In serial communication, the bits (the ones and zeros) are sent one by one, i.e. consecutively, over a transmission channel (e.g. a cable or wireless connection).
The receiver must read these bits at exactly the same speed as the transmitter sends them. We call this synchronisation. If this synchronisation is lost, the receiver can no longer interpret the data correctly.
To prevent this, the sender adds a code word to the data stream at regular intervals. This code word serves as a reference point so that the receiver can get back “in step” with the sender.
Because the code word must not occur in the actual data, the data stream is encoded in a certain way. This makes the code word that serves as a reference point for synchronisation unique and prevents any misinterpretations when detecting the code word.
The code word together with the corresponding data bytes is called a frame.
Synchronous versus asynchronous
When these frames are sent continuously — i.e. without pauses — we refer to this as synchronous transmission. Even when there is no data, (empty) frames containing the synchronisation word continue to be sent. This ensures that synchronisation is maintained at all times.
In asynchronous transmission, on the other hand, there may be pauses or blank periods between frames.
An important difference between the two types is that in synchronous transmission, the time between sending and receiving remains constant.
In asynchronous transmission, this time can vary depending on the length of the pauses between frames.
This variation in timing is called jitter.
The problem with DAB+
With DAB+, the data (the music bits) is transmitted synchronously and serially between the transmitter and the receiver. The receiver synchronises on the short pauses of approximately 1.5 ms between each frame. These pauses are called the NULL signal (a type of code word) and serve as a beacon to keep the receiver in sync with the transmitted frames.
The data stream consists of so-called transmission frames, which are transmitted synchronously one after the other.
The problem: asynchronous feed
The difficulty arises at the transmitter input, where the DAB+ signal is usually fed in via the public internet.
The transmitter receives a data stream via ETI or EDI:
• ETI stands for Ensemble Transport Interface (the original standard).
• EDI (Encapsulated DAB Interface) is a modern variant that encapsulates the ETI signal in IP packets via the internet, based on the DCP protocol.
The problem is that these internet connections are asynchronous: the TCP/IP packets do not always arrive at exactly regular intervals.
When the incoming data speed temporarily falls below the speed at which the transmitter sends its transmission frames, the transmitter has no data to transmit.
To compensate for this, the transmitter has a buffer. This buffer temporarily stores data so that small delays do not cause a problem.
However, if the delays become too long (e.g. due to network congestion or jitter on the internet), the buffer can run out, causing the transmission to falter or interrupt.
The solution: a synchronous connection
A number of DAB+ transmitters usually also have a so-called E1 input (according to ITU standard G.703) or ETI-NI (RJ45 or BNC).
Unlike ETI or EDI, an E1 connection is a synchronous 2.048 Mbps connection, meaning that the timing between the transmitter and the source remains perfectly constant. This makes E1 connections more reliable for an operational DAB+ system, especially in SFN architecture with multiple transmitters that need to run perfectly in sync.
The BBC, for example, uses such E1 connections in the UK.
In practice, however, they are difficult to obtain from operators, and today they are often emulated via synchronous fibre optic networks such as SDH (Synchronous Digital Hierarchy).
In summary
| Type of communication | Characteristic | Application with DAB+ |
| Synchronous | No pauses between frames, fixed timing | Transmission frames from the transmitter |
| Asynchronous | Pauses or variable timing (jitter) | EDI/ETI input via the internet |
| E1 (synchronous) | Constant timed connection | Reliable professional solution (as used by the BBC) |
Summary
Communication between DAB MUX and DAB transmitter plays an important role in transmitter synchronisation, particularly when multiple transmitters are used in SFN configuration.
The best solution is therefore synchronous data transmission between DAB MUX and DAB transmitter. These are equivalent to the synchronous transmission signal at the transmitter output.
If DAB transmitters become out of synchronisation, they suddenly become a serious source of interference for other DAB transmitters broadcasting on the same channel.
In large parts of the transmission area, reception can become impossible if one transmitter goes out of synchronisation.
For practical reasons, the use of synchronous transmission is sometimes not possible (also a larger investment for the radios). Public networks (Internet) can cause unexpected delays due to overload.