How Long Can HDMI Cables Be Run?

[Jason Dunn]

<div class='os_post_top_link'><a href='http://www.bluejeanscable.com/articles/how-long-can-hdmi-run.htm' target='_blank'>http://www.bluejeanscable.com/artic...an-hdmi-run.htm</a><br /><br /></div><p><em>"One of the glorious things about the traditional analog video formats is their robustness over distance. Our customers have run analog component video for hundreds of feet without so much as a booster box to keep the signal together. When DVI and HDMI first hit the market, many people had trouble running signals over even modest distances; 15 feet would be reliable, and anything longer was a gamble. That situation has improved, though HDMI will never rival analog video for reliability over distance. Here, we address why that is, and what you can expect if you need to run HDMI cable over distance."</em></p><p><img src="http://images.thoughtsmedia.com/resizer/thumbs/size/600/dht/auto/1230654032.usr1.jpg" style="border: 0;" /></p><p>I've never really understand the reasons why digital cables couldn't have longer run distances than analog - you'd think analog signals would be subject to more degredation - but <a href="http://www.bluejeanscable.com/articles/how-long-can-hdmi-run.htm" target="_blank">this article does a good job</a> of explaining what they've discovered through their testing. They don't go into the physics of how and why the digital signal degrades, but what matters is what they've discovered in their tests - and how just because you see a 50 foot cable on a store shelf doesn't mean that it will work the way you think it should work. If you're going shopping for HDMI cables, this article is a must-read.</p>

[doogald]

I'd have just read wikipedia (http://en.wikipedia.org/wiki/Hdmi#Cable_length).

[Sven Johannsen]

I've never really understand the reasons why digital cables couldn't have longer run distances than analog - you'd think analog signals would be subject to more degredation
Just basically, if you are talking about actual digital signals, i.e 1's and 0's, to transmit them you have to support much higher frequencies than you do for analog. Analog sound, that we can hear, up to around 20KHz, degrades over copper more at the higher frequencies than the low, but not so badly that it distorts too much for the average ear. When you digitize that sound, and get 64, 128, 256Kbit digital streams, the copper has to not only propagate that 'frequency' but also a good number of harmonics of that frequency for the 'square' wave data to be reproduced at the far end, in such a way that the receiver can accuratly assess the bits. The degredation happens faster when you are trying to propagate square waves (digital), than more sinusoidal analog sound. When you get to the frequencies required by digitized video, you get even more degredation in the square wave over distance.

[EscapePod]

Although I personally haven't tried it, the Tributaries HXC5 1080p video over ethernet device is supposedly good for transmitting the vid up to 50 Meters.

See more info at:
http://news.cnet.com/8301-17938_105-9821022-1.html

[djdj]

My 35-ft high quality cable is right on the edge of not working in my home theater. If the cable is run too close to an AC line or other source of interference it doesn't work at all. I'd hate to see what would happen if I were to try and run it any farther.

[Yanis]

Just basically, if you are talking about actual digital signals, i.e 1's and 0's, to transmit them you have to support much higher frequencies than you do for analog. Analog sound, that we can hear, up to around 20KHz, degrades over copper more at the higher frequencies than the low, but not so badly that it distorts too much for the average ear. When you digitize that sound, and get 64, 128, 256Kbit digital streams, the copper has to not only propagate that 'frequency' but also a good number of harmonics of that frequency for the 'square' wave data to be reproduced at the far end, in such a way that the receiver can accuratly assess the bits. The degredation happens faster when you are trying to propagate square waves (digital), than more sinusoidal analog sound. When you get to the frequencies required by digitized video, you get even more degredation in the square wave over distance.

All true. However there is theoretically no reason that a digital signal cannot be transmitted to infinity. There are three reasons for signal degradation in digital signals, all of which can be corrected.

First there is simple loss - also call attenuation. Corrected by amplifying the signal. Pretty easy to understand.

Second there is noise. Any unwanted introduced "stuff" will make the 0s and 1s harder to see. This can be corrected by reforming the signal. This is typically called a repeater. The 0s and 1s are decoded and the digital signal regenerated.

Finally there is mis-shaping of the digital wave form as Sven indicated. There are a number of technical terms associated with this distortion of the signal but there are two techniques to overcome this. First is a passive technique called equalisation. This does a pretty good job of reshaping the signal, and if you combine this with an amplifier to compensate for the loss then you can increase the cable length considerably without having to resort to a repeater. Qualithy cables have a known level of distortion and it is dependant on length. So if you have cable X of length N meters then you can easily add an equalisation circuit to compensate.

In fact both analogue and digital is subject to the same issues as above and the same techniques are used to preserve the digital signal as are used to preserve the analogue signal. The only difference is that any harmonic, intermod distortion or noise in an anologue signal cannot be easily corrected, and anu conversion process that an analogue signal goes through will subject it to some sort of distortion. Thus Analogue will always be subject to the limitations of the medium whereas a digital signal can always be reproduced without loss.

[Yanis]

Very informative article but really - I feel like I am transported back into the dark ages with all this talk of feet. At least supply the measurements in Metric so that the majority of the planet do not have to perform mental gymnastics to get it into an understandable system of measurement.

[Sven Johannsen]

Very informative article but really - I feel like I am transported back into the dark ages with all this talk of feet. At least supply the measurements in Metric so that the majority of the planet do not have to perform mental gymnastics to get it into an understandable system of measurement. Hey, most everyone's got two feet with them all the time. It's measurement I can relate to.;)