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http://www.cadac-sound.com/i/megacomms/megacomms-digital-audio-protocol/17/

Cadac’s MegaCOMMS digital audio network has been designed to meet the requirements of the most challenging applications. MegaCOMMS is a robust, TDM (time division multiplex) system. Control data is embedded within the data stream, so that no audio channels have to be 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 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. 

For larger, true audio networks, a MegaCOMMS router is required which provides low-jitter clock synchronisation and flexible routing capability for up to 3072 channels of 24-bit, 96 kHz audio. The addition of a router into a system only adds an additional sample (approximately 10us) of propagation delay. MegaCOMMS is also capable of providing automatic fail-over to redundant spare connections, so providing peace-of-mind for critical applications. Most Cadac MegaCOMMS devices (consoles, large-format stageboxes and network bridges) support dual-redundant connections, with automatic seamless change-over in the event of cable failure.

The hardware “hub” for a large-scale network is the Cadac MegaCOMMS router – the CDC MC Router. This device has 12 pairs (send and return) of MegaCOMMS ports, easily identifiable from glowing colour coded BNC sockets. The MegaCOMMS router can connect up to 12 MegaCOMMS devices, and a MegaCOMMS device can be a console, stagebox or network bridge. 

The CDC MC Router in conjunction with its control software, NetCOMMS, allows the fast and intuitive creation of powerful MegaCOMMS networks.  Via a PC via using a simple graphical user interface Router maps can be created, edited and saved before uploading to the CDC MC Router via the Ethernet connection or as part of a wireless network.

The CDC MC Router also has the useful function of providing “gain compensation” in that if any of the connected mic amps have their analogue gain adjusted, the router will automatically compensate for the change in audio level to any and all other devices connected. This gain compensation process takes just a single sample (about 10us.) for the router to make the necessary adjustment, and so is completely inaudible in operation. This facility allows a stagebox’s analogue inputs to be shared between a number of consoles, and for multiple consoles to be connected to a common set of I/O in order to provide multiple mixing systems or multiple fully dual-redundant systems. In these large scale applications, the MegaCOMMS router provides clock for all consoles and I/O devices.

Cadac have also designed network bridging devices to enable MegaCOMMS to connect to other protocols. The network bridges feature dual-redundant power supplies and dual-redundant connections to other MegaCOMMS units, as well as asynchronous sample-rate conversion to other popular protocols such as MADI (AES10), via the CDC MC MADI Newtork Bridge, and DANTE via the CDC MC Dante Network Bridge.

MegaCOMMS networks are clocked at 96 kHz, but Cadac do provide for SRC (sample rate convertor) to other clock speeds and conversion to more widely adopted protocols such as MADI and Dante.

Feature Summary

• Up to 128 bi-directional channels of 96 kHz / 24-bit audio
• Latency on a MegaCOMMS link is a single sample @96 kHz (around 10us.)
• Sub 0.4 millisecond latency from analogue inputs on stage, through all the console’s processing, to analogue outputs on stage
• Uses robust RG-6 co-axial cable
• Cable runs of up to 150 meters / 492 feet
• Via the CDC MC Optical bridge runs can be up to 2 km / 6,561 feet on single mode fibre