This article is from the Audio Professional FAQ, by with numerous contributions by Gabe M. Wiener others.
The best, quick answer is what cables you should NOT use!
Even though AES/EBU cables look like orinary microphone cables, and S/P-DIF
cables look like ordinary RCA interconnects, they are very different.
Unlike microphone and audio-frequency interconnect cables, which are
designed to handle signals in the normal audio bandwidth (let's say that
goes as high as 50 kHz or more to be safe), the cables used for digital
interconnects must handle a much wider bandwidth. At 44.1 kHz, the digital
protocols are sending data at the rate of 2.8 million bits per second,
resulting in a bandwidth (because of the biphase encoding method)
of 5.6 MHz.
This is no longer audio, but falls in the realm of bandwidths used by
video. Now, considerations such as cable impedance and termination become
very important, factors that have little or no effect below 50 kHz.
The interface requirements call for the use of 110 ohm balanced cables for
AES/EBU interconnects, and 75 ohm coaxial unbalanced interconnects for
S/P-DIF interconnects. The used of the proper cable and the proper
terminating connectors cannot be overemphasised. I can personally testify
(having, in fact, looked at the interconnections between many different
kinds of pro and consumer digital equipment) that ordinary microphone or
RCA audio interconnects DO NOT WORK. It's not that the results sound
subtly different, it's that much of the time, it the receiving equipment
is simply unable to decode the resulting output, and simply shuts down.
Fortunately, there is a ready solution for S/P-DIF cables. Any store that
sells high quality 75 ohm RCA video interconnect (or "dubbing") connectors
also sells high-quality S/P-DIF interconnects as well. They may not know
it, but they do. This is because the signal and bandpass requirements for
video and S/P-DIF cables are the same. National chains such as Radio Shack
sell such cables, and the data seems to indicate that they are good digital
interconnects.
For AES/EBU, there are fewer, less common solutions. Companies such as
Canare make excellent cables. Professional audio suppliers and distributors
may be good sources for such cables. If you are handy with a soldering
iron, then you can purchase 110 ohm balanced shielded cable and make your
own (which I have done quite successfully). Cables such as Alpha Twinax,
Carol Twin Coaxial, Belden 9207 twin axial, and the like, all work well for
this application. Use high-quality XLR connectors (be warned that these
cables are 0.330 inches in diameter and are a VERY tight fit in the
neoprene strin reliefs of many connectors: warming them in hot water makes
them pliable enough to work well).
As to how long these cables can be, it's hard to say. However, a couple of
general rules apply.
S/P-DIF was NEVER intended to be a long-haul hardware interconnect. The
relevant specifications talk of interconnect lengths less than 10 meters
(33 feet). In fact, many pieces of equipment cannot tolerate cables even
that long, due to the excessive capacitance and possibly induced common
mode interference.
AES/EBU is more tolerant of longer runs because it is balanced (thus more
immune to interference) and it's run at a higher signal level (5 volts
instead of 0.5 volts). The standards "allow signal transmission up to a few
hundred meters in length."
The reality is that much is highly dependent upon the actual conditions at
hand. The requirements are that the received signal fit within certain
requirements of rise time/period and voltage level, the so-called "eye
diagram". In other words, regardless of what kind of cable you use, if it
can't move the voltage at the receiver far enough soon enough, it simply
isn't going to work.
Another complicating factor is that both protocols allow a degree of
multi-drop capability. This means a single transmitter can drive several
receivers (the last of which must be terminated with the proper termination
impedance). However, implementing multi-drop puts more stingent
requirements on impedance matching. [Dick]
 
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