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Long Digital Coax Cable = No Good?

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A short 22 foot coax should be no problem, but you are getting close to the distance that you really need a 75 Ohm cable and a good one at that.


From a Belden bloger:


we were having a discussion about how long a crummy cable one can get away with using to carry digital audio data. Thanks to Larry Miller for reminding us that the critical number is the rise time and gave an empirical rule of thumb (1/6 wavelength) for when the cable goes into the transmission line mode and needs to be the correct impedance to avoid getting into trouble.


I normally tell people that the critical distance is 1/4 wavelength which then varies depending on the sampling rate (and resultant bandwidth) of a cable.


If you go by the 1/4-wave numbers the critical distance (after which the impedance is important to match) is:

44.1 kHz = 5.6448 MHz= 44 ft.

48 kHz = 6.144 MHz = 40 ft.

96 kHz = 12.288 MHz = 20 ft.

192 kHz = 24.576 MHz = 10 ft.

Hope this helps set a limit!

Steve Lampen


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