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Directional HDMI cables?

 
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Curt Palme
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PostLink    Posted: Thu Apr 03, 2014 4:38 pm    Post subject: Directional HDMI cables? Reply with quote


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So here's a new one on me. Could be a fluke, but it's worth posting I think.

Had a customer buy a G90 off me, and he bought a generic 40' HDMI cable off eBay, and it didn't work. I told him to try a short 6' cable, and the Moome card came to life. I told him to buy a decent HDMI cable from Blue Jean or similar, not some no name one. Before he did, he tried swapping ends of the cable, in other words, installing it backwards, and the darn thing worked! Not really quite sure why, but maybe Monster does have something going with the arrows on their speaker cables?

(I'm kidding of course with that last sentence!!!) Still, if you're in a bind with a nonworking HDMI cables, it's something to try...
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CIR Engineering




Joined: 25 Aug 2008
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Location: Chicago USA & Berlin Germany


PostLink    Posted: Thu Apr 03, 2014 10:40 pm    Post subject: Reply with quote

Redmere HDMI cables are becoming very common place these days and they are most certainly directional. This is because they are "active" cables that decode the signal on the input and then recode to HDMI on the output side. If the HDMI cable has arrows on it (seriously) it is probably a Redmere cable.

It's better to avoid Redmere cables when possible because they cause more delay during the HDMI sync and hand shake between devices. But in situations where regular HDMI passive cables won't work (like often with 4k) than Redmere can be an excellent solution without the use of additional hardware.

craigr

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VideoGrabber




Joined: 09 Apr 2006
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Location: Michigan


PostLink    Posted: Sun Apr 06, 2014 3:57 am    Post subject: Reply with quote

CIR Engineering wrote:
It's better to avoid Redmere cables when possible because they cause more delay during the HDMI sync and hand shake between devices.


This is good to know.

CIR Engineering wrote:
Redmere HDMI cables are becoming very common place these days... they are "active" cables that decode the signal on the input and then recode to HDMI on the output side.


?? This seems odd. I understand the role of an active cable, in boosting signal strength and increasing S/N. But why is it necessary to put a decoder on the front? Once you've done that, sync delay is inevitable.

If they really do decode the signal on the input, can I just cut off the output, and rewire it to a non-encoding HDMI connector? Would make for a fairly cheap HDCP stripper.

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PostLink    Posted: Sun Apr 06, 2014 4:06 am    Post subject: Reply with quote

BTW, I'm no expert on Redmere (really haven't been paying much attention), but I had thought the purpose was to provide adaptive EQ and deskew of the high-speed bitstream, so that thinner, cheaper cable could be used. I had never heard anything about decoding before now.
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ecrabb
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Joined: 13 Mar 2006
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PostLink    Posted: Sun Apr 06, 2014 8:33 pm    Post subject: Reply with quote

This is definitely a deep subject, but I'll chime in. Not because I'm any sort of expert (far from it), but because I sat through a lunch seminar, and I learned about HDMI signal transmission. I can't begin to do it justice, but I'll try to remember the high points. Back to that in a sec.

I'll just warn you now… This will probably be long, and I'll probably meander a bit because I don't want to spend time editing. Sorry in advance.

First, as Craig mentioned, there are active HDMI cables that have silicon in them, so there are most definitely directional cables. They're not snake oil like the passive (but supposedly directional) speaker or line-level interconnects. Redmere is but one active cable solution; there are several others.

OK, back to the seminar. I was at a big distributor show in Atlanta, and they hosted Jeff Boccaccio from DPL Laboratories, essentially the only independent cable testing lab. His talk was a little over an hour. It was very technical, very much so for an hour-long lunch talk, but it was nevertheless super-interesting and fascinating.

Not to contradict what Craig was suggesting, but from my understanding, there's no "decoding" going on in active cables, Redmere included. There is silicon in the source end of the cable. The silicon "power harvests" from the HDMI, actively amplifies and skews the signal to provide higher s/n ratio and flatter frequency response over the length of the cable. If the cable is working properly, there should be no difference in sync/handshake time. But, let's detour for a second.

My guess - and this is only a guess - is that Craig's probably seen some substandard Redmere cables, and/or there were other issues in the systems where he observed the sync/handshake delays. In Jeff's talk, he made a couple of comments about Monoprice that weren't overly positive. He didn't go into any detail, but I got the sense that his issue wasn't because their products are junk (they actually sell some good stuff). I think his concern was because you don't know what you're going to get. They carry so many products, and such a wide variety of products, at so many price points, that some are good and some are crap. I digress.

Jeff went into great detail on how they do their testing, from scoping the cables, the equipment they use, the process, how they work with their manufacturers and the actual factories to improve standards and communication, etc.

For instance, he said the test equipment they use test the cables would fit on a single round banquet table we were eating from, but he said it was over a million dollars' worth of equipment. He said just the scope probes were $40k each, and they need two obviously. So, it's easy to see why a company like Monoprice doesn't have their own equipment to test samples of their product.

He told another story about a manufacturers' product they were working with. They manufacturer sent them some development prototypes of an active cable they were developing, directly from the contract factory in China. The cables were outstanding - the best they'd tested. They gave the US manufacturer the green light, and the factory started manufacturing. They then retested the first batch from manufacturing, and they were an epic fail. They tore the cables apart and found that they'd switched from the $5/ea silicon that was used in the prototype to $1/ea silicon. So, back to Monoprice They're not tested, so who knows what we're getting? They could be good sometimes and crap others.

He also went into great detail talking about the frequency of the signals, dB loss over a length of cable, and the problems that are going to start creeping in as we move toward not just 4k, but 4k/60p. From there, he talked about the difference kinds of devices and how they're going to work (or not) with HDMI 2.0 and all the trouble that's going to cause. I don't remember all the nomenclature, but there are "dumb" devices which aren't profoundly affected by HDMI revisions, so things like passive and active cables, and even active repeaters. Those are basically RF devices that receive a signal, amplify/eq, and transmit them again. Then, there are devices like splitters or Cat5/6 extenders which do decode/re-encode the HDMI signal. They're good because their output is a perfectly spanky clean "new" HDMI signal no matter what it was looking like on the input side. The negative though is then there will be compatibility issues with (for instance) 4k/60, HDMI 2.0, etc.

Jeff then went on to say he thinks the situation is only going to get more challenging, not less so. He said 4k is coming, customers are going to want it, they're going to have to sell it, and they'd better know how to make it work right. It looked to me like the basic premise of his talk was telling the installers they need to know their stuff and spec and use the right products, because if they don't, it will either bite them, or their competitors will do it better.

I'll see if I can get ahold of his slide deck and share it with the group. It was definitely fascinating stuff.

Oh, I almost forgot the best thing I learned! I should all be using good cables that have been tested, not Monoprice. I was always under the impression that good cables were expensive, but not so. I checked out Jeff's site after the show, and some of the cables they've tested and approved are actually quite affordable.

http://www.dpllabs.com/page/dpl-approved-products

A Perfect Path HD-1000-25 like I'd need for the run from my prepro to projector is only $68 on Amazon. Good, tested, and include locking connectors. What's not to like?

http://www.amazon.com/Perfect-Path-HD-1000-25-Ethernet-Channel/dp/B003VV99OS/curtpalmecrtp-20

Perfect Path was on hand, the products looked really good, and from the sounds of it, they're very supportive of Jeff's efforts with testing and standards for the industry.

I'm not tearing everything apart tomorrow, and I'm not going to spend hundreds of dollars on HDMI cables, but as I replace gear, I'm going to start putting in better cables. It makes no sense to spend $10k or $20k on a system and put $2 cables in if there are good, well-engineered, tested, quality-controlled alternatives for reasonable money.

Cheers,
SC
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ecrabb
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PostLink    Posted: Sun Apr 06, 2014 8:43 pm    Post subject: Reply with quote

BTW, if you're not familiar with Jeffrey, he's one of the authors of HDMI Uncensored.

SC
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PostLink    Posted: Sun Apr 06, 2014 9:43 pm    Post subject: Reply with quote

ecrabb wrote:
This is definitely a deep subject, but I'll chime in. Not because I'm any sort of expert (far from it), but because I sat through a lunch seminar, and I learned about HDMI signal transmission. ...

I'll just warn you now… This will probably be long...


Thanks, Steve. That was interesting reading.

1) I know that the equipment required for testing is expensive stuff. I've seen what Agilent (for example) has to offer. I have a bunch of scopes. My best quality scope has better than 1 GHz bandwidth, and the 4 probes I got for that are 3.5 GHz. And that's not good enough.

2) My first comment mentioned boosting signal strength, and my second adaptive EQ. They're basically the same thing, with the second indicating the boost is frequency selective, and adaptive indicating it's signal dependent. However, I didn't think it required any decoding (removal of HDCP) and recoding. That's why I thanked Craig for bringing it to my attention.

You suggested some alternative reasons why he may have experienced delayed handshake synchronization, even if no decode/encode was going on. I'll leave it to him to consider if he thinks those are plausible.

3) I found the story interesting about the outstanding prototype cables from China, which turned into an epic fail in production. That's an all too common phenomenon. In fact, unless a purchaser makes special provisions to lock down a production configuration, manufacturing details and quality can change sometimes on a monthly basis. I think that's the biggest problem Monoprice has, which is generally good-quality stuff, at very good prices.

[offTopic]
This isn't all that different from how outsourcing high-tech US engineering projects to other countries works. I can't say anything specific, but there are companies (Asian, Indian, etc.) that secure contracts all the time, that do so by fronting the work with their A-team engineers. Once that's been accomplished, and contracts signed, the actual work is turned over to the B-team. How do I know? Because I'm a US engineer that winds up working with both teams... and there's a huge difference. And part of the reason I have a job is fixing (when possible), or developing workarounds (more often) to compensate for the quality (or lack thereof) of the B-team.
[/offTopic]

4) I did some looking on-line, and see that the RedMere technology was developed by an Irish company, with a fabless facility in Ireland (doh), and manufactured by Fairchild Semi over here, in Maine. It's their RM1689 chip, and from what I could tell, is only on the front-end of the cable (but I could be wrong). Here's what EE Times had to say in 2007...

"The RM1689 is an low power device that can allow the use of low-cost HDMI cabling and can draw power for its operation from the HDMI signals themselves. The RM1689 is embedded within the HDMI connector and enables the 3.4-Gbits per second operation required to pass the rigorous v1.3 testing for CAT2 cables compliance.

The use of thin flexible HDMI cabling is attractive to manufacturers of consumer devices such as gaming consoles, digital cameras and portable AV products, RedMere said. RedMere's RM1689 enables cable manufacturers to reduce costs on a number of fronts including a reduction in the copper content of up to 80 percent and the use of lower cost dielectric materials within the cable structure. The embedded de-skew technology enables yield and throughput improvements during the cable manufacturing process. The de-skew capability allows cable assembly customers to use lower-cost twisted pair bulk cable rather than the parallel pair construction used in today's high performance cables."


I guess that 'embedded in the HDMI connector' doesn't rule out having one on each end, with one a transmitter and the other a receiver. I just don't know. However, even if there are both, that doesn't imply that decoding must be going on.

5) HDMI is a set of TMDS signals. This means it uses differential signal pairs, and there are 4 of them (TMS0,1,2, and a Clock). Since these signals travel over separate wires, that's where "skew" can come in with cheaper cabling. Skew simply being a differential delay of one signal with respect to the rest. It's like if you had a parallel port, and all the bits didn't show up at the same time. If that phase shift or delay exceeds certain thresholds, HDMI stops working. And it's a big part of what RedMere tries to compensate for.

6) These cables are one of the few where Directional has any meaning. There's a lot of bogus nonsense out there, but this isn't it. Redmere isn't going to work if you plug them in backwards. So if you use any HDMI cables with arrows on the end you should comply with those labels. Worst-case, even if they're not Redmere (or a similar technology), they'll always work in the labeled direction, even if there's nothing active inside (and they would have worked in the other direction too).

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ecrabb
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PostLink    Posted: Sun Apr 06, 2014 10:05 pm    Post subject: Reply with quote

You're clearly an experienced engineer Tim, and I'm most definitely not. I don't even play one on TV. But, from my laymen's understanding, I think we're on the same page.

Oh, on the Redmere issue. From what Jeffrey was saying in his presentation, all these active technologies (Redmere included) must be optimized for each length and type of cable. Apparently, they don't just throw the same piece of silicon into the head-shell of any old HDMI cable and they're done (like I would have thought). According to Jeffrey, a manufacturer that's making an active cable has to run simulations and do testing, and load the right equalization "program" into the silicon to compensate for the cable impedance.

I think that's where the problems probably start for a company like Monoprice (and others to be honest). How tightly are they managing QC? How much and what kind of testing are they doing on their products to insure target quality stays at a certain level? I can't imagine trying to manage all that offshore manufacturing and trying to keep anything resembling a consistently high level of quality. "Nightmare" doesn't even begin to justify it. It's a pain in the ass just to get stuff manufactured in the US!

Jeffrey did say that any HDMI cable manufacturer should be able tell you exactly what signal loss their cable exhibits, and if they're using active gain to counteract it, what that gain is. He said if they couldn't or wouldn't answer the question, that you should find another vendor. Remember, he was talking to big installation pros that do hundreds of installs every year, not DIY guys that hang up a projector and watch it for years.

I guess this all really makes the case for a good testing and certification program (with ongoing production sample testing).

SC
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