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Will The Transition To IP Disrupt Live Production Workflows? Part 2

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Continued from Part 1...

Accuracy and Reliability is Key
One reason for PTP's suitability to broadcast applications is the resilience provided by the use of the Best Master Clock Algorithm (BMCA). The BMCA allows the most accurate Master to automatically take over the duties of Grandmaster when the previous Grandmaster loses its GPS lock, gets disconnected from the network, or is unable to act as Grandmaster for any reason. The BMC Algorithm uses a number of criteria to determine which Master should be Grandmaster including the following abridged list: User Definable Priority; Clock Class (e.g. GPS vs free running) and Clock Accuracy. It is also worth noting that if any GPS synchronised Master loses GPS lock, it will of-course itself become free running. Once GPS lock re-acquired, unless the Master’' local oscillator phase-lock loop (PLL) is driven slowly to re-synchronise with the GPS clock, then the system can suffer from what is known as "Sync Shock" when the Master's clock frequency suddenly changes. Whilst this may be acceptable in some IT applications, this is of course highly undesirable in a video production application. In the case of the SPG8000A, the "Stay Genlock" feature is designed specifically to avoid the problem of Sync Shock through careful control of the PLL.

Although in theory it is possible to use a Master with software-based time-stamping, in the case of live video production applications, it is highly unlikely that such a device could be devised with the necessary clock accuracy required for synchronous video processing. A hardware time-stamped Grandmaster device such as the SPG8000A is locked to GPS (or GLONASS or both to provide greater constellation resilience), with the Grandmaster’s local oscillator being phase-locked to the GPS reference. This local oscillator is the reference clock used with dedicated hardware for the precise timestamp of the incoming PTP messages and PTP sync packets. A dedicated hardware approach is unaffected by operating system behaviour or network traffic latency.

Although the transition to IP is seen by some as inevitable, not all equipment is available with IP interfaces. It must also be considered that the investment in SDI equipment has been so considerable, that the use of hybrid IP/SDI networks is likely for the foreseeable future. For broadcast applications, it is essential that the PTP Grandmaster provides support for the application specific video and audio PTP profiles, such as SMPTE 2059 and AES67, as well as traditional SPG features including black burst, tri-level and SDI out. All the above protocols must be referenced to the same GPS clock, or such a hybrid IP/SDI network would be inoperable. It must also be considered that a broadcast live production network is entirely reliant on a stable reference and any timing and synchronisation devices “must work”.

Although many equipment vendors have IP enabled equipment at an early stage of development, both equipment vendors developing IP video equipment and broadcasters and other content providers producing content are reliant on the availability now of an accurate and reliable timing and synchronisation solution. To refine functionality, the SPG8000A had an unprecedented 12 month period of field trials prior to being launched on to the market.

Although the concept of carrying uncompressed (or lightly compressed) video over IP is perceived as being very new, and indeed revolutionary, the precedent for the broad adoption of IT infrastructure for live production facilities has in fact been in place for many years. IT infrastructure began to be adopted almost fifteen years ago for compressed video distribution using MPEG-2 transport streams over IP. IT infrastructure is also in industry-wide use as the distribution and control component for file-based workflows. In both these cases, Tektronix provided technology support for early adopters; with TS over IP test equipment as well as with the first file-based QC tool on the market.

As such, Tektronix was involved with the earliest adopters of compressed video over IP and file-based QC and continues to be closely engaged with these latest developments with baseband video over IP.

We are at the beginning of a long term transition to IT-based infrastructure and those involved in the production and facility side of video have little experience with the new technology, but conversely are extremely experienced using SDI and all the issues associated with its use. This coupled with a huge investment in existing technology and workflows implies that the transition will take place gradually, making it likely that hybrid SDI/IP infrastructure will be in place for some years. Such production facilities will require equipment that is able to operate seamlessly and reliably in such a hybrid environment. The companies best placed to provide equipment that meets those requirements are those who have experience both of the challenges of the live production environment, as well as extensive experience of the challenges associated with the distribution of video over IP networks.
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