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Posts: 17117
Location: Pacific Northwest
Joined: August 25, 2002
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The following text taken from Allied Wire & Cable (see link below):
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AES EBU specs for digital audio cable require a sampling rate from 32 KHz to 193 KHz, bandwidth from 4.096 MHz to 24.5 MHz, and an impedance of 110 ohms ± 20%. These are very broad specifications, allowing everything from 88 ohm cables to 132 ohm cables. However, standard analog audio cables have impedances ranging from 45 ohms to 70 ohms. There is a lot of room for difference between the AES EBU digital audio cable impedances and the standard analog audio range, which could cause jitter and signal reflection problems. Those problems could then lead to receiver bit errors. To avoid these issues, 100 ohm to 120 ohm twisted pair cables are recommended.
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With such a broad 110 ohm ± 20% range specification, albeit with the vague notion of a tighter tolerance being recommended, I find the sonic benefit of my experiments outweigh the concern about conforming to AES/EBU specifications, which seem a bit arbitrary. The 75 ohm characteristic impedance tolerance of ±3 ohm is far more strict, including the strict rules of terminating RCA and BNC connectors for the purpose, which if not followed can cause impedance mismatches. Achieving an actual ±3 ohm tolerance for a typical 75 ohm S/PDIF digital cable build seems even less likely than achieving a broader 110 ohm ± 20% tolerance.
The following is a layman's perspective regarding my home-brew XLR digital cable builds for my own personal use:
While the 75 ohm characteristic impedance issue of an RCA connector or BNC connector termination involves a mechanical structure that maintains a precision relationship of the dielectric structure between the center conductor and the shield from end to end, an XLR connector has little to do with maintaining the dielectric structure of of a twisted pair. The termination of a balanced digital cable vs. a balanced analog cable is identical, with no particular termination method having to do with the mechanical structure of an XLR connector to cause a characteristic impedance mismatch. So a nominal 110 ohm characteristic impedance of a balanced digital cable seems to be unaffected by the structure an XLR connector.
As for the cable geometry, the key point of my AES/EBU digital cable design is to maintain a consistent and precision twist ratio, with the same loop area for each twist, without any gaps along the entire length of a twisted pair that will last the test of time. This is easy to do when a medium gauge solid core conductor with a low-mass dielectric is implemented for the task. My favorite wire being VH Audio's 21 AWG OCC solid core copper hookup wire with low-mass AirLok dielectric for the purpose. Not only does the wire sound great, it's also the easiest wire to tightly twist in a precision manner for noise cancellation purposes, and to firmly keep its shape without gaps. All the twisted pair needs is a simple outer wrap of heavy duty PTFE Teflon tape to help ensure that the tightly twisted solid core conductors keep a consistent geometry from end to end, without any gaps forming, and to provide an important level of cable resonance control.
A typical commercial AES/EBU digital cable design maintains a consistent geometry with the use of fillers (with added dielectric mass) that act as a former for a twisted pair to maintain a consistent geometry, which is especially needed for a typical pro audio stranded conductor with thick insulation, since that type of conductor does not easily keep its geometry within the moderate constraints of a typical cable jacket. So a typical AES/EBU digital cable design tends to need an opposing pair of rods or a bonding agent to keep the conductors evenly and firmly twisted together from end to end, whereas I find a medium gauge solid core conductor with low-mass insulation fulfills the need of a precision geometry that can be consistent from end to end without fillers for a balanced digital cable application. Furthermore, an audiophile-quality XLR connector is just as vital for audiophile performance rather than a typical pro audio XLR connector such as those from Neutrik and Switchcraft, which are unacceptable for an audiophile application, in my experience. The same applies to a pro audio 110 ohm AES/EBU digital cable such as those from Canare and Mogami. Finally, I break the pro audio rules by not using a ground conductor nor shielding for my AES/EBU digital cable builds, since they are unnecessary and degrade audiophile performance for a short 0.5 meter length home audio application, to my ear.
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