Whether they broadcast video (television or streaming) or audio (radio) content, time synchronisation is paramount for broadcast networks.
Indeed, if equipment is not perfectly synchronised, various errors can occur such as audio/video desynchronisation, or editing errors caused by inverted frames.
These issues can be amplified due to poor network quality or very important traffic. It is therefore important for time synchronisation to be optimum in order to offer a satisfying user experience.
PTP: accuracy for broadcast networks
A synchronisation protocol offering high accuracy and operating over long distances is required to meet this need. As a matter of fact, broadcast networks must address a large number of clients spread across a country or more.
PTP (Precision Time Protocol) is a time synchronisation protocol which offers excellent guarantees and operates over long distances. It has been first standardised in 2002 under the name of IEEE-1588. It has been reviewed several times ever since.
PTP has been introduced to offer greater accuracy than NTP (Network Time Protocol) which only offers theoretical accuracy in the microsecond range. As PTP uses hardware timestamps, it can reach theoretical accuracy in the nanosecond range. In practice, the accuracy is less than a microsecond, which is sufficient for broadcast networks.
How does PTP work?
PTP operates on a master-slave mode via a master clock which will broadcast its time reference across the entire network.
In order not to overload the master clock of a network, the latter can be taken over by “boundary clocks” which act both as slaves for the master clock and as masters for clocks situated below them in the hierarchy.
Two types of messages will be broadcast. To begin with, the master clock broadcasts its timestamp, and the transmission delay is being estimated by the network slave clocks at the same time. Then, once a slave knows the network-induced offset between the clock and itself, it can synchronise its clock when it receives timestamps from the master clock.
The specificity of IP-based networks
Traditionally, operators would use analogue networks to broadcast audio or video content. As each channel could only carry a single stream, synchronisation needs were reduced and were provided by algorithms designed to operate on this type of support such as black and burst or tri-level sync. Recently, operators have moved away from SDI (Serial Digital Interface) to IP (Internet Protocol) for costs and easier maintenance purposes.
However, since SDI networks are more reliable than IP-based networks, this change implies paying more attention to packet management on the network as well as to synchronisation to avoid errors. As a result, time accuracy becomes paramount.
PTP is able to meet these challenges. Its high accuracy enables it to synchronise a large number of machines efficiently. Although audio and/or video frames leave the source in the correct order, there is no guarantee that packets will not encounter delays or be lost in the process. With accurate timestamping, the receiver will be able to put them back in order to broadcast the sequence in order to the user.
PTP technical aspects
The PTP configuration comes with a mechanism of profiles that allow a set of protocol parameters to be defined. These different profiles ensure that PTP will operate at its best for the application in question. As such, there is a profile dedicated to broadcast networks. It enables to make the most of PTP in order to avoid errors as much as possible. This profile has been defined by the Society of Motion Picture and Television Engineers and is named SMPTE 2059-2. Below are the parameters defined by the profile:
| Parameter | Default | Minimum | Maximum |
|---|---|---|---|
| Domain number | 127 | 0 | 127 |
| Announce interval | 250 ms | 125 ms | 1 s |
| Sync interval | 125 ms | 1/128 s | 500 ms |
| Delay request interval | Sync interval | Sync interval | 32 x Sync interval |
With the use of dedicated profiles, PTP also ensures synchronisation suitable for 4G and 5G mobile networks. These networks are increasingly becoming a challenge for broadcast networks as consumer habits evolve. More generally, PTP is now used in the Open Radio Access Network (Open RAN or O-RAN) standard.