What length digital cable?

Doesn't a lot of this have to do with the nominal impedance of any given cable?

...Specifically, it is my understanding that an increased length of IC has increased impedance (as seen by the components).

If one considers that as frequency increases so does impedance this would suggest a limit to the high frequency information that could be accurately transmitted via cables with higher impedances. So length might have more to do with characteristic impedance. I understand this to be true with data cables like cat 5 – Specific impedance parameters are supposed to be critical to the speed within a computer network.

Anyone else more versed on this? Would love to get the straight goods. I often see 75ohm or 110ohm impedance cables specified or recommended for digital. I have also read that 24/96khz has different cable requirements compared to 16/44.1khz. This also makes one consider what requirements would be for even faster bit-rates such as those found in DVD & DVD-A.

Oops!!! I had it totally backwards – look below for the correct information – I was confusing capacitance and impedance. Sorry.

Important considerations for digital cables whether audio or not:

a) Impedance
(Ohms) represents the total resistance that the cable presents to the electrical current passing through it. At low frequencies the impedance is largely a function of the conductor size, but at high frequencies, conductor size, insulation material and insulation thickness all affect the cable's impedance. Matching impedance is very important. If the system is designed to be 75 Ohms, then the cable should match that impedance, otherwise error-producing reflections are created.

b) Attenuation
Is a ratio comparing power input to output. It is measured in decibels per unit length (db/ft), and provides an indication of the signal loss through the cable. Attenuation is very dependent on signal frequency. A cable that works very well with low frequency data may do very poorly at higher data rates. Cables with lower attenuation are better.

c) Shielding
Is normally specified as a cable construction detail. For example, the cable may be unshielded, contain shielded pairs, have an overall aluminum/mylar tape and drain wire or even a double shield. Cable shields usually have two functions: the first to act as a barrier to keep external signal from getting in and internal signals from getting out and the second to be a part of the electrical circuit. Shielding effectiveness is very complex to measure and depends on the data frequency within the cable and the precise shield design. A shield may be very effective in one frequency range, but a different frequency may require a completely different design. System designers often test complete cable assemblies or connected systems for shielding effectiveness.

d) Capacitance
In cable is usually measured as picofarads per foot (pf/ft). It indicates how much charge the cable can store within itself. If a voltage signal is being transmitted by a twisted pair, the insulation of the individual wires becomes charged by the voltage within the circuit. Since it takes a certain amount of time for the cable to reach its charged level, this slows down and interferes with the signal being transmitted. Digital data pulses are a string of voltage variations that are represented by square waves. A cable with a high capacitance slows down these signals so that they come out of the cable looking more like "saw-teeth", rather than square waves. The lower the capacitance of the cable, the better it performs at higher frequencies.

This information can be found at http://www.quabbin.com/tech_briefs/tech5.html