Multichannel AES Connectivity, Part One of Two: Ups and Downs of DB-25s
Anyone who has had the task of wiring together audio equipment that uses the 110-Ohm balanced version of AES3 knows that the size and expense of the connections can be a force to be reckoned with. The AES/EBU specification describes the use of a standard three-pin XLR connector, and equipment with plenty of rear panel space will generally feature this connector. However, as channel count has risen and rear panel space has diminished, several new connection techniques have come on to the scene, and maddeningly some are not compatible.
Arguably the most common higher-density AES connection is the convenient 25-pin D-subminiature (DB) connector. First popularized by Tascam Corp. for use with its DA-88 eight-channel digital audio tape recorders, the “Tascam format” carries eight AES pairs (16 audio channels): four in and four out over a single DB-25 connector. A flip cable could be used to connect two machines together for dubbing (i.e. outputs of one machine to inputs of the other and vice-versa). There is also a “Tascam” style analog connection scheme using the same DB-25 connector to carry up to eight channels of balanced audio.
While sometimes challenging to solder, the DB-25 connector is easy to source and is very convenient for handling multichannel audio. Unfortunately, not all manufacturers caught on to the idea that this popular format had become a de facto standard and started releasing products that used DB-25 connectors with incompatible pinouts. Further, it is not at all uncommon to find only an included adapter cable and no pinout for the device! This means that connecting gear together requires custom DB-25 cables adding to the time and expense of multichannel audio, and may add to the headache of troubleshooting missing or misconnected channels; this becomes truly challenging with 5.1 channel audio. So, beware of the DB-25 connector as it can be a speedy friend or friendly fire.