Much has been said over the past few years about the benefits of moving to use more ‘IP’ in broadcast, most of which has focussed on simply replacing the existing SDI connections with IP ones.
This paper will look at where the use of IP can enable innovative ways of working that would not be possible or practical without IP. The paper will pay specific focus to remote production as this is an area where latency cannot be avoided but if it is embraced can lead to more flexible methods of production which could drastically change the costs models for live production of outside events.
What is ‘Time’ anyway? The terms ‘Time’ and ‘Timing’ mean many things to many people in broadcast but whatever the interpretation, understanding exactly where a frame of video or sample of audio belongs is what makes television work.
For many, time can simply mean ‘time-of-day’, for others timing refers to the measure of frequency and phase. Systems generating video frames, sampling analogue signals or handling multiple signals together all rely on having some sort of reference signal to enable them to derive accurate frequency and phase alignment to ensure their processing occurs at a predictable and stable time over lengthy periods of operation.
Since the introduction of HD broadcasting it is becoming common for systems to be designed to handle a mix of frame rates (e.g. 1080i/25 and 1080p/50). With increased sharing of media on the global market as well as online, more complicated mixes of frame rates can be encountered too. It is typical in systems designed to handle a variety of frame rates that time is measured as an absolute value with at least millisecond (ms) accuracy.
SMPTE ST-2059 defines how the IEE’s Precision Time Protocol (PTP) should be used in broadcast systems. PTP time addressing provides a mechanism for identifying time down to nanosecond granularity with a time range of 136 years (2^32 seconds). The standard defines how absolute PTP values equate to timecode labels commonly used in broadcasting and allows the expected video signal phase to be calculated for all standard formats. Essentially the use of PTP clocks on an IP network replace the need for both time-of-day timecode distribution (e.g. LTC) and other reference signals (e.g. BlackBurst).
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