World Broadcasting Unions Digital Radio User Manual - page 12
DIGITAL RADIO GUIDE
TERRESTRIAL TRANSMISSION SYSTEMS - DRM
12
The combination of these techniques results in high quality sound in a narrow channel
with robust reception in an intended coverage area with relatively low transmission power.
In addition, source coding schemes using lower bit rates than that used with AAC/SBR
are included for lesser levels of audio quality if the AAC/SBR quality level is not desired
by a broadcaster. For example, a broadcaster may want to transmit two or more “speech”
only programs. These would not require the full performance of AAC/SBR.
4.1.2
Brief Description of the DRM System
(1)
Overall design
Figure 4.1: Transmission Block Diagram
source
encoder(s)
pre-coder
pre-coder
pre-coder
MUX
energy
dispersal
energy
dispersal
energy
dispersal
channel
encoder
channel
encoder
channel
encoder
OFDM signal
generator
modulator
audio data
stream
SDC
information
FAC
information
data
stream
FAC
SDC
MSC
normal prot.
[high prot.]
DRM transmission signal
flow of information
normal prot.
[high prot.]
cell
interleaver
normal/[high]
protection
OFDM cell mappe
r
pilot generator
Figure 4.1 depicts the general flow of different classes of information (audio, data,
etc.) after their origination in a studio or control centre (that would be depicted to
the left of the figure) to a DRM transmitter exciter/modulator on the right. Although
a receiver diagram is not included in the figure, it would represent the inverse of
this diagram.
There are two classes of basic information:
•
the encoded audio and data that are combined in the main service multiplexer;
•
information that bypasses the multiplexer that are known as fast access
channel (FAC) and service description channel (SDC), whose purposes relate
to identification and control for a transmitter and for appropriate decoding
selection within a receiver.
The audio source encoder and the data pre-coders ensure the adaptation of the
input streams onto an appropriate digital format. Their output may comprise two
parts requiring two levels of protection within the subsequent channel encoder.
The multiplex combines the protection levels of all data and audio services in a
proper format within the frame structure of the bit stream.