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I am planning on running some very long digital cables (HDMI, DVI, DisplayPort). I want to make absolutely sure that as long as the cable works with my desired settings (resolution and hz), there's absolutely no way I am losing any image quality (aka correct colors and image sharpness).

I have actually searched quite a lot online about this issue, and most people would claim that since the cable is digital it either works or not. Either you just see a perfect image, or you get very noticeable sparkles/blackouts, or well, no image at all.

A few people however, say that this "either works or not" stuff is a myth. This post for example, explains very well how even on a digital cable, data can still get corrupted:

As the length of the cable increases, not only does the signal strength decrease but so does the differentiation between each subsequent bit. If the signal quality is so bad that the machine at the other end cannot tell where one bit starts and another ends, it can guess (based on the signal strength) an incorrect value. The resulting signal is still digital, is it not? And yet it is incorrect

It is not so clear though, what would be the actual visible effect of such corruption.

  • Define “very long”. – Daniel B Sep 8 '14 at 4:54
  • The length I will be using is irrelevant. This is more of a principal question about digital cables, and whether it's even possible for one, for any reason, to give me a lower image quality without me knowing it. – SportySpice Sep 8 '14 at 9:48
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What would be the actual visible effect of such corruption?

The "either it works or not" isn't really accurate. The problems are a matter of degree, although degraded digital signals manifest in ways that tend to be noticeable, even when mild. The title mentions color and sharpness, but those are not the main victims of a degraded digital video signal. Typical symptoms are things like:

  • visual noise and artifacts
  • odd corruption of the image
  • image breakup or "pixelation" (image breaks up into rectangular blocks)
  • the frame freezing

An uncorrupted/undegraded signal is general very clean and sharp (photographic), so any degradation or corruption tends to range between noticeable and unviewable. If you subscribe to a digital TV source, you have probably seen examples when there were problem with the signal being fed to you.

Is it possible to get a lower image quality without me knowing it?

It's impossible to rule out that there could be degradation so slight that it would not be noticeable. If you were doing studio work, you would probably be using instruments to monitor the signal (and definitely wouldn't be messing with cheap, extremely long cables because they are a potential source of problems that can be easily eliminated). However, because a good image is so good, and the nature of the corruption is noticeable, your eyes would be a pretty good instrument for detecting these kinds of degradation.

How seriously should I take the cable length limits?

The cable length limits are based on signal loss and degradation limits. No cable delivers a perfect signal. The specifications are designed around being able to effectively use a signal that is degraded within acceptable limits. If the signal is within the spec, any equipment designed to use that spec should be able to use the signal properly. If the signal loss or degradation is outside the specified limits, some equipment may still be able to use the signal, so how noticeable it is could depend on the device connected at the receiving end. If the signal is degraded to the point where the data can no longer be discriminated, nothing will fix that.

DVI is good for up to 16 feet without a repeater; HDMI in the range of 33 to 45 feet. You can go farther with high-end cables and other tweaks. It is possible to make a cable that will have lower losses and less degradation, but it gets very expensive.

There are explicit tests the cables are supposed to pass to be in compliance with the spec. A cable of any length should pass those tests, in which case you would not see degradation. When you talk about seeing cheap cables at twice the length limit, there is little chance that they meet the spec. If an extremely long cable merely has the right connectors at each end and doesn't pass the tests (which would be an expected outcome for a cheap cable), you are likely to notice a degraded image.


A couple of sources for further reading:

http://www.bluejeanscable.com/articles/how-long-can-hdmi-run.htm

http://www.datapro.net/techinfo/dvi_info.html

  • I already know this, however, this doesn't really answer my question. There are pretty cheap cables from china(aliexpress.com) that will even go 20m(66 feet) without any repeater. The question is, is simply seeing an image on my monitor is a good enough indication that everythig works, or is it possible I am getting an inferior picture quality without even knowing it. This is more of a principal question about digital cables, and whether it's even possible for one, for any reason, to provide a lower image quality without the user knowing it. – SportySpice Sep 8 '14 at 9:58
  • The first link discusses explicit tests the cables are supposed to pass to be in compliance with the spec. The quick answer is that a cable of any length should pass those tests, in which case you would not see degradation. If an extremely long cable merely has the right connectors at each end and doesn't pass the tests, you might see a degraded signal. It would not be surprising to find extremely cheap cables that don't comply with the specs. – fixer1234 Sep 9 '14 at 19:00

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