Digital interfaces
This page lists the most common digital interface standards used to connect between digital mixers, recorders, editing systems and microphones etc.
Contents
ADAT
ADAT is an acronym for Alesis Digital Audio Tape, a format for recording 8 tracks of digital audio onto Super VHS magnetic tape. It also is used as an abbreviation for the ADAT Optical Interface or the ADAT Lightpipe which carries 8 digital audio signals via a single fibre optical cable.
The ADAT Optical Interface uses TOSLINK connectors at each end, making them identical to SPDIF optical cables. However, the data streams of the two protocols are totally incompatible. The ADAT Optical Interface supports up to 8 channels of uncompressed audio at 48 kHz, 24 bit.
The protocol can carry higher bit rates with a reduced channel count and is also “hot pluggable” i.e. devices do not need to be powered down to change connections.
AES/EBU
This is a balanced connection using RS422, usually with XLR-3 connectors wired as normal i.e. pin 1 is screen, with the signal on 2 & 3. AES/EBU carries two channels of digital audio and an integral synchronising (Word Clock) signal. The driver output impedance is about 110 Ohms, producing between 3 V and 10 V peak-to-peak. The receiver impedance was originally set at 250 ohms, allowing up to four receivers to be driven from one source, but due to reflections caused by impedance mismatch it was decided to revise the specification to point-to-point connections at 110 ohms (AES-3-1992).
A single subframe is 32 bits long, and comprises 4 bits for synchronisation (to identify channel A or B), 4 auxiliary bits (which may be used for low-quality speech) and 20 bits for audio, with the remaining 4 bits comprising a validity bit (V), a user bit (U), a channel status bit (C) and a parity bit (P). If required the 4 auxiliary bits may be added to the 20-bit audio to produce a 24-bit system.
The system was designed to use existing professional audio connectors and cable e.g. microphone leads. With twin screened cable lengths of up to 100m can be used, longer with equalisation. However Star Quad microphone cable is not suitable, except for very short (less than 10 m) runs, due to the increased capacitance of star quad cable.
SPDIF
The Sony/Philips Digital Interface is effectively an unbalanced ‘consumer format’ version of the AES/EBU interface, and usually terminates in a Phono connector; it is not suitable for long lead lengths. Category codes are inserted to indicate the type of usage e.g. CD, Sony PCM-601, DAT etc.
Peak-to-peak voltage is about 500 mV, so interfacing to professional equipment can cause problems – although the AES/EBU receiver should operate down to 200 mV. The main problems, however, are incompatibilities in data format, and a format converter should always be used.
SDIF
The Sony Digital Interface SDIF-2 uses 75 ohm co-axial cable via BNC connectors to transmit a single audio channel at 16 (or 20) bits. A separate word clock signal is used, so three cables are necessary for stereo.
SDIF is used mainly for interconnecting PCM-1610 and 1630 machines and also for PCM-701 and DAT machines and several types of audio workstations.
The Sony PCM-3324 and -3348 multitrack machines use SDIF but with an RS422 interface and D-type connectors; a single BNC connector carries word clock as before.
TDIF-1
This a proprietary TEAC Tascam digital audio format which uses a 25-pin D-type connector to carry 8 inputs and 8 outputs.
Mitsubishi ProDigi
This format is similar to SDIF but incompatible, and uses multiway D-type connectors with separate pins for each audio channel and for word clock etc. There are two 16-channel interfaces (Dub A and Dub B) using 50-way connectors for the 32- channel ProDigi multitrack machine, and a 2-channel interface (Dub C) using 25-way connectors for the stereo machine.
MADI
The Multi-channel Audio Digital Interface was the result of collaboration between Neve, Sony, SSL and Mitsubishi to produce a multi-channel interface which would be transparent to AES/EBU data. MADI uses a single 75 ohm cable with BNC connectors for distances of up to 50 metres to carry 56 channels of 20 or 24 bit digital audio. Fibre optic cable can be used for greater distances. The link is asynchronous, with buffers at both ends, so that the data can be re-clocked to the separate AES/EBU synchronising signal. The data rate is fixed at 125 Mbits/sec.
Yamaha
This manufacturer-specific interface uses an 8-pin DIN connector and carries 2 channels of 24-bit audio multiplexed on an RS422 balanced line.
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