Audio Global Viewpoint – February 2021
PTP Locking IP OB Trucks And Studios – Why?
As we progress through our IP journey the technology now available is providing us with the opportunity to rethink many of our working practices. But with these advances there seems to be some working practices emerging that defy logic. To me, one of these is PTP locking an OB truck with the studio.
Around about the mid 1980s viable frame synchronizers became readily available. Although they were expensive, and were significant pieces of hardware, they succeeded in synchronizing free running video and audio to the studio that was neither frequency nor phase synchronized.
Although frowned upon by the purist engineers who were unhappy with the theoretical degradation of the signal (one that few could measure never mind see or hear), the frame synchronizers worked wonderfully for OB trucks and greatly simplified many SDI systems.
As SDI expanded throughout the industry frame synchronizers became ubiquitous and any self-respecting MCR or studio would have more than enough devices to synchronize every outside source including OB and SNG trucks.
Then came along IP. I like IP a lot and in the right hands it can transform our approach to television and provide more flexibility, scalability and resilience then ever imagined. It’s fair to say, that to fully leverage ST2110 then we need an external time reference by using a PTP grand master clock. A system that is capable of providing sub-nanosecond accuracy and is great for keeping latency low.
In a network with multiple PTP source clocks the BMC (best master clock) algorithm choses which is the most accurate PTP clock and declares it the grand master. Thus, enabling all other devices such as cameras, sound consoles, and vision switchers to effectively slave to it. In these applications I accept we must have accurate localized timing for ST2110 to work efficiently and a PTP clock synchronized to a GPS type service is one of the most accurate methods of providing our timing accuracy.
Then the argument goes on, and this is where I have the problem, that the IP OB trucks PTP grand master must be GPS locked to satellites to make the trucks video and audio signals synchronous to the studio (assuming the studio is also GPS locked). Why? I ask. We haven’t made OB trucks frequency locked to studios for at least 30 years (probably more). Why are we doing this now?
I accept the OB truck must have a localized video frame frequency that is in the same ballpark as the studio, as was achieved by placing an SPG in the OB truck, but does it really have to be frequency and phase locked to the studio? Why not just put an IP frame synchronizer in the MCR or studio and allow the OB truck to free run? I remember that most of the SNG trucks I worked in during the late 1990s were just free running and relied on the MCR frame synchronizer to make the video and audio synchronous to the studio.
Surely free running PTP master clocks are close enough to meet our “frame frequency ballpark” needs? SPGs certainly are.
I suppose that if the OB trucks PTP clock and studio PTP clock were synchronized through a common GPS source then they would achieve frequency and phase lock, resulting in a theoretical minimum latency. But would anybody notice? Especially when we consider the number of video frames compression uses in returning the signal back to the studio. We don’t do this with SDI, so why put ourselves through the pain of trying to achieve it now?
I don’t think this level of accuracy is necessary and we’re trying to solve a problem that doesn’t need solving, that is, achieving frequency lock between an OB truck and studio. By doing so, we are greatly and unnecessarily complicating the system. An IP video frame synchronizer in MCR would easily facilitate the necessary studio synchronization allowing the OB truck (or any other outside source) to free run.
Please email me and let me know if you agree or not. Let’s keep it simple!
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