The AVN-GMCS is a PTPv2 grandmaster clock for use with AoIP applications. IEEE1588-2008 PTPv2 (precision time protocol) is used to synchronise all the nodes within a network. To achieve this one of the nodes must become the master clock and distribute time packets to the others. The AVN-GMCS is designed to perform this role simply and accurately, enabling sub micro second synchronisation between all nodes.
RAVENNA (of which AES67 is a subset) allows for the distribution of audio across a network. For this to be possible, each of the nodes needs to be time synchronised with one another. Ravenna uses PTP time stamping to achieve this, which distributes the network time but also works out the latency involved in the delivery and adjusts the time at each node accordingly. Unit configuration is achieved easily either with the front panel controls or the webserver, including the setup of the PTP profile.
In normal operaton, the unit has PTPv2 time stamping resolution to 8nsec. The AVN-GMCS uses a combination of a GPS receiver, a PLL (phase lock loop) and a specialist on-board clock device to create the precise, low jitter clock signals required to drive the time stamping circuitry of the physical transceiver, also providing holdover if the GPS signal is lost.
The specialist on board clock is available in three different types: TCXO, OXCO and CSAC (Chip Scale Atomic Clock, Caesium), which vary in both price and accuracy:
AVN-GMCS – TCXO Temperature Compensated Oscillator accurate to 1 part per million (worst case 1 sec gain/loss every 11.5 days). *
AVN-GMCOS – OCXO Oven Controlled Oscillator accurate to 0.01 parts per million (worst case 1 sec gain/loss every 3.1 years). *
AVN-GMCCS – SAC Quantum Atomic Clock accurate to 0.00050 parts per million (worst case 1 sec gain/loss every 63 years). *
A separate clock input can act as an alternative reference source to GPS. Clock outputs, driven from the physical transceiver, can be used to provide media clocks for external equipment local to the AVN-GMC when it is in both master and slave states. The clock outputs are available as AES-3id and word clock at 32, 44.1, 48, 96, 176.4 and 192kHz. A variable PPS output is also available to provide an external clock accurate to the PTP system timing.
The built-in webserver, or front panel LCD display, can be used to configure the unit. Front panel LEDs show the synchronisation status, GPS lock and the status of the AC and DC power supply inputs.
4 general purpose outputs indicate critical states for the unit using a 9 way D-type connector mounted on the rear panel. Pull down when active pins are supplied for GPS lock status, external sync present, AC power present and DC power present.
The unit has a front panel power button and dual power connectors – an IEC mains input and a 12V DC input, which allows the AVN-GMCS to be used for both studio and mobile installations. Moreover this allows for a secondary power source to reduce the effect of power down events. In any case, the unit monitors the status of both power sources and displays this on the front panel. In power off situations, a super capacitor is used to keep the GPS receiver powered in a low power mode for more than 20 hours, enabling the receiver to regain lock immediately rather than having to cold start.
* This figure represents the holdover accuracy should the GPS signal be lost – this is an approximation based on 1st year stability figures.
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