The way towards digitisation

In the last few years, innovative technologies have changed many areas of both our private and professional everyday life. Vinyl records, telephones and typewriters have been replaced by CDs, mobile phones and computers to the largest extent. Digitisation has played a decisive role in this process. The demands on quality and operating comfort have increased along with the achievements. Existing technologies are faced with users' increased demands. Today, radio must be gauged by the CD and the new media in terms of sound quality and operation. With the "Digital Audio Broadcasting" technology, the digital age has now also dawned for the radio, and the requirements exerted on this medium can be fulfilled.

Digitisation

In contrast to analogue processes, where the information to be transmitted is presented in the form of oscillations, coding in Digital Radio is effected in long chains and in the digital information units `0` and `1`. Thus a data stream is formed which can contain different kinds of information like sounds, texts, images or software. The traditional concept of radio is thus expanding towards multimedia radio.

Data compression

Digital Radio uses the effects of the human auditory system. The human ear does not perceive sounds which are below a certain minimum volume. Data which are below the so-called 'threshold of audibility at silence' can be filtered out. This is possible, because in a digital data stream, for each information unit its volume relative to other units is stored as well. Moreover, in a sound signal, at a certain limit value the more quiet parts are superimposed by the louder ones. All the sound information in a piece of music which falls below the so-called auditory masking threshold can be filtered out of the signal to be transmitted. Both psychoacoustic effects lead to a considerable reduction of the data stream to be transmitted without a perceivable difference in sound for the listener. The technical term of this process is MUSICAM.

Transmission optimisation

The data compressed in this way are now prepared for broadcasting. The signal to be transmitted is split up across subcarriers (max. 1536) in a frequency block (1.5 MHz). The broad scattering of data over the frequency spectrum makes the data less sensitive to interference. As a rule, only a part of the information gets lost in the event of interference. The original signal can be reconstructed by means of digital correction methods, and the interference remains unnoticed by the radio listener. In addition, Digital Radio signals are provided with a special guard interval, which uses the effect of multipath reception in a positive way. The reflections of signals at houses and hills or mountains which cause interference in FM lead to an improvement of the reception signal in Digital Radio. These system components are summarised under the term COFDM.

Reception technology

The data stream which has been prepared and transmitted in this way is now received by the Digital Radio receiver equipped with a simple rod aerial. The so-called Viterbi decoder performs corrections of any errors which may arise in the receiver, based on mathematics. Already during transmission, up to three additional error protection bits are added to each digital information unit (bit). With the corresponding algorithms, each arriving information unit can be checked for correctness and corrected, if required.