Integrated Systems Digital Network. A digital telephone network hawked by the telephone monopolies as they can run the long-haul links over their existing analog plant. As the end connections are ultimately digital, there is no modem, only a codec. ISDN services include audio and data transmission, as well as the other usual telephone company add-ons such as caller ID, call waiting, etc. The basic unit of ISDN is the B-Channel (bearer channel) which are 64kbps (yes, that’s a whopping 64 kilobits per second) of bi-directional datastreams of audio, video, or computer data. There is an auxiliary D-Channel which is used as a control circuit, but is used by the APT codec to carry SMPTE data. With ISDN, one pays per B-Channel, and once allocated to the user, the costs of transmission on the multi-channel circuit are fixed, regardless of the actual data rate used over the circuit. However, with a multi-channel installation, the circuits are not guaranteed to travel the same route to the destination, resulting in varying transmission delays, resulting in garbage at the destination node. Many codecs are now able to handle the multi-channel circuits, but providing timing to SMPTE timecode resolution is unfeasible so, in practice, either mixes are downloaded at the receiving end and rebuilt using SMPTE timecode for synchronization, or to send the monitor mix, and in real-time, simultaneously record the performance back onto the master tape, also via ISCN. The overdubbed audio will be offset by a fixed time constant, so the delay remains fixed, and the tracks are re-synced after recording. There are three commonly used types of ISDN audio codecs: Dolby Fax, APT x100, and Musicam (MPEG). Dolby Fax is used by the movie studios and high-end recording facilities, with about 300 installations world-wide; it is based on AC-2 encoding; it requires four B-channels. APT (Advanced Processing Technology, owned by SSL, Inc.) developed the data reduction called “x100.” This format is widely used by broadcasters, and has been adopted as the encoding standard of DTS. APT offers the shortest processing delay of all of the common ISDN data-reduction systems, and the highest bandwidth of up to six B-channels, with concurrent SMPTE time-code. The MUSICAM format uses MPEG and is again widely used by broadcasters; it requires from one to six B-channels. Depending on where and who you are, ISDN is either the wave of the future, or the backwash of the past (as it’s all the telephone companies have to offer).