Digital Coax Cable - Does the Length Make a Difference

@kijanki - you beat me to the punch. :).

Digital cables have signals with very fast rise and fall times. In order to reduce reflections from interfering with the signal transmission, they are spec'ed with a fixed impedance (75 ohms in the case of S/PDIF) driving a load with the same impedance. Any variation in the impedance will result in a signal reflection. 

If everything were ideal, the cable length wouldn't matter. The problem is that there are minor variations in the impedance through the connector, PC traces, etc. at the ends of the connection. This will result in some reflection, although in a well implemented system, it is fairly minimal. 

It takes about 5ns for the signal to travel through a 1m cable, so the reflection will propagate back to the source end of the cable and then back to the destination in 15ns since it's traveling through 3 lengths of cable. 

As I understand it, S/PDIF rise/fall times are typically in the 20ns range, which means that with a 1M cable, the reflection from the start of the square wave is going to arrive around the same time that the primary signal is going passing through the threshold where the signal is sampled. This can cause a perturbation in the signal which interferes with the timing - essentially adding jitter. 

If you use a really short cable, the reflection reaches the destination early enough in the rise (or fall) of the signal to not interfere with the timing. Likewise, if the cable is long enough, the reflection will arrive after the signal has passed through the threshold voltage so it won't cause a problem. 

This is why it's recommended to use an extremely short cable (such as you'd get inside a CD player, or a long enough cable that the reflections don't interfere. The general guidance I've read is that using a cable of 1.5M to 2M will avoid this issue almost all the time. 

Shorter USB cables will not be a problem. The biggest difference is that the USB clock is not used to generate the DAC sample clock on modern DACs, so any jitter introduced by reflections from the connection will not cause DAC clock jitter. 

That's not to say it will not have any impact since it can contribute additional noise, but it's likely that other noise transmitted through the USB connection will have a larger impact. And a longer USB cable provides more opportunity for noise to be picked up in this connection.