User Manual

SGD-SB2025NT-TUM, Part 2
Jan 12 Page 68 SOLAR (SYNC) TIMING
7 SOLAR (SYNC) TIMING
7.1 G ENERAL
The purpose of this section is to give the user an understanding of the way in which Solar operates
in the context of packet timing and buffering, which is core to the task of delivering consistent and
reliable audio over an IP network. There is more information on the specific subject of IP network
changes and the effects on Solar operation in Section 11 – Troubleshooting & Maintenance .
7.1.1 Synchronising Signals
The first and fundamental issue to be appreciated is that to synchronise multiple signals, which
each pass over different bearer circuits, means that each signal must be delayed by an amount
such that its total end-to-end delay is the same as that of the signal that passes over the route of
longest delay. In other words, hold back the quicker signals until they are each as slow as the
slowest, since it is not possible to make the slower signals travel faster. This is a core principle
and is just the same for Solar as it is for a conventional analogue borne simulcast system.
7.1.2 Network Jitter
There is an additional aspect about the use of IP that is different to conventional bearers and that
is “Network Jitter”. On a stable and robust IP network, IP packets are likely to arrive at a fairly
consistent rate. However, even on good, reliable networks there will be times when packets get
delayed slightly longer than the average, for a variety of reasons. To cater for this situation, it is
necessary to hold packets received at the destination unit in a store or buffer for a short (variable)
time, then output the data at precise intervals to iron out these variations and produce a steady
contiguous audio stream. Larger variations in the packet delivery times require larger buffers,
however, it would be poor practice to use large buffers as a means to overcome a poor quality or
unstable IP network. The user must be satisfied that the IP network is working correctly at the
outset before attempting to optimise Solar ‘Sync Timing’.
7.2 S OLAR I NTERNAL T IMING
The 1PPS timing signal is used to generate very accurate timing frames of 20 ms duration and it is
to these frames that Solar is synchronised – this is the function of the PLL in the NI. Therefore,
each 1 second period will contain 50 such frames, which are numbered 0 to 49 with frame 0 (zero)
starting on the rising edge of the 1PPS signal. Since all internal timing events are linked to the
frame clock, Solar’s timing parameters are in 20 ms steps.
7.2.1 Determining the Timing Values
In order to operate correctly, the timing settings must be sufficient to provide buffer periods that
can cater for the longest delay exhibited on the system. It should be recognised that the longest
network delay may not occur during normal operation but under network fault conditions if a
network re-route is applied (see Section 11.1 – Network Latency and Faults for further
information on this topic).
The initial approach for commissioning is to set timing periods that will probably prove to be
significantly longer than the network delays. However, these settings will need to be optimised, as
it is likely to result in the end-to-end delay on the audio signal being longer than necessary, which
may be noticeable especially for T/T operation.
7.2.2 PAT Reports
To be able to select the optimum timing values, the average time taken for data packets to arrive at
each unit are calculated and displayed on the TM Sync Timing page. These times are labelled
‘PAT’ and are calculated once a second, based upon the time taken for the packet originating in
frame 0 to arrive at the destination unit. The ‘PAT TM’ figure indicates the time of packet arrival at
the TM, the ‘PAT’ NI figure indicates the time of packet arrival at an NI. Both figures are rounded
up to the nearest 1 ms.