Digital Audio Interfaces 57518.2 MADI (AES10–1991) Serial Multichannel Audio Digital Interface ....................
The MADI standard is a serial transmission format for multichannel linearly represented
PCM audio data. The specifi cation covers transmission of 56, mono, 24-bit resolution
channels of audio data with a common sampling frequency in the range of 32 to 48 kHz.
Perhaps this is conceived more easily of in terms of 28 stereo “ AES ” audio channels (i.e.,
of AES3–1985 data) traveling on a common bearer, as illustrated in Figure 18.6. The
MADI standard is not a “ networking ” standard; in other words, it only supports point-to-
point interconnections.
18.2.1 Data Format
The MADI serial data stream is organized into frames that consist of 56 channels
(numbered 0–55). These channels are consecutive within the frame, and audio
data remain, just as it is in the original digital audio interface, in linearly coded,
2’s-complement form, although this is scrambled as described later. The frame format
is illustrated in Figure 18.6. Each channel “ packet ” consists of 32 bits (as also shown in
Figure 18.6 ), in which 24 are allocated to audio data (or possibly nonaudio data if the
nonvalid fl ag is invoked) and 4 bits for the validity (V), user (U), channel status (C), and
parity (P) bits as they are used in the AES3–1985 standard audio interface. In this manner
the structure and data within contributing dual-channel AES bit streams can be preserved
intact when traveling in the MADI multichannel bit stream. The remaining 4 bits
Table 18.1 Salient parts of the signal and interface specifi cation
General
Transmission data format Electrically equivalent to AES
Impedance 75 ohms
Mechanical BNC connector
Signal characteristics 1 V, measured when terminated
Output voltage in 75 ohms
DC offset 5 0 m V
Rise/fall time 30 to 44 nS
Bit width (at 48 kHz) 162.8 nS