Audio Engineering

576 Chapter 18

per channel (called, confusingly, mode bits) are used for frame synchronization on the
MADI interface and for preserving information concerning A/B preambles and start of
channel-status block within each of the contributing audio channels.

18.2.2 Scrambling and Synchronization

Serial data are transmitted over the MADI link in a polarity-insensitive (NRZI) form.
However, before data are sent, they are subjected to a 4- to 5-bit encoding, as defi ned
in Table 18.2. MADI has a rather unusual synchronization scheme in order to keep
transmitter and receiver in step. The standard specifi es that the transmitter inserts a
special synchronizing sequence (1100010001) at least once per frame. Note that this
sequence cannot be derived from data, as specifi ed in Table 18.2. Unusually, this sync
signal need not appear between every frame, as Figure 18.6 illustrates. This sync signal is
simply repeated wherever required in order to regulate the fi nal data rate of 100 megabits/
second specifi ed in the standard.

18.2.3 Electrical Format

MADI travels on a coaxial cable interface with a characteristic impedance of 75 ohms.
Video-style BNC connectors are specifi ed. Because the signal output is practically DC
free, it may be AC coupled and must sit around 0 V 100 mV. This signal is specifi ed
to have a peak-to-peak amplitude of 300–600 mV when terminated—this choice of
amplitude being determined by the practical consideration that the signal could be
directly derived from the output of an ECL gate.

18.2.4 Fiber-Optic Format

Oddly, the MADI standard did not defi ne a fi ber implementation, despite the fact that the
copper implementation was based on a widely used fi ber interface known as FDDI

syn

c

syn

c

syn

c

syn

channel n channel n 1 channel n 2 c

56 channels

1 frame

Figure 18.6 : Data structure of MADI, multichannel audio interface.