Transmission Format
MADI links use a transmission format similar to Fiber Distributed Data Interface (FDDI) networking (ISO 9314), which was popular in the mid-'90s for backbone links between LAN segments. Since MADI is most often transmitted on copper links via 75 ohm coaxial cables, it more closely compares to the FDDI specification for copper-based links, called CDDI.
AES10-2003 recommends using BNC connectors with coaxial cables (see part 7.1.4) and ST1 connectors with optic fibres (see part 7.2.1). The specifications about fibres can provide a range of up to 2 km.
The basic data rate is 100 Mbit/s of data using 4B5B encoding to produce a 125 MHz physical baud rate. This clock is not synchronized to the audio sample rate, and the audio data payload is padded using "JK" sync symbols.
The audio data is almost identical to the AES3 payload, though with more channels. Rather than letters, MADI assigns channel numbers from 0–55 or 0–63. The only difference is that frame synchronization is provided by sync symbols outside the data itself, rather than an embedded preamble sequence, and the first four time slots of each sub-channel are encoded as normal data, used for sub-channel identification:
- Bit 0: Set to 1 to mark channel 0, the first channel in each frame
- Bit 1: Set to 1 to indicate that this channel is active (contains interesting data)
- Bit 2: notA/B channel marker, used to mark left (0) and right (1) channels. Generally, even channels are A and odd channels are B
- Bit 3: Set to 1 to mark the beginning of a 192-sample data block
Sync symbols may be inserted at any subframe boundary, and must occur at least once per frame (0.45% minimum overhead.)
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