Time (and What You Need to Know About It)Time
Time plays an essential role in electronic systems, and certainly in the operation of DAB+. A basic understanding of the various time scales is necessary to fully grasp synchronization within a Single Frequency Network (SFN).
DAB+ employs various mechanisms to enable this synchronization. Which ones are used depends on the manufacturer’s implementation and the network configuration.
In this chapter, we discuss the key concepts needed to understand the time references within DAB+.
The SI-second
The SI second is the official unit of time in the International System of Units (SI). It forms the basis of virtually all modern time systems and is essential for applications in which accuracy and synchronization are of great importance.
The SI second is defined as 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine energy levels of the ground state of the cesium-133 atom. This extremely stable reference is realized using atomic clocks.
Atomic Time (TAI)
TAI (Temps Atomique International) is the international atomic time and is based entirely on the SI second. Unlike UTC, TAI does not include leap seconds, and time passes completely continuously.
PLEASE NOTE:
There is an important distinction between TAI and the “TAI-based” time used in the EDI interface. For example, TAI starts counting from January 1, 1958! The EDI standard for DAB+ makes an important adjustment here and starts on January 1, 2000!
Within the EDI standard, time is represented as the number of SI seconds that have elapsed since January 1, 2000, at 00:00:00 TAI. This is therefore not a complete TAI time scale, but a TAI-based time measurement with a fixed reference date.
This continuous time scale forms the basis for the synchronization of DAB+ stations within an SFN network.
TAI is a fully continuous time scale based exclusively on atomic clocks.
UTC time
UTC (Coordinated Universal Time) is the international standard time displayed on our clocks worldwide. As the Earth’s rotational speed is not entirely constant, UTC is adjusted by adding (or, in theory, removing) leap seconds. In this way, UTC remains closely aligned with the Earth’s rotation.
Because of these leap seconds, UTC is less suitable as a time reference for precise synchronisation applications such as a DAB+ SFN. For this reason, a continuous atomic time reference is used for synchronisation.
However, UTC remains essential for applications where a human-readable date and time format is required, such as log files, error messages and event logging.
UTC also attempts to keep pace with the Earth’s rotation. When the Earth’s rotation slows slightly, a positive leap second is added to UTC. As a result, UTC always lags slightly behind TAI.
UTCO (UTC Offset)
In order to derive UTC from the TAI-based time, the UTCO (UTC Offset) field is included in EDI messages. This field consists of 8 bits and contains the number of leap seconds that have been introduced since 1 January 2000.
Using this offset, the UTC time can be easily calculated:
UTC = TAI-based time − UTCO
The combination of a continuous atomic time and the UTC offset makes it possible, on the one hand, to achieve extremely accurate SFN synchronisation and, on the other hand, to display log files and events in the familiar UTC time.