Now that you know all about analog cables and connections — including some of the elusive TRS, XLR, TS and RCA words — you're ready to move on to the other audio language: digital. But you need to learn S/PDIF, AES/EBU, Toslink and a few other new nouns to speak this language fluently. If analog cables and connectors are like English, then digital is like Chinese — there are more words to master, and some contain only subtle differences in “meaning,” though they may look similar.
Digital audio uses one of two loosely defined inter-connection types: those that carry stereo signals and those that carry multiple channels. One aspect that differentiates digital audio from analog is that digital stereo and even multichannel signals typically travel down a single cable. Perhaps the most common of stereo digital pipelines are S/PDIF, AES/EBU and Toslink. Each is a standard in its own right, though they may share the same cable type. AES/EBU (often simply shortened to “AES”) is usually found on XLR-type connectors that look identical to standard mic connectors, yet AES/EBU requires special cables that possess a higher impedance of 110 ohms. AES connections typically transport stereo signals whereas XLR cables always carry mono analog signals. S/PDIF (often pronounced “spidif”) transmits a similar stereo protocol via a different cable type, yet it refers to the digital formatting rather than the cable itself. S/PDIF can be transmitted across both coaxial (RCA-type) and Toslink (fiber-optic) cables.
Toslink, on the other hand, does refer to a cable type, and these light-based cables are used for a variety of digital-audio connection protocols, including S/PDIF, various multichannel surround formats and Alesis' trademark ADAT Lightpipe standard. ADAT Lightpipe can transfer as many as eight channels of digital audio simultaneously (unidirectionally) through a single cable. Similarly, Tascam's competing TDIF (pronounced “tidif” or “tee-dif”) carries a max of eight channels bi-directionally, also via a single cable, yet TDIF uses 25-pin D-Sub cables (similar to serial-style PC cables) that conform to special TDIF specs.
AES/EBU (Audio Engineering Society/European Broadcasting Union) and S/PDIF (Sony/Philips Digital Interface) are similar; S/PDIF was adapted from the (at the time incomplete) AES standard. They do, however, possess differences in their data formats (notably in how blocks of data are structured) and possess significant electrical differences. Still, AES/EBU inputs and outputs most often appear on XLR connectors. Rarely will you find switchable S/PDIF-AES/EBU on RCA coaxial connectors. When there are four or more AES connections on a device, DB25 connectors (similar to TDIF) are sometimes used, which transmit four pairs of simultaneous stereo I/O. S/PDIF is most often found on RCA or Toslink connectors, but can be found — albeit rarely — on BNC (Bayonet Neill-Concelman or sometimes, erroneously, British Naval Connector) jacks. BNC is the single-pin, locking-connector type used for cable television.
Tascam's TDIF (Tascam Digital Interface) and ADAT Lightpipe are similar in that they both carry as many as eight channels of digital audio, but the similarity pretty much stops there. First, ADAT is a one-way route whereas TDIF can transport eight channels in both directions simultaneously. Both can support sample rates as high as 96 kHz, yet they use entirely different cables and data formats. Maximum cable runs vary widely with each digital audio format. AES/EBU is the most practical, as it can stretch as far as 100 meters. Cable runs can be 10 meters with coaxial S/PDIF and only 5 meters for optical.
It pays to be familiar with the different data and computer connections, as those worlds collide with digital audio. For starters, many soundcards and other devices have word-clock input and output, which are typically found on a pair of BNC connectors. Although the in-depth topic of digital audio sync — with its myriad related topics, such as SMPTE and MTC — will be reserved for a different column, in short, word clock is used for synching multiple digital audio or audio/video sources together.
Next, modern computer interfaces such as USB and FireWire present plenty of opportunities to transfer digital audio and data such as MIDI. USB devices and computer ports come in versions 1 and 2. Most version 2 computers are backward-compatible, but, typically, the opposite is not true. FireWire is also found in two versions: 400 and 800 megabits per second, also known as 1394a and 1394b. (For the basics of USB and FireWire as audio interfaces, see “Phantom Power” in the May 2003 issue.)
Finally, this discussion wouldn't be complete without a brief discussion of Yamaha's newer mLAN (Music Local Area Network). This next-generation protocol uses the wide-bandwidth FireWire interface to transport multichannel high-resolution digital audio, video, MIDI, timecode and basic data all through one standard FireWire cable. And even though it bears Yamaha's trademark, mLAN is an open-source platform that is swiftly being adopted by many gear manufacturers.
For clear descriptions of about any type of cable or technology for audio, video, computers and a lot more, check out http://whatis.techtarget.com, which features an alphabetized techie encyclopedia.