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How much to spend on an HDMI cable
One of the most contentious topics on any AV related bulletin board these days is the one around whether spending money on an HDMI cable is worth it. I may as well make my position clear and say I think it is and here's why.
Firstly it seems odd to me that after spending hundreds of pounds , perhaps more , on a couple of items of home entertainment the one piece of equipment joining them together should not be given some consideration. Maybe it's the use of the word digital and the overlaid assumption that digital means perfect, but all digital means is that the signal can be represented in numeric form and if we are to be pedantic and talk about a binary digital transmission it just means that those 'digits' would be limited to being represented as either 0 or 1.In the absence of anything else to go on, however, all that can assure you is that the resultant transmission of that single bit of information is either wholly right or wholly wrong.
So the fact that the transmission is digital is not , in itself , an assurance of its integrity.
The advantage of a digital transmission however is that it is relatively easy for the engineers to apply an error correction protocol to the digital transmission stream, if they want to. The downside of an error correction protocol is that it takes time to compute and consumes bandwidth , the more bandwidth is used on the error correction the less there is for the data signal you actually bought the cable to transfer, be it video or audio. HDMI transmits its signals using TMDS which in turn uses the 8b/10b protocol to try and ensure an even number of 1's and 0's are passed ( a long sequence of 1's may cause the receiver to become 'desensitised' , to them or to put it more accurately and to quote Wikipedia "To be specific, bit errors can occur when a (relatively) long series of 1's create a DC level that charges the capacitor of the high-pass filter used as the AC coupler, bringing the signal input down incorrectly to a 0-level").
In the world of computer networks where error correcting protocols are more common a receiver that received a data packet that was so degraded that it could not be recovered could signal the transmitter and ask for it to be sent again. In the world of home cinema this is not an option, if the packet is lost, for any reason, then there is no way to get it sent again, because by the time you would know that there is a problem the next packet is already here , such is the nature of the continuous data stream associated with home entertainment. This is obvious when you think about TV/radio/satellite broadcasts but it is just as true with Blu-Ray or your PVR etc,
For this reason HDMI does not employ any error correction protocol to the video data stream.
When a digital signal is run through a cable, the edges of the bits round off ( remember these are actual voltages not the simple illustrations you might see in a text book) , and the rounding increases dramatically with distance. At the same time, poor control over impedance results in signal reflections , where portions of the transmitted signal bounce off of the display end of the line and echo back down the cable, and 'mix' with the later information in the same bitstream before it has a chance to arrive at your display. At some point, the data become unrecoverable, and with no error correction available, there's no way to restore the lost information.
In Summary.
Where the critics of spending money on an HDMI cable have a point is that at relatively short distances and low data rates most, but not all, data packets are 'OK' where they have it wrong is assuming that "because it is digital" none of the packets are 'failed' , this is simply wrong 'failed' packets happen all the time and given the "unrestartable" nature of the data stream this will have a transient effect on what you are looking at or listening to . It's just that at greater rates and distance the proportion of 'failed' packets grows to a point where their effect on the picture/sound becomes increasingly blatant, prior to the data stream collapsing all together.
Since we are limiting this discussion to the HDMI cable only and asking ourselves what it can do to reduce it's contribution to these 'failed' packets , I have to admit it is just a cable there is nothing active about it. The basis of its contribution therefore happened long before you bought it i.e. with the thought that went into its design, the care with which it was manufactured and the raw materials it is made from. Like it or not , and who does, all these things cost money.
So how much should I spend ?
Remember this above all , an expensive cable won't 'fix' a poor setup but cheap one can certainly undermine a good one. Determining how much to spend is not an absolute thing it does have to have some context to it , my own feeling is that for most people a good starting point would be 5% of the combined value of the equipment you are joining together ( this is just my figure , I'm sure plenty will disagree with it, and feel free to come up with your own ). So for example a #300 Blu-Ray player and a #700 TV would have a combined value of #1000 , spending 5% or #50 for a cable would seem reasonable to me , notice that is is for a 1m cable only .
This is not the end of the matter however , there are some factors to consider that might move you above or below this. For example ;
- Is the cable going to be built in or , or laid somewhere inaccessible ? Cables in these locations are likely to have a lifespan beyond the current items they are joining together. Our expectations for our domestic entertainment equipment are increasing all the time, you don't want to find yourself compromised when the next generation requires a data rate that your hard to reach cable cannot sustain. Even though it is no guarantee , this might encourage you to spend more than 5%.
- Even given what you have spent on it , the items being connected aren't used very often and you might wish to spend the money you saved on that cable to spend on another cable which will be used more often. This might bring you below 5%.
- How visible is it ? Let's be honest cables aren't the most attractive things and aesthetics are important. Why spend a lot of money on an attractive TV and then have an ugly cable ? Could take you either way , but most likely above 5%.
A simple process then might be to sum up the value of the equipment you are joining together, apply some percentage you are happy with. This will get the basic price for a 1m cable, adjust this figure up or down against any other criteria you think are important. This should position you against a brand or model you are happy with and then select within that the length of cable you require.
About the Author
Doug has been selling HDMI cables and advising people on their home cinema designs foe 3 years. For a range of HDMI cables a selection of high quality audio and visual cables vist http://www.avprime.co.uk/
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