HDMI interseptor - Page 1
Forum section: electronics / rf-microwave

author: oliv3r
author_rank: Frequent Contributor
timestamp: September 03, 2023, 10:00:08 am
content: Hey all,I'm working on an HDMI pass-through PCB (to get the CEC line out of it) and want to influence the HDMI signals as little as possible.Looking at LCSC there's not many options in terms of HDMI connector variants.So I'm looking at a few options.* Use identical connectors on opposite sides, the downside is to use via's the get the signal from one end to the other.* Use identical connectors side-to side, and route the signal between them as normal diff pairs, where one connector exists the signals on top and the other on bottom of the pads, and goes around it. The downside is to have a longer traces* Use reverse connector in combination with a normal connector, and mount them opposite of each-other. The up/downside of this is that things become really close, so it's hard to actually match impedance (but isn't that true for all).Since ideally, I'd like to be able to pass-through the full 600+ Mhz of HDMI 2.x, I understand this becomes difficult (on a 2 layer 0.8mm board, but not sure how 4 layers would help here).Here's my current solution, i may have to add half a mm maybe of space between the connectors to give the board a bit more room, but i'll have to see. (See attachment).Is this 'okay'-ish? Any pointers?I'll probably make the shielding pair/pins of each diff channel their own ground, without anything inbetween, and manually do the 'ground plane' for them, so ignore the big ground plane for now.

author: Njk
author_rank: Frequent Contributor
timestamp: September 03, 2023, 05:29:59 pm
content: If CEC is all you need, what's the reason for PCB? You can sacrifice one cable to source the CEC line directly from the cable connector and that'll be the least intrusive option. If PCB is really necessary, think about active repeater chips like TI TMDS171x, TMDS181x etc. that has retiming capability, especially when you have to deal with 600+ MHz clock in HDMI 2.x

author: oliv3r
author_rank: Frequent Contributor
timestamp: September 05, 2023, 02:42:41 pm
content: Well for one, it will sit between the PC and the TV, but I hear what you are saying, splice the cable, but then that's not a very neat solutionnor is it to offer to someone else of course.But adding any IC, would add significant cost and design effort, and you still have to route the pins in/out of the HDMI connector, so that overhead you'll always have?

author: mojoe
author_rank: Frequent Contributor
timestamp: October 08, 2023, 12:48:32 am
content: Just a suggestion - what about one of those cheap HDMI "splitters" that you can get on Amazon? You might be able to tap off the signal you need. I haven't opened one, but they have active devices inside, and many don't pass the protection signal (forgot what it's called). This is useful for home recording via HDMI.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: October 10, 2023, 07:35:05 am
content: The signal integrity considerations that come to a high-speed 2.x HDMI line are non-trivial. I'm not sure you can get away with the connector you have designed there for 2.0, I'm quite confident it won't work for the line rate of 2.1.With every generation it becomes harder to get good reliable cables because of the increased signal integrity aspects.

author: Smokey
author_rank: Super Contributor
timestamp: October 10, 2023, 08:19:44 am
content: 4 layer small boards like that are almost as cheap as 2 layer.How about two of the same right angle through hole connectors facing away from each other, but one on the top side and one on the bottom side. That should make straight through routing. Route pairs on top and bottom, both inner are ground.

author: oliv3r
author_rank: Frequent Contributor
timestamp: October 15, 2023, 09:43:59 am
content: you are correct, JLCPCB offers 2€ for 2 OR 4 layer boards!I have change the board to 4 layers now, and did a ground poor on every layer. I've put the power lines on one of the inner leyers.The HDMI interconnect I actually also put on the inner layers, so that I could have ground pours above and below.I'm sure it's not perfect, but the alternative, is an SMD version, and i'd have to snake the traces around to get the pins to match. Finding an SMD reversed connector seems impossible from lcsc, so I gave up on that avenue.So it's now a choice of lesser evils. Very very short traces, but maybe violating some design recommendations, or use via's ...Using the same connector top/bottom is certainly something that has occurred and I will certainly entertain that idea. But with this new idea 4 layers depicted in the image, what would be the impact of using via's from a HF aspect?So below, is the board now with top/bottom layers hidden, so there's ground pours ontop and below of these traces on the top/bottom.Also I understand one prefers to keep signals ontop, but I think with these simple/short traces, the exception is acceptable ...Obviously, I have no equipment nor knowledge how to properly do 600MHz traces proper ...

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: October 15, 2023, 10:36:58 am
content: You seem to misunderstand that line rate of HDMI. Earlier you mentioned HDMI 2.x. 2.0 used a TMDS of 600 MHz, but the actual data was much higher clockspeed (I think 10x?), so 6 Gbit/s per line. With HDMI 2.1 they ditched this entire TMDS nonsense, and went for four lines of data instead of three lines and wasting a pair for TMDS signals, and then went to 12 GBit/s.This is significantly faster than 600 MHz - even if you only care about datarate/2, you are still dealing with DC-3 GHz in the case of HDMI 2.0, and DC-6 GHz in HDMI 2.1. I don't even know if you can meet the signal integrity requirements of such a line rate with through-hole connectors like that.TMDS is a kind of 'pixel' clock - with every TMDS pulse, you transmitted 3x10 bits of data (3 lines, each their own color/YCrCb/whatever)

author: Smokey
author_rank: Super Contributor
timestamp: October 15, 2023, 09:06:23 pm
content: Quote from: TheUnnamedNewbie on October 15, 2023, 10:36:58 amThis is an interesting. If you search Digikey for HDMI 2.1 rated connectors, there is only one through hole part returned. All the rest are surface mount... hmm... :https://www.digikey.com/en/products/detail/cui-devices/HD01-19-RA-TH/15761304https://www.cuidevices.com/product/resource/hd01-19-ra-th.pdfHere are some docs about HDMI layout. The TI one has some good pictures of Via return current paths.https://e2e.ti.com/cfs-file/__key/telligent-evolution-components-attachments/00-138-01-00-00-10-65-80/Texas-Instruments-HDMI-Design-Guide.pdfhttps://ez.analog.com/video/w/documents/687/hdmi-layout-guideline