Technical aspects of DRB audio and data transmission

How does Digital Radio work ?

This technical explanation was presented by the Digital Radio Study Group at the recent Forum in Sydney:.


Generation of a Digital Radio Signal

Figure 1 is a block diagram of a conceptual digital radio generator, based on the Eureka
147 model. Each audio or data service is coded individually at the source level, error
protected and time interleaved in the channel coder. The services are then multiplexed
in a main service channel multiplexer, according to a pre-determined, but adjustable,
multiplex configuration. The multiplexer output is combined with multiplex control and
service information to form the transmission frames. Finally, the signal is modulated,
typically with Orthogonal Frequency Division Multiplexing (OFDM). The signal is then
transposed to the appropriate radio frequency band, amplified and transmitted.

Reception of a Digital Radio Signal

The digital radio ensemble is selected in the
tuner, the output of which is fed to the demodulator (typically OFDM demodulator), and
channel decoder to eliminate transmission errors. Multiplex control and service
information is passed to the user interface for service selection and to set up the receiver
appropriately. The main service channel data is further processed in an audio decoder to
produce the left and right audio signals, or in a data decoder as appropriate.

Audio Compression

Digital radio systems reduce the audio source data rate by means of digital audio
compression techniques, typically a low data rate sub-band coding system enhanced by
a psychoacoustic model. Due to the specific behaviour of the inner ear,
the human auditory system perceives only a small part of the complex audio spectrum.
Only those parts of the spectrum located above the masking threshold of a given sound
contribute to its perception, whereas any acoustic action occurring at the same time but
with less intensity and thus situated under the masking threshold will not be heard
because it is masked by the main sound event.

To extract the perceptible part of the audio signal the spectrum is split into a number of
equally -spaced sub-bands. In each sub-band, the signal is quantised in such a way that
the quantising noise matches the masking threshold. Digital Radio standards do not
generally prescribe the audio encoder – only the format of the coded bit stream and
actions to be taken by the decoder. This approach has the advantage of allowing for
future improvements in the encoder (ie further data rate reductions) without the need to

change existing decoders (ie receivers). This approach, however, means that different
encoders, although compliant with the standards, may afford different audio qualities.


Program Associated Data

Each audio program contains a variable amount of Program Associated Data (PAD)
which is used to convey information together with the sound program. Typical
examples of PAD applications are dynamic range control information, a dynamic label
to display program titles or lyrics, speech/music indication and text with graphic

Independent Data Services

In addition to PAD, general data may often be transmitted as a separate service,
typically in the form of a continuous stream or in packet mode, where individual packet
data services may have much lower capacities and are bundled in a packet
sub- multiplex. Typical examples of Independent Data Services are a Traffic Message
Channel, correction data for Differential GPS, paging and an electronic newspaper.

Conditional Access

Digital Radio systems may include Conditional Access (CA). Usually, the CA can be
applied to any or all of the services within an ensemble. The CA system includes three
main functions: scrambling/unscrambling, entitlement checking and entitlement
management. The scrambling/unscrambling function makes the service
incomprehensible to unauthorised users. Entitlement checking consists of broadcasting
the conditions required to access a service, together with encrypted codes to enable
unscrambling for authorised receivers. The entitlement management function distributes
entitlements to receivers.

Service Information

The following elements of Service Information (SI) can generally be made available to
the listener for program selection and for operation and control of receivers:

· basic program service label (ie the name of a program service);

· program type label (eg news, sports, classical music);

· dynamic text label (eg the program title, lyrics, names of artists);

· program language, time and date, for display or recorder control;

· switching to traffic reports, news flashes or announcements on other services;

· cross-reference to the same service being transmitted on another frequency or
channel or via AM or FM and to other services;

· transmitter identification information (eg for geographical selection of information).