Why bother with PTP and SMPTE ST 2059?

Since the beginning of TV synchronising all the elements has been critical. It all started with having to synchronise the beam in the video cameras with the beam in the TV. Naturally, when you want to bring more than one camera together, you need to make sure all the cameras are in the same point of the picture at the same time otherwise the picture will jump around when you mix between them, known in the trade as a non-sync cut.

Whilst timing can require very precise system designs, distributing black and burst is not so hard 70 years on, although it’s fair to say that large systems are more difficult due to complexity. Some stations will want to run tri-level sync to support an HD workflow at, say 23.98fps, which will leave them with two timing systems. If any audio workflows are needed, they are likely to need word clock, and perhaps others need DARS (word clock over AES3). For a larger broadcaster, then, it’s not hard to find yourself with several timing infrastructures each needing their own cabling and distribution amplifiers (splitters). Although redundancy in timing systems is certainly possible, many of these systems aren’t redundant. Sometimes when you lose timing, it’s very obvious, but sometimes the symptoms are confusing and seem unrelated. This means when an amplifier does fail, it can take a long time to determine the cause and then also find the DA.

PTP is a network-based timing distribution system standardised by the IEEE as 1588-2019. It’s not simply a distribution of a synchronising pulse, like black and burst, it’s a time distribution system. Like NTP, used in general computing, PTP provides a way of setting the time on each device. In PTP’s case, this is a very precise time, hence the name Precision Time Protocol. Being based on UDP, it can be delivered over the same network cabling already in use.

PTP can replace all previous synchronisation signals because it the distribution of time and not pulses. If all devices have the same time, then they can infer when the pulses should be based on that time. This is what the SMPTE standard ST 2059-1 does. Defining equations for each signal type, ST 2059-1 allows equipment to know when the beginning of that type of signal should be happening. SMPTE ST 2059-2 defines the PTP parameters to be used which should ensure that all devices are able to interoperate.

It’s important to remember that PTP is a time distribution system which employs bi-directional messaging to calculate precise timing. Whereas black and burst was a waterfall, cascading distribution system which could be installed and nearly forgotten, PTP should be monitored to ensure the system is continuing to work – both the infrastructure and the number of connected end-points.

In summary, then, PTP drastically simplifies timing around your facility in terms of physical infrastructure and future flexibility. It will also, probably, but cheaper if not during project installation, in the long run. This is balanced against the added understanding and training needed to configure, diagnose and maintain a PTP system.

Title image by George M Groutas, used under the Creative Commons attribution 2.0 licence

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