有人说HDMI线只要通效果都一样（线圣高级副总裁答疑16页229楼）（请大家文明讨论）

护戒使者

有没有英文好的给翻译下大概意思[s:18]

HiViUser

Impedance, in particular, becomes a really important concern anytime the cable length is more than about a quarter of the signal wavelength, and becomes increasingly important as the cable length becomes a greater and greater multiple of that wavelength. The signal wavelength, for one of the color channels of a 1080p HDMI signal, is about 16 inches, making the quarter-wave a mere four inches--so impedance is an enormous consideration in getting HDMI signals to propagate along a cable without serious degradation.

Impedance is a function of the physical dimensions and arrangement of the cable's parts, and the type and consistency of the dielectric materials in the cable. There are two principal sorts of cable"architecture" used in data cabling (and HDMI, being a digital standard, is really a data cable), and each has its advantages. First, there's twisted-pair cable, used in a diverse range of computer-related applications. Twisted-pair cables are generally economical to make and can be quite small in overall profile. Second, there's coaxial cable,where one conductor runs down the center and the other is a cylindrical "shield" running over the outside, with a layer of insulation between.Coaxial cable is costlier to produce, but has technical advantages over twisted pair, particularly in the area of impedance.

It's impossible to control the impedance of any cable perfectly. We can, of course, if we know the types of materials to be used inbuilding the cable, create a sort of mathematical model of the perfect cable; this cable has perfect symmetry, perfect materials, and manufacturing tolerances of zero in every dimension, and its impedanceis fixed and dead-on-spec. But the real world won't allow us to build and use this perfect cable. The dimensions involved are very small and hard to control, and the materials in use aren't perfect; consequently,all we can do is control manufacturing within certain technical limits. Further, when a cable is in use, it can't be like our perfect model; it has to bend, and it has to be affixed to connectors.

So, what do we get instead of perfect cable, with perfect impedance? We get real cable, with impedance controlled within some tolerance; andwe hope that we can make the cable conform to tolerances tight enough for the application to which we put it. As it happens, some types of impedance variation are easier to control than others, so depending onthe type of cable architecture we choose, the task of controlling impedance becomes harder or easier. Coaxial cable, in this area, is clearly the superior design; the best precision video coaxes have superb bandwidth and excellent impedance control. Belden 1694A, forexample, has a specified impedance tolerance of +/- 1.5 ohms, which is just two percent of the 75 ohm spec; and that tolerance is a conservative figure, with the actual impedance of the cable seldom offby more than half an ohm (2/3 of one percent off-spec). Twisted pair does not remotely compare; getting within 10 or 15 percent impedance tolerance is excellent, and the best bonded-pair Belden cables stay dependably within about 8 ohms of the 100 ohm spec.


If we were running a low bit-rate through this cable, it wouldn't really matter. Plus or minus 10 or 15 ohms would be "good enough" and the interface would work just great. But the bit rate demands placed on HDMI cable are severe. At 1080i, the pixel clock runs at 74.25 MHz, and each of the three color channels sends a ten-bit signal on each pulse of the clock, for a bit rate of 742.5 Mbps. What's worse, some devices are now able to send or receive 1080p/60, which requires double that bitrate.

Impedance mismatch, at these bitrates, causes all manner of havoc. Variations in impedance within the cable cause the signal to degrades ubstantially, and in a non-linear way that can't easily be EQ'd or amplified away. The result is that the HDMI standard will always be faced with serious limitations on distance. We have found that, at 720p and 1080i, well-made cables up to around 50 feet will work properly with most, but not all, source/display combinations. If 1080p becomes a standard, plenty of cables which have been good enough to date will fail. And it gets worse...

[s:8][s:8][s:8]

[ 本帖最后由 HiViUser 于 2008-8-23 13:00 编辑 ]

HiViUser

In June 2005, the HDMI organization announced the new HDMI 1.3 spec.Among other things, the 1.3 spec offers new color depths which require more bits per pixel. The HDMI press release states:

"HDMI 1.3 increases its single-link bandwidth from 165MHz (4.95 gigabits per second) to 340 MHz (10.2 Gbps) to support the demands of future high definition display devices, such as higher resolutions, Deep Color and high frame rates."

So, what did they do to enable the HDMI cable to convey this massive increase in bitrate? If your guess is "nothing whatsoever," you're right. The HDMI cable is still the same four shielded 100-ohm twisted pairs, still subject to the same technical and manufacturing limitations. And don't draw any consolation from those modest "bandwidth" requirements, stated in Megahertz; those numbers are the frequencies of the clock pulses, which run at 1/10 the rate of the data pairs, and why the HDMI people chose to call those the "bandwidth" requirements of the cable is anyone's guess. The only good news here isthat the bitrates quoted are the summed bit rates of the three color channels -- so a twisted pair's potential bandwidth requirement has gone up "only" to 3.4 Gbps rather than 10.2.

[s:31][s:31][s:31]

[ 本帖最后由 HiViUser 于 2008-8-23 13:01 编辑 ]

九十九龙马

[s:16] 头大如斗 乌云盖顶

HiViUser

What's to be Done?

It's unlikely, given the whole hearted way in which the consumer electronics industry has embraced HDMI, that this interface will disappear anytime soon. We're stuck with it. Given that that's so, what can a person do to avoid problems with video dropouts and outright signal failure?

First, limiting run lengths is a good idea whenever it can be done.If you don't need to put your sources at one end of the room and the display at the other, by all means avoid doing so.
         
Second, if run lengths can't be limited, consider relying on analog component or RGBHV signals for your distance runs; these formats aremuch more robust (in large part because they run in coax rather than intwisted pairs) and can be run hundreds of feet.

Third, eliminating unnecessary switches, couplers, and adapters mayhelp; as bad as the impedance mismatch problems are in the cable itself, those problems are even worse when the cable's conductors mustbe split out to join to a connector, or when the signal must travel through connections that can't be kept at 100 ohms.

Fourth, there are some things that can be done in cable design. Most of the HDMI cables on the market were Chinese-made, and many of them were designed more for low cost than forhigh performance. Better design in cable construction and connector need to be reviewed for ultimate performance of superior quality.

[s:97][s:97][s:97]

[ 本帖最后由 HiViUser 于 2008-8-23 11:36 编辑 ]

xtal

165MHZ还是不要做梦了
到现在为止,总共也没有几个型号的显卡能够跑到146MHZ时钟的.

HiViUser

HDMI Myths and Misconceptions: (HDMI 神話 和 誤解)

"Because HDMI is a digital signal, it doesn't degrade when run over a long distance like an analog signal does, because it's just ones and zeros."

Yikes! Not true at all. To explore this issue calls for a bit more detailed discussion.

First, it's true that if a digital video signal stays intact from one point to another, there's no degradation of the image. The digital signal itself can degrade, in purely electrical terms, quite a bit over a distance run, but if at the end of that run the bitstream can be fully and correctly reconstituted, it doesn't matter what degradation the signal suffered--once that information is reconstituted at the receiving end, it's as good as new.

That's a big "if," however. Ideally speaking, digital signals start out as something close to a "square wave," which is an instantaneous transition from one voltage to another; these transitions signal the beginnings and ends of bits. (In practice, such transitions aren't strictly possible, and trying to achieve them can generate harmful noise; consequently, high-order harmonics are usually filtered out which results in the wave starting out squarish but not-quite-square.) A square wave, unfortunately, is impossible to convey down any transmission line because it has infinite bandwidth; to convey it accurately, a cable would have to convey all frequencies, out to infinity, all at the same level of loss ("attenuation"). What happens, therefore, in any run of cable is that a digital signal starts out looking relatively nice and somewhat square, and comes out the other end both weaker and rounded-off. The transitions that mark the edges of bits get smoothed and leveled to the point that, far from that ideal square wave, they look like relatively gentle slopes. Portions of the signal lost to impedance mismatch bounce around in the cable and mix with these rounded-off slopes, introducing an unpredictable and irregular component to the signal; crosstalk from the other pairs in the HDMI bundle also contribute uneven and essentially random noise. As a result, what arrives at your display doesn't look very much like what was sent.

Now, as we've said, up to a point, this won't matter; the bitstream gets accurately reconstituted, and the picture on your display is as good as the HDMI signal can make it. But when it starts to fail, it starts to fail conspicuously and dramatically. The first sign of an HDMI signal failure is digital dropouts--these are colloquially referred to as "sparklies"--where a pixel or two can't be read. When these "sparklies" are seen, total failure is not far away; if the cable were made ten feet longer, there's a chance that so little information would get through that there would be no picture on the display at all.

The shame is that, with HDMI, this is prone to happen at rather short lengths. When DVI was first introduced (same encoding scheme, same cable structure, but a different connector from HDMI), it was hard to find cables that were reliable in lengths over 15 feet. The fact that these multipin cables aren't economical to manufacture in the US and so were being produced exclusively in China, too, didn't help; Chinese cable manufacturers are very good at keeping costs down, but not the best at keeping tolerances tight. Today, a good HDMI cable can be relatively reliable up to about 50 feet, but because different devices tolerate signal degradation differently, it's impossible to say categorically that a 50 foot cable will work; it's only possible to say that it will work with most devices.

Why is that? Well, it all has to do with bad design. The designers of the HDMI standard didn't really think much about the cable problem, and it shows. This topic is fairly complex in itself, so we've split it out into a separate article: What's the Matter with HDMI Cable?

Analog video signals, contrary to what seems to be the conventional wisdom in home theater circles, are extremely robust over distance. We have run component video for hundreds of feet without observable degradation; the bandwidth of precision video coax, rather than being horribly overtaxed like that of an HDMI cable, is greatly in excess of what's needed to convey any HD signal. It is true that an analog signal degrades progressively with length; but that degradation, in practice, is so slight and slow that it rarely gives rise to any observable image quality loss in home theater applications.

[s:68] [s:68] [s:68]

九十九龙马

原帖由 xtal 于 2008-8-23 11:36 发表
165MHZ还是不要做梦了
到现在为止,总共也没有几个型号的显卡能够跑到146MHZ时钟的.

那蓝光机的显卡为什么可以实现？

xtal

原帖由 九十九龙马 于 2008-8-23 11:40 发表


那蓝光机的显卡为什么可以实现？

24HZ逐行输出啊,靠平板电视内部电路整倍数倍频

HiViUser

What Makes one HDMI Cable Better than Another, and Does it Matter? (甚麽能令 HDMI 線优於其他，又是否重要呢？)

HDMI cable quality is a bit complicated, and unfortunately, it's hard to judge from a spec sheet, especially because very few manufacturers provide any useful product specs. There are a few things to bear in mind.

At present, to our knowledge, most of the HDMI cables are built in China. Don't let the fact that an HDMI cable bears a U.S. brand name lead you to believe that that HDMI cable contains American products, American labor or American know-how; none of them, other than ours, do. And China may be an easy place to get a good price, but it is not a good place to get a leading-edge technological product; for top-quality data cables (and HDMI is a data cable).

The Chinese source problem makes it very hard to get a spec sheet, and very hard to know what that spec sheet means, when dealing with an HDMI cable. Most vendors of HDMI cable in the US don't know what attributes would make a good HDMI cable, and since they don't participate in the manufacture beyond specifying jacket printing and the shape of the molded connector, they don't really have much reason to find out. The result is that most citations to product spec that one finds in connection with the sale of HDMI cable are references to the product's wire gage. Wire gage is somewhat meaningful, but judging HDMI cable quality by comparing wire gage is like judging automobile quality by comparing engine block length--a very, very inexact way of looking at the problem.

The primary work of an HDMI cable is done by the four shielded twisted pairs which carry the color, sync, and clock signals. The designers of the HDMI standard made an inexcusable error of judgment in running these signals balanced, in twisted pairs, rather than unbalanced, in coaxes; attenuation (the tendency of the signal to get weaker with distance) is much greater, and impedance and timing are harder to control, in twisted pairs than in coax. Control of the cable impedance is critical to keeping the rounding of the bit edges under control; the more the impedance wanders off of spec, the more the signal will round, and the closer the cable comes to failure. Where a coaxial cable's impedance can be controlled within two percent of spec, it's a challenge to keep a twisted pair any tighter than about 15% plus or minus.

The HDMI signal will fail if attenuation is too high, or if the bit transitions become excessively rounded so that the receiving unit can't reconstitute them accurately. There's no really reliable benchmark for just how much attenuation is acceptable, or how round the shoulders can be, before the "sparklies" will start. (Yes, there are specs for these things in the official HDMI spec document, but real-world devices vary so much that meeting the spec is no guarantee of success, while failing it is no guarantee of actual failure.) But while wire gage has something to do with the former, it's really the latter that's important; and wire gauge has nothing to do, at least directly, with impedance control.

[s:18] [s:18] :Q :Q

HiViUser

Transmission line impedance, in any cable, is dependent on the cable's materials and physical dimensions. For purposes of an HDMI cable, these are:

1. the shape and size of the paired wires;
2. the thickness, and dielectric properties, of the insulation on the paired wires;
3. the dimensions of the shield over the pair.

These seem, in principle, like simple things to control--that is, untilone spends a bit of time in a wire and cable factory and finds out just how many little problems there are. Wire is never perfect; its dimensions and shape vary from point to point, and small dimensional variations can make for significant impedance changes. Wire can suffer from periodicity (in fact, strictly speaking, it not only can, but always, at some level, does) because it's been drawn over a wheel that was microscopically out-of-round, and that periodicity will cause thewire to resonate at particular wavelengths, which can really wreak havoc.


The plastic dielectric has to be consistently extruded to the correct diameter (and thousandths of an inch matter here!); if it's foamed, it needs to have highly consistent bubble size so that one side of the dielectric isn't airier than another, or one foot airier than the next. The two wires in the pair need not to wander in relation toone another; as they "open up" or are pressed tightly together because of tensioning on the wire-twisting machine (or tension applied to thecable by other handling, or by shield application, or...), or because the finished cable is being flexed, the impedance changes.


The shieldis a factor in the impedance as well, because both signal wires havecapacitance to the shield, and if the foil is wrapped more tightly inone place and more loosely in another, that, too, will cause impedanceto vary. (And these are just a few of the obvious problems;manufacturing processes involve other problems that nobody not involved in manufacturing would ever think of. For example, the lube that's used to assist in wire drawing needs to be washed off the wire before dielectric is extruded over it; what if the side from which a jet of cleaner is fired at the wire gets cleaner than the opposite side, andthe dielectric winds up conforming differently to one side of the cable than the other? What about the other thousand things you and I, not working in a wire factory, have never even begun to think about?) As a result, although every manufacturer's HDMI cable is built to meet anominal 100 ohm characteristic impedance, every foot of every cable is different from every other. The best one can do is to hold impedance within a range, centered on 100 ohms; the official HDMI spec calls for 100 ohms plus or minus 15%, which for a coax would be horribly sloppy.The tighter that tolerance can be kept, the better the performance willbe.

Worse still, impedance is not a one-dimensional characteristic. HDMI cable operates over an enormous frequency bandwidth, and impedance in atwisted pair is frequency-dependent (in a coax it is, too, but far, farless so). A twisted pair's impedance will rise relative to frequency;how much it will do so, and how evenly and regularly, will depend uponsubtle physical characteristics. So, strictly speaking, no cable can actually be within tolerance for impedance over the whole operating range of the cable; it can only be within tolerance by the method thespec designates for measurement.

Impedance control is important for another reason: timing. As impedance varies, so will the time it takes a signal to travel down thecable. Electricity travels at nearly the speed of light; how close tothe speed of light it travels depends on the dielectric, and isreferred to as the "velocity of propagation." The objective, in putting together the four pairs in an HDMI cable, is to have them be identical;but in actual practice, each pair in a four-pair set will have its own delay. If the delay of one pair is sufficiently greater than the delayof another pair, the receiving device will not know which "red" pixel belongs to which "blue" and "green" pixel, or if the clock circuit isoff, it may be impossible to time any of the color signals reliably.Since this delay depends on the consistency and dimensions of the dielectric, and the consistency and dimensions of the dielectric are important factors in impedance, the same requirement for consistent impedance applies here; if impedance is too inconsistent, timing will be too inconsistent, and the whole system will fail.

[s:33][s:33][s:33]

[ 本帖最后由 HiViUser 于 2008-8-23 12:00 编辑 ]

HiViUser

One way of looking at cable performance is to chart the attenuation fora given length of cable against frequency. For any cable, attenuation (measured in dB) will increase with frequency; this attenuation comes from a few factors. Loss to resistance goes up with frequency, because higher frequency signals are able to use less and less of the cross-section of the wire (this is known as "skin effect") and so have less copper to travel through. Losses to reactance -- capacitance andinductance -- also increase with frequency. Then, what we call "return loss" adds the most irregular, and difficult-to-control, component to the loss. "Return loss" is the loss to impedance mismatch, and is socalled because it represents the portion of the signal which is lost when, upon encountering a change in the impedance of the circuit (this may be a change in impedance along the cable, or a change of impedance on entering or leaving a connector, or a circuit board trace, or encountering a different impedance than expected at the load end of the circuit), it reflects back along the cable towards the source rather than being delivered to the load. While basic resistive and reactive losses are pretty reliable and have a definite relationship to frequency, return loss can be quite irregular. A graph of return lossagainst frequency, rather than showing a nice, consistent curve, is characterized by sharp, spiky lines. Why is this? Well, return loss hasto do, more than anything else, with those manufacturing tolerances andtheir impact upon impedance. Every wire, at some level, has some periodicity, and so resonates somewhat at some unintended frequency.Every dielectric extruder fails, at some level, to extrude the dielectric consistently; every spooler that winds wire or dielectric-covered wire, every wire twister, every unreeler thathandles that wire as it goes back into another stage of processing,every foil-wrap and drain-wire machine, every planetary cabler (which bundles and twists the pairs together with one another), every jacket handler and extruder--all of these machines, in all of these processes,apply microscopic irregularities to the cable which show up as returnloss. Return loss can't be eliminated, at least not in a real-worldcable; but it can be, within limits, made as small and as consistent across a range of frequencies, as possible.

[s:20][s:20][s:20]

[ 本帖最后由 HiViUser 于 2008-8-23 11:55 编辑 ]

HiViUser

Generally speaking, devices handle very linear or predictable losses very well. If one knows that one part of a signal will come in a thousand times weaker than another part, it's easy to "EQ" the incoming signal to boost the weak part to match the level of the strong part.But return loss can't be EQ'd out because it's too uneven and unpredictable.

Return loss, not resistance, is the critical consideration indetermining the quality of an HDMI cable; if one were comparing cables with similar resistance, capacitance, and inductance values against one another, and consulting a chart of attenuation relative to frequency,what one would generally see would be that cables with superior returnloss characteristics would show a flatter attenuation curve than theothers. This is very important in HDMI because the required bandwidth for an HDMI signal is enormous, and the higher the frequency, the harder it is to control return loss.

Generally, in looking at HDMI cable products currently available onthe market, we've found that these issues get overlooked. Instead oftrying to control impedance well, which will result in flattening the curve on the attenuation chart, manufacturers generally try to control resistance. Why? Well, resistance is a lot easier to control. Bigger wire (smaller AWG number) has less resistance, and choice of materials can play a role, too (silver-plated copper is lower in resistance thanbare copper, and bare copper is lower in resistance than tin-plated copper, for example). But as the frequency demands placed on the cable increase, bigger wire doesn't really help all that much (and, for awhole slew of reasons having to do with manufacturing process control,it can actually hurt), because it's not the total loss that's limiting performance; it's the non-linear component of the loss that's the real problem. With return loss specs not generally available for Chinese-sourced cable, one often can't get a good idea exactly whatbasis there is for comparison between two HDMI cables.

[s:30][s:30][s:30]:victory::victory::victory:

[ 本帖最后由 HiViUser 于 2008-8-23 11:54 编辑 ]

xtal

TMDS传输技术很古老了,当年缓存很贵,因此只做成准串行,
结果JITTER的容差搞的要求很高
(必需小于每个数据位周期的1/4,而纯串行是1/2)

然后HDMI发明的时候本来就是存心不良,嫌DVID的接插件太贵,想省钱.
然后就改了一下接插件,其他I/O部分硬件电路纹丝不动,编码电路部分就加入了音频信号的打包还有高色深支持.
但是好死不死的,居然设计这个规范的时候,不把接插件厂家扯进来,结果居然是一个没有锁扣的接插件
笑死了

九十九龙马

原帖由 xtal 于 2008-8-23 11:45 发表

24HZ逐行输出啊,靠平板电视内部电路整倍数倍频

PC显卡也可以这样啊