Cadac CDC five_02-01-2020_Rev-7.docx
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MegaCOMMS: Data Protocol
Communication between the remote MegaCOMMS I/O devices and the control surface is via a
proprietary Cadac high speed protocol called MegaCOMMS, using high-speed 75 Ohm coaxial cable
terminated in BNC connectors.
MegaCOMMS is a robust, TDM (time division multiplex) system. Control data is embedded within the
data stream, so that no audio channels are sacrificed for this purpose. The high bandwidth available
means that the current implementation of MegaCOMMS can carry 128 channels of 24-bit, 96 kHz
audio, plus control data, plus clock, bi-directionally, up to 150 metres (492 ft) via a pair of RG-6 coaxial
cables.
In addition to audio and control data, MegaCOMMS provides for accurate, phase-aligned clock
distribution, by embedding timing markers in the data stream. This allows reliable, low-jitter
synchronisation of all hardware elements within a network.
The simplest implementation of a MegaCOMMS network is the straightforward console-stagebox
configuration. In this application, the console provides the clock and the stagebox synchronises itself
once the connections are made. Total through-system propagation delay for this system, including all
console processing and A-D / D-A conversions is an astonishing 37 samples (@ 96 kHz), or just under
400us. This compares with the many millisecond propagation delays usually found in most other similar
systems.
Two interconnection paths are provided, A and B, each of which requires a transmit and receive cable.
The maximum capacity of each path is 128 audio channels in each direction.
MegaCOMMS can also be converted to an optical format from BNC via our converters, when distances
over 150m are required. Maximum distance over optical is 2km via single mode fibre,