dCS - The Jitter

dCS writes: "If a product is locking to the clock signal of an external source, such as a CD transport connected to a DAC, interference picked up by digital audio cables between the products can smear the transition times of the clock data within the signal – essentially, changing the point in time where a 0 changes to a 1, or vice versa."

"Balanced lines help reduce interference induced in the cables. This is why the AES/EBU format uses 110Ω twisted-pair, shielded cable. This effectively shields the conductors in the cable from most electromagnetic interference (EMI) and flushes any that it picks up to ground, eliminating it from the signal. Any EMI that gets through to the conductors gets phase-cancelled, because each conductor is exactly 180 degrees out of phase with the other. (Since the pairs are run together, any noise will be induced in both conductors in phase with each other and, thus, cancelled.)

It’s important to ensure that a cable has the bandwidth needed for digital signals. The square waves carrying the signal have a very fast rise time between the low and high states (the 0s and 1s). A fast rise time translates to a very high frequency – up in the megahertz range. For this reason, it’s advisable to use good quality 110Ω cable for AES transmission and 75Ω cable for S/PDIF transmission that is specifically designed to carry digital audio data.

When a digital signal is passed through a cable, the cable will, to a degree, act as a filter. A poorly designed cable, which is unfit for use with the interface it is designed for (such as AES3, for example) could potentially filter out high frequencies from the signal before it reaches the DAC from the source device.

This causes an interaction between any two consecutive data bits within the signal, called intersymbol interference. Depending on the relationship between the first and second of any two bits, the transition between the two can be temporally smeared. The ideal clean vertical line of the square wave becomes more sloped, meaning the exact moment a 0 changes to a 1 or vice versa can be blurred. In short, jitter can be introduced purely from the interactions within the data itself.

If the timing data in the audio signal is being used to lock the DAC’s clock to the source’s clock, this intersymbol interference will have a negative impact on sound quality, as it can introduce jitter to the DAC’s clock. However, if the audio system is making use of a Master Clock, and the timing information embedded in, for example, the AES3 signal is no longer being used, the effects of intersymbol interference are negated. While the same filtering effect in the cable and interactions within the data occur, the intersymbol interference does not cause jitter. This is because the Word Clock signal being sent from a Master Clock is regular and does not change like an AES signal does.

It is worth noting that as the PLL used in a dCS product is slow acting, and the clock recovery circuits used are very capable, the effects of intersymbol interference are minimised in cases where the DAC needs to lock to the clock information embedded in the audio signal (such as instances where a Master Clock is not available).

The next post will discuss clock synchronisation, such as how to utilise a Phase Locked Loop to synchronise two different clock domains, like those in a DAC and connected Transport." 

James Cook
dCS: Head of Customer Service