July 14, 2024

HDMI and Output Control

Tim Lee at Tech Liberation Front points out an interesting aspect of the new MovieBeam device – it offers its highest-resolution output only to video displays that use the HDMI format.

(MovieBeam is a $200 box you buy that lets you buy 24-hour access to recent movies. There is a rotating menu of movies. Currently video content is trickled out to MovieBeam boxes via unused broadcast bandwidth rented from PBS stations. Eventually they’ll use the Internet to distribute movies to the devices.)

This is a common tactic these days – transmitting the highest-res content only via HDMI. And it seems like a mistake for Hollywood to insist on this. The biggest problem is that some HDTVs have HDMI inputs and some don’t, and most consumers don’t know the difference. Do you know whether your TV has an HDMI input? If you do, you either (a) don’t have a high-def TV, or (b) are a serious video geek.

Consider a (hypothetical) consumer, Fred, who bought an early high-def set because he wanted to watch movies. Fred buys MovieBeam, or a next-gen DVD player, only to discover that his TV can’t display the movies he wants in full definition, because his TV doesn’t do HDMI.

Fred will be especially angry to learn that his MovieBeam box or high-def DVD player is perfectly capable of sending content at higher definition to the inputs that his TV does have, but because of a bunch of legal mumbo-jumbo that Hollywood insists upon, his set-top box deliberately down-rezzes the video before sending it to his TV. Just imagine what Fred will think when he sees news stories about how pirated content is available in portable, high-def formats that will work with his TV.

The official story is that HDMI is a security measure, designed to stop infringers. It’s been known for years that HDMI has serious security flaws; even Wikipedia discusses them. HDMI’s security woes make a pretty interesting story, which I’ll explore over several posts. First I’ll talk about what HDMI is trying to do. Then I’ll go under the hood and talk about how the critical part of HDMI works and its well-known security flaws. (This part is already in the academic literature; I’ll give a more accessible description.) Finally, I’ll get to what is probably the most interesting part: what the history of HDMI security tells us about the industry’s goals and practices.

Officially, the security portion of HDMI is known as High-bandwidth Digital Content Protection, or HDCP. The core of this security design is the HDCP handshake, which takes place whenever two devices communicate over an HDMI cable. The handshake has two goals. First, it lets each device confirm that the other device is an authorized HDCP device. Second, it lets the two devices agree on a secret encryption key which only those two devices know. Subsequent communication over the cable is encrypted using that key, so that eavesdroppers can’t get their hands on any content that is distributed.

In theory, this is supposed to stop would-be infringers. If an infringer tries to plug an authorized video source (like a MovieBeam box) into a device that can capture and redistribute video content, this won’t work, because the capture device won’t be able to do the handshake – the authorized video source will recognize that it is unauthorized and so will refuse to sent it content. Alternatively, if an infringer tries to capture content off the wire, between an authorized source and an authorized TV set, this will be foiled by encryption. That’s the theory at least. The practice is quite different, as I’ll describe next time.


  1. Mark McKenna says

    @Sewell Direct:

    Heh — “Psst, wanna buy a signal boosting cable from me to counter problems you wouldn’t have if you didn’t use HDMI in the first place? BTW, you should really thank the HDMI people for a nebulous and unfounded reason.”

  2. New to HDTV have only 2 hdmi inputs what can I do to have more hdmi inputs. is there some kind of y cables or box or something. thxs for helo hdtv novice

  3. “High Definition wouldn’t be so prevalent if it wasn’t for HDMI”- Sewell Direct

    That’s simply not true.

  4. So why can’t consumers launch a class action lawsuit against manufacturers selling equipment deliberately disabled to work with equipment some of the same manufacturers sold just a few years ago with the promise of being able to watch HD? If you bought a HDTV and cannot view it, what good is it to have bought it? MANY folks were sold on HD “ready”. Hmmm….Not really HD ready if the only method to get it there doesn’t exist on the TV.

  5. Don’t forget that HDMI signals lose quality the longer they go.
    We started selling the Gefen line of Super Booster HDMI cables that have built in active repeaters. The longest one from them is the 150 ft. length cable that still maintains 1080p.

    As far as the guy with the super long post up above-
    High Definition wouldn’t be so prevalent if it wasn’t for HDMI.

  6. I heard somewhere that all Blu-Ray devices will only offer their full res in HDMI- can anyone confirm this?

  7. Voice of Wisdom and Reality says

    Folks, I have been in the CE business for several years, understand the contracts and agreements for all this stuff, and have been designing DVI and HDMI circuits, so I know a thing or two about all of this. The author has it pretty much correct, but the blog post is short enough to leave out a lot of the causes of these problems. I will try and fill in a few blanks.
    HDMI is the creation of Silicon Image, and SI is pretty much the sole beneficiary of this technology. It is a completely unnecessary technology.

    HDMI does not offer a better image to the consumer – Side by side tests by many consumers confirms this. In some instances a display manufacturer has botched up their implementation of component video, and as a result HDMI may look better, however there are an equal number of reverse cases where HDMI looks worse. Analog video and related transmission circuits are highly refined, and are perfectly capable of resolutions beyond 1920×1080.

    HDMI transmission distance is less than component video – HDMI uses high speed serial digital lines, which uses 1.4Gbs for 1080p, which requires a cable bandwidth of at least 3GHz. Component video has a video bandwidth of 74MHz for 1080p, and requires a cable bandwidth of 150MHz. HDMI uses twisted pairs, and component uses coaxial cable. Anyone with an engineering background can plainly see why HDMI is length limited to maybe 30 feet is your are lucky. Fiber optic solutions to go 300 feet exist, at a cost of about $2,000. Component over coaxial cable on the other hand can go 300 feet with a quality Belden RG6, for a fraction of the cost.

    HDMI is compatible with less displays and less interconnecting equipment, as the author and others have noted.

    HDMI consumer marketing includes simple things like audio and video in one cable (saves a $2 audio cable), and uncompressed video (from a compressed source device).

    In summary, there is NO REAL CONSUMER BENEFIT TO HDMI.

    Silicon Image, knowing they could not promulgate HDMI by themselves, smartly created HDMI LLC with leading CE manufacturers, based on the promise of HD content from Hollywood. The big CE manufacturers know they will sell equipment based on the big HD carrot. Rather than create an well thought out, well engineered, and best for the industry solution, they cobbled together a solution with existing property to make the vested parties happy, at the cost of the unsuspecting consumer.

    However this was a short sighted plan, that will only hurt Hollywood, and hurt the industry. Consumers are becoming increasingly aware of these issues. If the issue is pushed (i.e. disabling of the analog video world), consumers will be forced towards other solutions, and possibly other content that does not use such restrictions.

    To answer another point made by Wes Felter and others as to why HDCP has not been cracked. Cracks and hacks are born of need. Currently there is no point in cracking HDCP, because once you have cracked it, you are left with plain HDMI, which is still too much data to do anything with. There is no cost effective way to record it. The bottom line is there are easier ways to get to the source content. You can use DeCSS if the content is DVD, and you can hack DirecTV boxes to get to transport streams if you want HD. In the short future, we will see DeACSS to allow copying of HD-DVD and Blu-Ray. So there really has been no need to crack HDCP, nor will there ever be.

  8. winemaker says

    With component video cables, you won’t be able to run very far without irrecoverable signal degradation. Besides that, it would be a pretty massive cable to cover 25 ft. Wire diameter and dielectric diameter must increase to mediate skin and dielectric loss.

    Once the dynamic range is diminished, you can’t get it back. Sure you can post-amp it, however, you will also be amplifying the noise.

    The industry needs a better quality cable.

  9. winemaker says

    HDMI is NOT a lossless transfer mechanism. The HDMI spec indicates that a BER of 10e-9 is the link quality, i.e. 1 bit per billion error free. On 1080p, this means you can have pixellation or other artifacts every couple of seconds. Cables have loss, i.e. skin and dielectric loss, which cause degradation via eye closure, and this doesn’t even begin to address things like far or near end crosstalk.

    Scaling will ALWAYS create artifacts. Especially on embedded text, aliasing filters only mitigate this somewhat.

    Motion blurring isn’t an artifact of the cable, usually an artifact of the chipset inside the TV. Macroblocking might be interpreted like this again due to bit errors inside the high speed digital link

  10. A “magnification blending filter” is exactly the same as an anti-alisasing filter.

    Not exactly, but similar. They’re both low pass filters, but in general an antialiasing filter is different from the corresponding perfect reconstruction interpolation filter. But I’m splitting hairs.

  11. Ozjuggler,
    A “magnification blending filter” is exactly the same as an anti-alisasing filter. If you up-sample a signal, you then need to eliminate the spurious frequencies that you’ve generated, just as you need to do before down-sampling. The filter’s frequency response is the same (assuming the same amount of up/down sampling), the only difference between anti-aliasing and up-interpolating is when the filter is applied relative to the change in sampling rate.

    You sound pretty full of yourself for someone who is obvously full of crap.
    “So who is it that doesn’t know what they’re talking about?”


  12. Andrew Pierce says

    VEGA: “And for the person wanting to run the video through your A/V receiver, don’t do it. Most receivers have crappy chips in there that will compromise the video anyway”

    HDMI is a digital transfer method. The very idea of HDMI is to allow you to transfer audio and video around between your electronics with zero degradation, regardless of the quality of your cables, the chips in your receiver, etc. That is to say, as long as the 1s and 0s arrive at their destination still distinguishable from each other the receiving device can reconstruct it EXACTLY as the sender issued it.

    The fact that the signal can be routed through your switch/receiver without loss is in fact one of the nicest benefits of an all digital implimentation, and an improvement over analog component cables.

    All of this _should_ be independent of the HDCP implimentation, but of course if we give the content the power to restrict what sort of devices their content can talk to, don’t expect your intrests to supercede theirs.

  13. All this hoopla over HDMI and what a superior technology it is overlooks one of the ugly limitations – that of cable length. If you have (as I do) most of your badass computers in your office and you have (as I do) your big TVs over in the living room, how then are you to get the nifty content from one to the other? Beaming it over a home network? Nuh-uh. That’s the thing that gives the Studio Weasels the heebie-jeebies.

    So fine; I’m supposed to use an HDMI cable from the computer to the big 16×9 HDTV. Sigh. So now I’ve gotta buy a long HDMI cable to link the box to the pretty thing.

    But wait! Because of the nature of the unshielded twisted-pair cables in HDMI, and cable that’s longer than 25 feet (uh, honey? can you get the tape measure?) starts to cause some serious degradation of signal. As in: massive artifacting. Motion blurring the way you get when you render to WMV for 56k. Warbling audio that sounds like a nursery of robot babies is singing faintly in the background.

    F*** the HDCP. And the HDMI. I’ll stick with the comp video cables, which have a strong, strong, robust signal that can punch thru and deliver the clear and hi-res pic that is supposedly the selling point of all this high-price-tag tech.

    I predict: lots of gear coming back to Best Buy within the 30 day return period in the next couple of years.

  14. Chicago Joe says

    Supercat said:
    “This would have allowed the creation of such players in Linux and other open-source systems since the computer wouldn’t need to decrypt anything and thus there would be no need to keep the computer software secret.”

    paulc said:
    “I suspect it’s deliberate colusion between Microsoft and the content providers to lock Linux out of the party…”

    I doubt it. Sony is a big supporter of both HDMI & HDCP and a lot of their “HD-capable” (and better) televisions are running a version of Linux…RTFM.

    Also, Lawrence is correct. HDMI can exist just fine without all the DRM crap that is HDCP. HDCP is the “technology” that is beset with security holes and is really keeping everyone from enjoying the content at it highest possible quality.

  15. I recently purchased a Sharp HDTV with an HDMI input and a Denon receiver to serve as an analog upconverter and HDMI router. When we got our Scientific Atlanta 8300HD cable box from Time Warner I discovered the SA wouldn’t put the picture out on HDMI to the Denon, just a message claiming the Denon wasn’t an HDCP device. Boy was I pissed.

    The good news is Time Warner just did a firmware refresh on our cable boxes and the SA will now put the picture out the HDMI jack to the Denon. I’m hoping all of this early HDCP crippling of legitimate users will go away in a similar fashion.

  16. Please stop writing about HDMI as though it’s the same thing as HDCP. HDCP is the problem; it was not included in the initial HDMI standards and HDMI is a really nice set of technologies- audio and video in one small package. If only we could line-item veto the HDCP bullshit.

  17. Supercat said:
    “This would have allowed the creation of such players in Linux and other open-source systems since the computer wouldn’t need to decrypt anything and thus there would be no need to keep the computer software secret.”

    I suspect it’s deliberate colusion between Microsoft and the content providers to lock Linux out of the party…

  18. VEGA babbled: “All upscaling it is going to do is double the existing pixels and put an antialiasing filter on it.”

    No, not an antialiasing filter – a magnification blending filter. Antialiasing is only useful when you *downsample* a large image to a small one. So who is it that doesn’t know what they’re talking about?

  19. Vega, you say: it seems the author of this story really didn’t know what they were talking about on certain issues. DVD’s and most movies can only go up to 480 i/p.

    Except the article clearly calls out next-gen and high-def for all DVD player references. These may not exist now, but they are coming, and they will supposedly incorporate the HDMI technology limitations that Ed is talking about. The author is ahead of you.

  20. VEGA: Ed is talking about the FUUUUUUUUUUUUUTUUUUUUUURE. I.e., HD-DVD, BluRay, XBOX 360, PS3, and beyond.

  21. It seems the author of this story really didn’t know what they were talking about on certain issues. DVD’s and most movies can only go up to 480 i/p. You can’t get HD out of that. All upscaling it is going to do is double the existing pixels and put an antialiasing filter on it. You have to start with HD to get HD. And most true HD stuff is downrezzed before it even leaves the broadcasting tower anyway to fit bandwidth specs. So your not getting the true HD as it was intended anyway. And if you don’t like HDMI, use DVI or the RGB. They both can be digital. Need to format convert? Run it all through a scaler box. That simple. And for the person wanting to run the video through your A/V receiver, don’t do it. Most receivers have crappy chips in there that will compromise the video anyway. Just run the video direct to your TV and put a macro on your remote to switch everything at once. You’ll get better quality.

  22. A breaking story today is that Michael Jackson has reached a deal to sell his 4,000-song catalog to Sony. The relevence of this news to the “HDMI and Output Control” is that content, music in this case, may be released in a proprietary (DRM) based technology. As Fred von Lohmann notes in his post, consumers may not realize that their content is being “disabled”.

    What struck me a particulary outrageous, while watching the news, was the reporting that the music would not work on a variety of devices because of “incompatiblity”. With this type of industry shilling by reporters, consumers will be left unaware that the RIAA and the MPAA are purposely preventing content from playing on hardware that is perfectly capable of playing the content.

  23. The requirements for viewing high-definition DVDs on a computer are even more outrageous. What makes that part of the requirements ironic is that the current implementation requires all sorts of fancy-shmantzy code on the computer (which is all proprietary, can’t run under Linux, etc.). A simpler approach would have been to provide a means by which the computer would exchange encrypted communications between the DVD drive and the monitor without having to decrypt them in the middle. A standard protocol could have been defined to do things like requrest that a DVD window be overlaid at a particular location on the screen, etc. This would have allowed the creation of such players in Linux and other open-source systems since the computer wouldn’t need to decrypt anything and thus there would be no need to keep the computer software secret.

  24. If the pleb isn’t expected to appreciate the lustre of the Emperor’s garb, then we’re left with the legit geeks with fat wallets and the illegit geeks without.

    Not exactly a mass market, especially if the illegit geeks are then inspired to facilitate the pleb producing legit hi res videos via http://www.videoegg.com

    How ironic, that Hollywood sells expensive low res and free culture supplies free hi res.

    Hollywood is anally retenting itself out of the market.

  25. I hate to raise this, but it may be that Hollywood is counting on people who are not “serious video geeks” not noticing that video is down-rezzed on analog outputs. Down-rezzed outputs are generally limited to 480p (DVD quality), which isn’t so bad. On screens below 50 inches or so, many casual viewers may not realize that they are not seeing all the resolution to which they are entitled, especially if they’ve never seen true HD in their living rooms.

    I’m reminded of my own parents, who were amazed at the increased video quality when I changed all the connections behind the TV to the higher resolution options (S-Video, etc). Never occurred to them that anything was wrong until I fixed it.

  26. Todd Jonz says

    There appears to be no end of “gotchas” with HDMI. It was only just recently I learned that, depending on the device, it may or may not be possible to pass an HDMI output through an HDMI router. The case in point is the current generation of DirecTV receivers, which reportedly cannot be connected to a switching A/V receiver, i.e. they must be connected directly to the television or monitor. (“Daddy, would you crawl behind the TV again and plug in my XBox?!”) Could this be DirecTV’s way of marketing its proprietary DVRs by precluding us from connecting our own DVRs in the HDMI daisy chain? I guess we’ll find out when their next generation of MPEG4-capable receivers is released this summer.

  27. Wes Felter says

    I regard the HDCP cracks as theoretical too, since I’ve seen no reports of anyone actually implementing them. If it’s so easy to extract the master key, why hasn’t it been posted on the Web yet?