, pin 1 is on the other end where you started. Sometimes manufacturers do not connect the TMDS shields individually to their respective pins but to common shield. It is unknown if that is allowed by the specs. Also it is highly unlikely that the pin you counted as 7 is missing a connection, it is either broken or if it is left out it should not be sold as a HDMI cable. \$\endgroup\$ – Justme Aug 6 at 5:52
  • \$\begingroup\$ I checked on wikipedia to see the numbering. Checked twice... Threefold... Still not believing. Specially the number 15, since it's serial data, AFAIK. But you're right: something is off. \$\endgroup\$ – Lucas BS Aug 7 at 10:09
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    \$\begingroup\$ It depends whether you are looking at the connector at the cable, or the connector at the device. Wikipedia shows the pin numbering of a receptacle, so if you take a cable out of the receptacle and turn it towards you, the pin numbers are obviously mirrored too. \$\endgroup\$ – Justme Aug 7 at 10:13
  • \$\begingroup\$ Indeed. I was imagining a male connector with the holes facing me. This picture confirms what you say: encrypted-tbn0.gstatic.com/… \$\endgroup\$ – Lucas BS Aug 7 at 10:36
  • \$\begingroup\$ Now that I corrected the numbering, I see the shields are missing. And the CEC pin, that other answers suggest is only recommended. When all starts making sense... \$\endgroup\$ – Lucas BS Aug 7 at 10:46
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Unfortunately, they are all important. Including cable shields. Except maybe the UTIL and CEC lines if you don't need the features they provide.

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Aside from what Justme wrote (accept their answer, it's correct and the critical point here):

HDMI's data lines are really high-speed. That's easy to imagine: assume you have 1920×1200 px screen you want to feed at 60 fps. That makes 138.24 million pixels a second. Each of these have 8 bit on each color channel, and then there's 8b/10b-encoding, so that you end up with 1.3824 billion bits per second per color channel.

That means the signals have high bandwidth, and that means that things like "I soldered together these twisted pair cables, altering the properties of the twisted pair and its shield" drastically reduce signal quality.

In short: won't work, buy a new long cable. If a cable in the length you need isn't available, there's a technical reason for that.

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    \$\begingroup\$ Extemely good point. I didn't think that someone would actually try to make a longer cable by soldering two together. It might not anyway work due to impedance mismatches. There are also different classes of cables, which just means the given length cable is marked to work at certain bandwidth when you buy it. Making a longer cable from two may not work at that bandwidth due to longer total length. The video signal contains other things than just active area, so minimum bandwidth for 1920x1200 at 60Hz needs minimum 154 MHz clock or 1.54 GBPS per lane at RB timing. \$\endgroup\$ – Justme Aug 4 at 15:16
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    \$\begingroup\$ I've seen a product that "extends" HDMI cables. It was simply a two-way HDMI connector: One end accepts the cable from the source and the other end accepts the cable that goes to the target. I bought one just for testing with my computer and screen projector (normally there's a single 20-m-long between the two and the projector works perfectly). Plugged a 10-m-long cable to one end and plugged another 10-m-long cable to the other end. Result: didn't work :) \$\endgroup\$ – Rohat Kılıç Aug 7 at 11:33
  • \$\begingroup\$ @RohatKılıç I bet I could have built the same functionality, half the price, wirelessly! \$\endgroup\$ – Marcus Müller Aug 7 at 11:51
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"I have two brands of broken HDMI cables that I need to join to make them longer." (italics mine).

No, you don't need to join two cables. Nobody needs to do that, and I advise against it. Why? Several reasons:

  • mechanical robustness of the splice
  • signal integrity of the splice + shields (impedance mismatch, reflections, etc.)
  • added capacitance of the cable length affects signal integrity
  • longer cables have to pay more attention to losses and need higher quality conductors

Given that you can get reasonably-priced, HDMI certified longer cables from Monoprice, Blue Jeans Cable, etc. why would you waste your time splicing broken cables? Are you so broke that you couldn't spend $17-20 on a longer cable?

In direct answers to your sub-questions:

  • Fundamental signals: Data and clock pairs, HPD, DDC (I2C), +5V and ground
    • CEC is optional, but recommended.
  • Shields? Yes, shields, at least on the diff pairs.

If you insist, what you should do instead is repair your cables as-is (keep the ends) and use a coupler to connect them. I don't recommend that either (see above about longer cables) but it will yield a better result than trying to splice them directly.

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