Originally Posted by InGear
I agree with Jim Williams and Bob Katz; here's some more detail and a caution about reading cable specs carefully:
When making studio cables, if you read the specs in some catalogs, you might conclude that analog snakes have lower capacitance than digital snakes:
Mogami's spec for 2932 analog 8-pair snake lists capacitance of 3.7 pF/ft interpair "partial", and 40 pF/ft wire-to-shield "partial". (Catalog page 24)
Mogami's spec for 3162 digital 8-pair snake lists an "effective capacitance value between inner twin" of 14pF/ft, and doesn't mention a wire-to-shield value. (Catalog page 50)
So at first glance it might appear that the analog cable is *way* better: 3.7 pF/ft vs. 14 pF/ft!
But not necessarily so -- 3.7 pF/ft is an engineering calculation for JUST THE WIRE PAIR. When you add shield capacitance it gets bigger (at least to 24 pF/ft by calculation of "wire-to-shield-to-other-wire").
To resolve this for real, I tested 30 feet of Mogami 2930 analog snake against 30 feet of Mogami 3162 digital snake. Here's the results on a Fluke multitester. Not guaranteed NIST-traceable accuracy, but hopefully with decent relative accuracy:
Mogami 2930 ANALOG snake: about 34 - 35 pF/ft within a pair in 1 channel
Mogami 3162 DIGITAL snake: about 25 - 26 pF/ft within a pair in 1 channel
Of course, your mileage may vary (and it would be interesting to see this comparison reproduced on laboratory-grade equipment).
In conclusion, at least by the above measure it IS better to install digital cables even if you'll be running analog signals over them.
Let me add to it: The capacitance between the conductors pair and the capacitance between a conductor and a shield are separate issues, and should be specified and viewed separately, because they impact the signal differently in different cases.
That is so because the impact of the capacitance is there only when one applies voltage change between conductors (or conductors and shield). In the case of balanced drive, the 2 inner conductors see the full differential voltage between them. But if the conductors pair are very close to each other (say tightly twisted pair for example) relative to the distance to the shield, the combined action of the pair tends to cancel much of field as it relates to the shield, making the cable to shield capacitance less important.
But take the same cable and use it in unbalanced mode. Now the impact of capacitance between the pair can be the same, but it is higher when one views the electric field between the signal conductor and the shield.
Generally, regarding analog vs. digital cable - There is no such thing as a digital cable. All cables are analog, as all signals are analog. An analog signal can be "anything", and a digital signal is "closer" to being restricted to having one of two voltage states, say 0V and 5V (or other levels). But in reality, the transition between states is not sudden, there is some time for the signal to rise and fall, the signals may have ringing during transitions, or exponentially rising, or.... all very analog stuff. In fact the whole idea of running curent on a conductor is very analog (unless one wants to get into quatum physics, examinimg an electron at a time, which has no place here, because we are dealing with a flow of huge numbers of electrons, and the current is the avarage of all the electrons, thus continues analog not discrete behaviour).
So the concept of analog cable vs. digital cable is an outcome of marketing and sales. Perhaps such a distinct division was done to help a customer buy the right product, but the fact remains, there is no such thing as digital cable. There are cables suitable for say digital audio, but the same cables may not work for say digital video... A cable for digital audio tends to be capable of higher frequencies (more bandwidth) then a cable for signals for analog audio bandwidth. Both cables are analog cables, they both carry analog signals.
The more in depth way to view cables is to view their characteristics, and find a cable that is well suitable for a given application. That could require both mechanical and electrical considerations.