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    Respected alanjg007's Avatar
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    (Part2 of 2)Why don?t expensive HDMI cables make a difference?

    Testing cables

    Scientific explanations are all well and good, but it's practical testing where the talking stops and the evidence starts. To prove the doubters wrong, we upped the ante and decided to test full motion video to prove that changing cables makes no difference. In order to test scientifically, we turned to our Digital Foundry TrueHD card, which captures the RAW and uncompressed HDMI signal. Crucially, it performs no error correction, so we can accurately compare the output from different cables and spot any errors.

    As an input, we connected a laptop via HDMI to the TrueHD card. We set Windows to use a resolution of 1,920x1,080 with 24-bit RGB colour, at a refresh rate of 50Hz (PAL). Note that the playback device doesn?t make any difference, as the HDMI signal is the same regardless of whether it?s a Blu-ray player, games console or PC. For test footage, we used the open-source film Tears of Steel. As our PC was set to 50Hz, the video played back at 25fps (25p), which is the PAL standard.

    In order to compare different cables, we needed to ensure that the captured footage was the same regardless of the cost of the cable. To do this we used the TrueHD capture card to capture a few seconds of Tears of Steel for each cable, saving the results in an uncompressed video file. We then used a frame grabber tool to take the same 50 frames (two seconds? worth) from each captured video file, saving them as uncompressed BMP files. To ensure that we captured the same frames, we started on the first frame of the spaceship taking off; it?s easy to spot as the frame before this is completely black.


    Once we?d got 50 frames from our range of test cables, we needed to compare them. Our rational was that if frame one from HDMI cable one was the same as frame one from HDMI cable two (and so on), then the cables had made no difference. In order to compare the files we generated an MD5 hash of each image, which can be thought of as a digital fingerprint, as is represented as a 32-digit hexadecimal number, such as 6add8f2f6edee97b042fcb629fe2f5e7. In short, if the MD5 hash of two files is the same, the files are forensically identical.

    In the event that we found any captured frames that were different, we had a second method to check for differences, ImageMagick?s Compare tool. This tool is designed to mathematically and visually show the differences between two files. It creates a new image, where pixels are colour red to show that there?s a difference between the two images. This allows us to see where the error is.

    Our first comparison was between an expensive 1m HDMI cable that costs ?70 and a 1m cable that costs ?4.99. In our first set of tests, 49 of the 50 frames we captured from the expensive cable were completely identical to the 50 produced from the cheap cable. With the one frame that was different, we got two completely different MD5 hashes.

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    We needed to find out where the error was, so we used Compare to highlight what the error was and found that there was a one-pixel error. To see if you can spot where the error is take a look at the images below and see if you can spot the difference: the top image is from the cheap cable, the middle image is from the expensive cable and the bottom image is from the expensive cable

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    Hard to tell, right? If when we crop in to the image (see below) to show the error, spotting a single-pixel error is incredibly hard. Look at the top-left of the right-hand image and look for the red pixel. This is the error, which is the difference between the cheap cable (left) and expensive cable (right). As if to prove our point about cable quality not making a difference, the single pixel error we found wasn't from the cheap cable's output; it was a dark grey spot on a single frame of the expensive cable's output.

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    Keep in mind that this is a single pixel amidst two million other pixels, displayed for 1/25th of a second. This is barely noticeable in the extreme circumstances we set up in our Labs, so the likelihood you'll spot it on a TV that has built-in error correction is next to zero. What's more, one frame in 50 is well within the error allowance of the HDMI standard.

    Next, we decided to compare a cheap 5m cable (?5.99) versus an expensive one (?130), as the argument is that cheap cables suffer over length. This time around we didn?t find one error on either cable. Throughout all of our testing, mathematically and scientifically, bar that one-bit error, we didn?t see any differences between expensive cables and cheap ones, proving that price doesn?t influence quality.

    Cable care

    Doing a bit more testing, we found that the only reason you may want to pay more for an HDMI cable is for better build quality. This isn?t because out-of-box you?ll see any difference, but because a more expensive cable will be more rugged and have a better-made connector. As proof, we tested a few cables that had been hanging around for a few years and had been rather mistreated and stored badly. These cables were showing signs of physical wear and, when we plugged them into our capture card with no error correction, there was an extreme number of errors in every frame. As such, this cable would no longer pass the HDMI certification, and so it is effectively broken.

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    Dozens of sparkling, coloured and white dots dancing across the screen showing just how badly damaged the cable was. However, even a cable in such poor repair would be hard to spot on a TV, as error correction will make those pixels impossible to spot even if they are technically wrong.

    Even so, this gives us two important bits of information. First, treat your HDMI cables with care and make sure that connectors are securely plugged in and there?s nothing putting pressure on them. Secondly, if you're buying an HDMI cable for a piece of kit that you move around a lot, such as a projector or a laptop you're constantly unplugging and plugging back in, it will pay to buy a cable with a more rigid cable and a stronger, less flexible joint between the cable header and the wire itself.

    Conclusion

    We were told that we hadn't tested for motion when we last ran our test, but we've since done that and still proved that there's no difference in quality between expensive and cheap HDMI cables. Our one concession on this matter is that more expensive cables have better build quality and are more rugged, so are a better choice if you're constantly unplugging a lead. Even so, there's no need to go crazy and spend a fortune. We're also happy to run a blind test for anyone that still believes that they can see a difference, although we don't think that anyone will be brave enough to do this.
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  3. #2
    Biatch Moderator berley's Avatar
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    far too much reading in part 1 and 2, can someone just summarise if your better off buying cheapo ones Hidden Content
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    with analogue there was definitely a difference but with digital, the cheapo is the kingo of the cabo (cable)

    If i were to pay over the odds for something which can be picked up at a fraction of the cost that had identical performance to the much more expensive one then i would feel very short changed!

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    Big Bad Boss Gazer's Avatar
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    This reminds me of the electrical place Currys ....when you buy a new Tv, DvD or a Freeview box they always try and sell you one of there leads priced around ?70 and you can buy the same lead for ?10 on ebay and have the same results.
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    DarkHours Moderator dx100-uk's Avatar
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    or the gold plated audio leads vs the cheap leads.

    load of ole crap..

    dx
    its nice here....

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