as an aside, are all HDMI cables the same in terms of connector interfaces?
My mum wants to buy an "HDMI" DVD player for her Sony HD TV, then get some HDMI cables for it (as they dont come with the player).
If I get, say, those 7dayshop ones, there's no chance I could accidentally end up with one that wont fit is there?
actually, capacitance on the cable will hack away at the trailing and leading edges of the waveform,(you're not actually sending 0's and 1's, it's a nominally square wave that represents 0's and 1's, being sent in an analogue format.)
Yes, but in digital signals the communication is "clocked" - the period during which each bit should be found is known, meaning you can identify a bit as a 0 or a 1 within that given timeframe, even if there is ringing and serious ramping on the signals at both ends. Add to that the fact that there is error correction transmitted too, you're looking at an all-or-nothing solution, distortion does not come into it with digital.
"Yes, but in digital signals the communication is "clocked" - the period during which each bit should be found is known, meaning you can identify a bit as a 0 or a 1 within that given timeframe, even if there is ringing and serious ramping on the signals at both ends. "
yup, I agree completely, that's exactly why I said "whether or not you can tell the difference is a different issue all together though. "
It's much more complicated than that. The data is transferred in packets with error correction, and will never get the the D/A's unless received correctly
Yeah but when an error is detected then the packet may need to be re-sent. You can only re-send a certain number of packets in any given time frame so there is a point where even with error correction, not every tile will have complete information.
To me it seems that digital cables either work or they don't, but my mate bought a pricey cable for his HD projector and he swears it gives him 'deeper blacks' 
The hifi mags seem to agree with him.
Yes all HDMI cables are the same in terms of connectors - it is a standard socket.
cheers m_f
yup, I agree completely, that's exactly why I said "whether or not you can tell the difference is a different issue all together though. "
Ah, now I read it as you meant it 
Yeah but when an error is detected then the packet may need to be re-sent.
Nope, it uses error correction, not error detection and resend - the data is encoded with extra "redundant" data that allows the receiver to recreate the correct info even if the packet appears to be damaged (to a fairly high degree). In this case the BCH form IIRC.
http://en.wikipedia.org/wiki/Error_detection_and_correction#Error_correction
and he swears it gives him 'deeper blacks'
The hifi mags seem to agree with him.
S'quite alright, I do it all the time
Right... the HDMI interface is designed to allow a certain error rate to occur and still work perfectly. There is enough error correction data supplied in the rest of the data that the erroneous bits can be corrected or ignored.
However, once you go over a certain error rate then the error correction cannot handle it and the whole thing grinds to a halt and you get a very distorted picture, or no picture at all. This is called the Cliff Effect.
The difference that a better quality cable will make is to allow you to use longer cable lengths before the error rate gets such that the picture dies entirely.
So yes, there will be less errors on a better quality cable, but this won't make any difference until you reach the threshold that stops a picture being shown at all!
So, if you use a cheap cable and get a picture, an expensive cable won't do anything different.
If you have a longer cable run, and a cheap cable gives you garbage, an expensive one might be able to give you a picture.
Simples...
Well done funkynick - only 10 minutes behind the thread 
but at least putting it into a normal-person-friendly package!
Hey... I'm also trying to work here!!
Nope, it uses error correction, not error detection and resend
Okay. However error correction can NEVER be 100% perfect (even if you send a second exact copy of the data then they could both still be wrong) so if you need to send a digital signal in a particular time frame then it is still possible for the transport medium to corrupt it to such an extent that a correct packet is not available in time.
But going back to the OP: nah as long as you don't go for the absolutely cheapest cable (which will probably be poorly made meaning the connections will break) then it will make **** all difference to the digital signals.
Don't underestimate the placebo effect. Just because there is no scientifically measurable improvement, doesn't preclude the owner of said "uber" cables from perceiving an improvement.
Interesting article here
http://www.audioholics.com/education/cables/long-hdmi-cable-bench-tests
At lengths less than 4 meters you can just about use silly string (OK, not really) and get HDMI to pass at any current resolution. At less than 3 meters you'll even extend that to 12-bit color and possibly the next crazy idea HDMI Licensing decides to throw at consumers. Don't spend a lot on these cables and if you want to save money you won't let anyone at a big box store talk you into buying from them.
12 bit?! Never heard of that before but obviously something new to worry about in a few years.
I was in a branch of Currys at the weekend and they had the same 1.5m HDMI cable - identical part number and all - for £25, £35, £40, £80 and £100.
We figured they were hoping that people who went in to buy a cable would find the "cheap" one and think they've got a bargain, and people who were buying a hugely expensive telly would just pick up the first one they saw and wouldn't care if it was expensive.
Or that Currys are just a bit rubbish.
but at least putting it into a normal-person-friendly package!
There are no normal people on this thread...
If I get, say, those 7dayshop ones, there's no chance I could accidentally end up with one that wont fit is there?
As mentioned, it's a standard connector.
My TV and Blu-Ray DVD combination make great play of the fact that the cable must be "something or other compliant" (read: expensive) in order for the two devices to talk to each other and be operated from one remote control. They work fine with the £3 cable.
I was in a TV shop the other day and asked about HDMI cables and the guy launched into the full sales spiel "they start at £25 but really you're best off with these at £60 because...."
but really you're best off with these at £60 because...."
I think you look like a mug?
I think you have money to spunk up a tree?
I think you weren't born with a functioning brain?
>so if you need to send a digital signal in a particular time frame
Statically that was correct, but if the whole lot's in spec then it's so statistically low as to be insignificant.
Tho' that's not how hdmi operates. Looking at the spec (I've a pdf of it here) it uses BCH for error correction on audio and control channels, and there's an additional level of coding for error reduction at the transmit/receive level.
There's some encoding done on the video stream data, but that doesn't appear to be for error reduction.
There *are* two categories of cable performance specified, called (funnily enough) Category 1 (data clock up to 340MHz) and Category 2(data clock up to 74.25MHz) - without looking further I'm guessing the latter relates more to NTSC/PAL (ie standard definition video) than anything else. That may be worth checking for on any cable you buy - tho' I don't know how/if they're marked up as that.
For compliant, read 'guaranteed to work as specified'. The spec specifies cable, connecter, transmit and receive performance, if everything is in spec it should work as intended
It doesn't mean non-compliant cables *won't* work, but a short run will probably work, but the same quality may be utter pants in a long cable length. I can't see a specied length in the spec, but it does specify attenuation and the two go hand-in-hand to a degree (the spec also covers cable equalisation where used). (Ah - funkynick's covered that bit...)
As a generalisation, there's lots of different error correction methods or ways to provide more robust transmission, they may work better in some circumstances than others.
Eg a cd - a scratch *along* an arc it will handly very badly, whereas a radial scratch it'll handle fine bacuse of the way the error correction's done.
>ah, phase ? That doesn't really make sense for impulse signals which are synthesised from numerous
Actually it does. What looks like a nice rising edge at the source may look really grotty at the receiving end depending on what the cable's attenuation/frequency and phase/frequency characteristics are.
There's an article/paper in scientific ammerican/new scientist/other publication on this.
Basicaly they created a synthetic (for want of a better word) signal and measured the output at the other end. The explanation as to why cheep cables don't work over long distances was quite in-depth, but was to do with the fact that the switching rate/frequency was so high that you no longer had a current flowing down the wire in the conventional sense, and it was in fact perfectly reasonoble to be sending the next bit of information down the wire before the previous one had reached the end.
They tested 3 manufacturers at three lengths, All 3 worked at <2m, although looking at the readings it was easy to spot the cheep one, the middling one was still pretty close to perfect.
At <5m the cheep cable was just sending jibberish, the middling cable would have been perfectly OK in most situations and the expensive cable was still good.
At 15m+ the expensive cable was the only one that worked, the signal looking comparable to the cheep one at <2m.
The hifi mags seem to agree with him.
Most on the ones I read tend to say there is no difference that can be noticed by human eye.
>ah, phase ? That doesn't really make sense for impulse signals which are synthesised from numerous
Actually it does. What looks like a nice rising edge at the source may look really grotty at the receiving end depending on what the cable's attenuation/frequency and phase/frequency characteristics are.
yes, but you don't measure rounding or delay in degrees. Phase only has a meaning at a fixed frequency.
I have a question. If you connect the x-box/blueray player to the tv with a good set of component leads then presumably the signal is D/A converted in the player, sent via the component as Analogue and then displayed? In which case, if the player is good, and has a better D/A converter than the TV then you could get a noticeably better picture this way than by connecting the player using HDMI, sending a digital signal and having it coverted to analgue by the cheaper D/A in the TV? Or have I got something wrong? (quite possible)
answer: flat panel TVs are digital all the way to the pixels :o)
Ah, so no D/A necessary. but...hang on....I don't see ones and zeros at each pixel so something converts the D to a colour and brigthness (or to a value for 3 clours) at each pixel surely!....that sounds like a D/A converter and I can't beleive the TV has 1680x1080 of them!
Am I being dense?
it appears I was wrong, it's only digital to the edge connector :o)
But 8 bit resolution isn't very demanding...
I bought expensive HDMI cables, because I am a mug. They're only useful over long distances.
Apparently analogue cables make little difference in most useages as well.
I was in a TV shop the other day and asked about HDMI cables and the guy launched into the full sales spiel "they start at £25 but really you're best off with these at £60 because...."
"....I get more money for it."
My brother works in Comet and his best commissions come from extra bits like cables. He could sell you a telly for £700 and a hdmi cable for £70 and still 'earn' far more in commission for the cable than the telly.
He could sell you a telly for £700 and a hdmi cable for £70 and still 'earn' far more in commission for the cable than the telly.
Mugs are those that spend that money on the stuff.
He could sell you a telly for £700 and a hdmi cable for £70 and still 'earn' far more in commission for the cable than the telly.
Its the rip off extended guarantee they try and sell you that they make the most on
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