A new EBU Technical Review article explains how PTP – the IEEE 1588 Precision Time Protocol – is used to ensure reliable synchronization of the devices in an IP-based production chain. It's the first of a series of articles that will explore the topic progressively in more depth.
For traditional SDI-based production the question of synchronization is relatively simple, since SDI is an inherently synchronous communication medium. Sync pattern generators are used within the broadcast facility to provide an absolute reference for frequency and time, delivered over every SDI link.
However, as the industry is taking its first steps away from SDI, looking to use IP networks for production, the task of synchronizing a multitude of elements connected via Ethernet is no longer as straightforward.
As explained in the new article, written by Thomas Kernen (Mellanox) and Nikolaus Kerö (Oregano Systems), media organizations have turned to PTP to solve the timing challenge. The IEEE protocol is already used extensively in telecommunications and industrial applications. It was specified in a generic manner so that its performance could be adjusted to suit the requirements of a specific application, via PTP profiles.
The two PTP profiles that are relevant for broadcast production are AES67 – for audio – and SMPTE ST 2059-2. The latter, for video applications, is an important element of the SMPTE ST 2110 suite of specifications for professsional media over managed IP networks (see tech-i 34, pages 8-9).
This first article explains the basic principles of time transfer with PTP and how accuracy is achieved. It also covers how the different devices in the network interact with respect to timing, with one node being "elected" to act as a master and the others as slaves.
Future articles will discuss how PTP messages are transported over an IP network, the implications of the broadcast PTP profiles, (SMPTE ST 2059-2 & AES67) and how the building blocks of a PTP node impact its accuracy. The authors will compare different PTP aware network devices with each other, putting special emphasis on covering the PTP management mechanism as it is extensively used in broadcasting applications.