Megacomms: Data Protocol - Cadac C Series Hardware Overview

MegaCOMMS: Data Protocol

Communication between the unit and the control surface is via a proprietary Cadac high speed protocol
called MegaCOMMS. The protocol uses high-speed 75 ohm coaxial cable terminated in BNC
connectors or depending on the unit, via fibre optic cable using the duplex optical LC ports.
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 100 metres (328 ft) via a pair of RG-6 coaxial
cables and up to 2km (1.24 miles) via single mode fibre on optical.
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-stage rack
configuration. In this application, the console provides the clock and the stage rack synchronises itself
once the connections are made. Total through-system propagation delay for this system, including all
console processing and A/D to 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.
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