Nemesis Audio
7
Technical Info – MADI and switching streams
MADI Basics
Multichannel Audio Digital Interface (AES10-91) is a common method of transporting a large number
of audio channels between devices. The protocol describes up to 64 channels of up to 24 bit audio at
a sample rate of 48kHz. The sample rate can be higher at the expense of channels, so 32 channels at
96kHz is also a possibility. With the S-Mux protocol even higher sample rates can be achieved by
placing consecutive samples in adjacent channels.
The symbol speed of MADI is 125Mbps, whilst the data rate is 100Mbps, the difference being the use
of a line encoding system known as 4B5B, which converts 4 incoming bits into 5 outgoing bits.
The data format is almost identical to the AES3 payload, though with more channels due to the use
of Time Division Multiplexing - the channel data being sent as 32 bit blocks (24 bits for audio, 4 mode
bits, and 4 for other purposes), in channel order from 1-64. There are also frame synchronization
symbols that make up the overall transmitted data to align transmitter and receiver. It is also
possible to send additional data over the MADI link such as control and MIDI information.
Due to use of NRZI encoding in the MADI stream, and the line 4B5B encoding system, at the physical
level there is no direct correlation between the digital transmitted signal and the original digital
audio signal. Sadly this makes switching the signal a harder task than in the equivalent analogue
world, as the chance of switching from signal A to signal B and expecting to see a similar level, or
identical bit pattern is near impossible. There is no ‘stream of 0’s’ for silence, for example, as the
levels of encoding will always be resulting in continuously variable numbers. This means that the
output of any physical-level switching system at the point of switch will introduce a temporary
corruption of data into the signal stream, and it will be down to the receiving device on how well it
copes with this corruption.
This leaves us with 2 methods for attempting to switch MADI sources into a receiving input:
1)
Directly switch the data between source A and source B and attempt to do it as cleanly as
possible.
2)
Decode both incoming MADI streams to the raw digital data level, switch those and use the
data to generate a new MADI stream.
Both options have advantages and disadvantages. Option 1 requires a much simpler design and
enables data to be transmitted in its native format (i.e. all source information is preserved as is,
including any additional control or MIDI data) which may be of considerable advantage as new or
different protocols are enabled over the MADI link, this method will always prove to be “up to date”.
The downside is that the switchover point will produce a temporary corrupt data stream. Some
receiving equipment (such as DiGiCo consoles) will look at the last received frame and mute that
sample if any corruption was found. If this muting does not happen a large click or bang may be
heard at the receiver, the size of which will be dependent on how well the receiver handles the
corruption, the frame position of signal interruption, and how cleanly the switch occurred. It should
be noted that this click or bang may also occur simply by connecting/disconnecting a signal to an