World Broadcasting Unions Digital Radio User Manual - page 13

DIGITAL RADIO GUIDE

TERRESTRIAL TRANSMISSION SYSTEMS - DRM

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The energy dispersal provides an ordering of the bits that reduces the possibility of 

unwanted regularity in the transmitted signal.

The channel encoder adds redundant bits as a means for error protection and

correction and defines the mapping of the digitally encoded information into QAM

cells, which are the basic carriers of the information supplied to the transmitter for

modulation.

Cell interleaving rearranges the time sequence of the bits as a means of

“scrambling” the signal so that the final reconstruction of the signal at a receiver will 

be less affected by fast fading than would be the case if “continuous” speech or

music were transmitted.

The pilot generator injects information that permits a receiver to derive channel-

equalization information, thereby allowing for coherent (includes phase information) 

demodulation of the signal.

The OFDM cell mapper collects the different classes of cells and places them on a 

time-frequency grid.

OFDM depends on each of many subcarriers carrying its own sinusoidal

amplitude/phase signal for a short period of time. The ensemble of the information

on these subcarriers contains what is needed for transmission. In the case of DRM, 

for a 10 kHz channel, there are hundreds of subcarriers.

The modulator converts the digital representation of the OFDM signal into the

analogue signal that will be transmitted via a transmitter/antenna over the air –

essentially phase/amplitude representations as noted above modulating the RF.

With a non-linear high-powered transmitter, the signal is first split into its amplitude 

and phase components for injection in the anode and grid circuits, respectively,

and then recombined (by the action of the transmitter itself set at the correct

differential delay time), and then recombined prior to final emission. This splitting is 

not necessary for linear amplification.

(2)

Distribution Interface

Referring to the extreme left of Figure 4.1, apart from audio and data applications

that are multiplexed, additional information is sent that is required  to instruct the

transmitter to select the correct mode, error protection level, etc. and to send

information in the transmission to the receivers to permit them to switch to the

selection of several variables to allow for proper decoding. (The boxes and arrows 

for this are not shown directly in  Figure 4.1.) In the aggregate, this collection of

information and the means to get it to the transmitting station is called the

“Distribution Interface” (DI). 

These signals can emanate from a studio, or from a more elaborate network

control centre, and be transmitted via land lines or via satellite circuits to the

appropriate transmitter station(s). These details will not be noted here, but can be

found in ETSI documents TS 102 821 and TS 102 820, both dated July 2003.

In terms of connections with other parts of the DRM system, these signals, as

appropriate, are placed in either the Fast Access Channel (FAC) or the Service

Description Channel (SDC) for transmission to receivers.

There are 4 categories of data associated with the Distribution Interface: