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Not like this which is about protocol, this is about physical.

The HDMI2.0 (18Gbps) fiber optic HDMI cables on market seems to be "cheap", like<$100.

Although optic fibers are very cheap now, but considering those cables contain 4 optic channels which require 4 transmitters and 4 receivers each at least with a rate of 4.5Gbps, and for comparison 10Gbps optical NIC is $100 with only one 10G transmitter and receiver.

Does those cables just connect the TMDS lines directly (or only with some general off the shelf cheap level shifter like ICs) to the optical tranceivers (or even just some off the shelf cheap ICs and a optical diode?) to get such low cost? If so, maybe a HDMI2.1 48Gbps transmitter/receiver can be DIYed (paired with off the shelf optic cables) which is not on market yet.

What is the recommended way to convert TMDS (two sub question:18Gbps and 48Gbps which may be a very different design) signal to optical signals with low cost?

Updated

Phase synchronization between channels is another problem, HDMI is a synchronized bus, the electric-optic and optic-electric converters should have fixed phase shift (and noise) that is compliant with HDMI requirement.

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  • \$\begingroup\$ 10Gbit SFP+ modules cost way less than $100: fs.com/products/15235.html Optical transcievers are very cheap to make. Probably that device costs so much because it includes 4 ~$15 transceivers. \$\endgroup\$
    – user1850479
    Commented Jan 21, 2021 at 1:26
  • \$\begingroup\$ @user1850479 The cable integrates all circuilts in a normal HDMI plug, I guess they are diode based. SFP+ sounds like the big module for NICs. \$\endgroup\$
    – jw_
    Commented Jan 21, 2021 at 1:38
  • \$\begingroup\$ @user1850479, also if it's $17 at qty 1, you can bet it's less than $8 if you're buying 10,000's of them. jw_: You wouldn't use an SFP+ transceiver inside an HDMI plug, but you'd use basically all the same parts that are inside the SFP+ transceiver. \$\endgroup\$
    – The Photon
    Commented Jan 21, 2021 at 2:52
  • \$\begingroup\$ @user1850479, it's interesting that the FS.com page you linked shows 136,000 parts in stock, but only "50 sold". \$\endgroup\$
    – The Photon
    Commented Jan 21, 2021 at 2:55

1 Answer 1

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Indeed ASICs for this purpose are used. Here is an example:

SL82821 is an ultra-low power four channel VCSEL driver designed for HDMI (48 Gbps) active optical cables

Features

  • Data rate up to 12 Gbps per channel
  • Supports all legacy HDMI 1.4 and HDMI 2.0 data rates
  • Four channels
  • Single supply voltage: 3.3 V
  • Dual supply voltages 1.1 V and 3.3 V
  • Externally adjustable VCSEL currents
  • Integrated temperature controller
  • TMDS and FRL compliant inputs
  • Programmable input equalizer
  • Available as a bare die

The bare die will be put on the PCB inside the connector of the cable directly such as: enter image description here

The photo is from here.

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  • \$\begingroup\$ jw_ - Hi, Unfortunately this self-answer is getting flagged by other site members, due to (a) being very short, (b) no part of the linked article is included in the answer (so if the link dies, it becomes useless), and (c) if this really is the conclusion to your question, then you should áccept it, to indicate that the topic is concluded. Therefore, as a minimum, if you don't want to add more detail to this post, at least áccept it to show that you (as the OP) consider it to be the final conclusion of your question. Thanks. \$\endgroup\$
    – SamGibson ♦
    Commented Aug 28, 2022 at 15:20
  • \$\begingroup\$ @SamGibson I didn't include the detail since the answer is the first sentence. And the example is FYI. Now I notice the example is important to be a answer to the OP but the link is not accessible now. \$\endgroup\$
    – jw_
    Commented Nov 2, 2022 at 0:49
  • \$\begingroup\$ jw_ - I see. So if the answer to your question is that first sentence of your self-answer, please accept your answer, as I linked in my previous comment, so that it's clear to readers that this is the conclusion of your question. Or is there a reason why you don't want to do that? I'm just trying to clarify the status of the question for future readers. Thanks. \$\endgroup\$
    – SamGibson ♦
    Commented Nov 2, 2022 at 1:01
  • \$\begingroup\$ @SamGibson The link is accesible now and I'v included the link detail. \$\endgroup\$
    – jw_
    Commented Nov 2, 2022 at 1:17
  • \$\begingroup\$ jw_ - Thanks, so now you can accept your answer (i.e. add the "green tick") to show this is your solution to your question. Since you have now added a photo, it is required to add a link back to the original source of the image (e.g. link to the original webpage containing that photo), unless you took the photo yourself. \$\endgroup\$
    – SamGibson ♦
    Commented Nov 2, 2022 at 1:25

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