8. Digital Audio Connectivity

These days more and more studio gear and electronic musical instruments are equipped with digital audio interfaces, in addition to conventional analogue connections. The most common stereo or dual-channel interfaces are AES-EBU, S/PDIF and Toslink. Let's look at these in turn:

AES/EBU

The AES/EBU (Audio Engineering Society/European Broadcast Union) system uses a differential (balanced) serial data stream that contains audio and other sub-code data. Serial simply means that the data is transferred sequentially, one digital bit at a time. It is a relatively high voltage interface using voltages of 3 - 10V to represent digital bits. The protocol uses XLR connectors and is considered a professional audio format. It can send data at almost 3 Mbits/sec, so special cabling must be used for long distances. Many engineers make the mistake of using mic cables (probably due to the familiar XLR connectors). These should not be employed as they have too much internal capacitance due to their heavy screening, and will degrade the signal over very short distances. Proper AES/EBU cables must be used.

S/PDIF

The S/PDIF (Sony/Philips Digital Interface Format) system uses unbalanced, low-voltage (0.5 - 1V) serial data transfer with slightly different sub-code definitions to AES/EBU. It uses RCA (phono) connectors and was originally intended as a consumer format. It too can send data at almost 3 Mbits/sec, so cabling is still an issue. Avoid standard audio cables for all but the shortest distances. Dedicated S/PDIF cables are available from most high street electrical stores.

Both these systems are known as self clocking or asynchronous. This means that the clock information needed to synchronise the devices at either end, is embedded in the data stream itself. Earlier transfer protocols such as SDIF-2 required a separate physical connection to establish a master clock with which to govern the transfer process. Generally speaking these days, the receiving device will need to see and lock onto the embedded clock data in order to reliably and accurately read the incoming stream. Problems such as pops, clicks and noise can usually be traced back to synchronisation - which is more often than not due to incorrect settings made within the devices involved - or sometimes to jitter, a drift in the timing accuracy of the embedded clock itself. Bad jitter problems can nearly always be attributed to using incorrect cables.

Toslink (Optical)

Toslink (Toshiba Optical Link) is effectively an S/PDIF signal causing an LED to flash on and off very quickly. The light passes down a length of optical fibre, and at the receiving end an opto-sensor regenerates the S/PDIF signal. Although Toslink avoids electrical interference and ground loop problems, it's not all good news. Cheap optical leads can introduce a lot of jitter to the signal, and only short runs are practical - it's worth paying for high quality optical leads if you need the long cable runs that this technology is able to provide.