6
\$\begingroup\$

I am looking to turn on and off the throughput of an HDMI signal via a microcontroller. Would four ICs such as the SN74LS08N AND gate be enough? Or do HDMI signals need something more complex?

\$\endgroup\$
16
\$\begingroup\$

Absolutely not.

First: HDMI uses TMDS signaling. This is not compatible with TTL logic gates; it's a current-mode differential protocol, very different from the single-ended outputs used in TTL logic.

Second: The TMDS signals used by HDMI run at over 1 GHz. This is far faster than the 74LS parts you're looking at can handle; indeed, I don't think it's compatible with through-hole parts at all. (The pins alone will have too much inductance for this fast of a signal.)

Consider using a dedicated HDMI switching part, such as the TI TMDS261B ("2 to 1 HDMI switch") or the Fairchild FDHDMI08 ("Wide-Bandwidth Differential Signaling HDMI Switch").

\$\endgroup\$
  • \$\begingroup\$ Can you cite a source about 1Ghz? Cables are required to pass a little above 75Mhz for 720p60. \$\endgroup\$ – Diego C Nascimento Jul 9 '16 at 0:40
  • 1
    \$\begingroup\$ @DiegoCNascimento 720p60 equates to at least 553MHz serial data rate for pixel data, and higher because of blanking times. The clock itself is only 1/10th of that because SERDES is used (30bit parallel data is serialised onto 3 cables). \$\endgroup\$ – Tom Carpenter Jul 9 '16 at 1:06
  • \$\begingroup\$ @DiegoCNascimento HDMI 1.4 transmits 10.2 Gbit/s across three TMDS channels - 3.4 Gbit/sec per channel. There won't be 1.7 GHz of transitions on each channel due to the 8b/10b encoding, but it's still over 1 GHz. A 74LS part certainly won't cut it. :) \$\endgroup\$ – duskwuff -inactive- Jul 9 '16 at 1:07
  • \$\begingroup\$ @DiegoCNascimento TDMS clocks symbols, which are 10 bits, rather than than clocking bits. Thus 75MHz means 75 million symbols/sec. The bit rate is 10x higher, at 750million bits/sec. \$\endgroup\$ – Cort Ammon - Reinstate Monica Jul 9 '16 at 1:09
9
\$\begingroup\$

No, the signal frequency is way too high for typical AND gates. The signalling is also differential TMDS.

You may be able to apply your AND gate to the HPD (Hot Plug Detect) signal. You can then emulate unpluggnig the HDMI connector. Most sources will then stop transmitting, which is what you want.

\$\endgroup\$
4
\$\begingroup\$

As you would need dedicated HDMI switching chips to do the switching, which might be difficult to source and a PCB with the right footprint, etc. Allow me to make an alternative suggestion which might be easier for a one-off project:

Get a cheap, ready-made HDMI switch. For example, this 5-way HDMI selector on eBay has an infrared remote control. You could use the microcontroller to generate the infrared signals, or use a contact closure to press the button(s) on the provided remote control.

Just a thought. Maybe it helps.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.