World Broadcasting Unions Digital Radio User Manual - page 63

DIGITAL RADIO GUIDE

TERRESTRIAL TRANSMISSION SYSTEMS - ISSUES

63

broadcasters can be expected to have significant flexibility in determining when

and how to phase out the analogue portion of the IBOC signal all together.

In addition, the simulcasting of audio material in some IBOC systems, while done

primarily to facilitate time diversity, can also mitigate the undesired digital receiver

behaviour experienced in cases of severe signal obstructions or extreme cases of

interference.  In these cases, systems without time diversity (such as Eureka 147)

exhibit what is called a "cliff effect" failure, in that the audio signal is perfect one

second, and completely gone ("muted") the next.  In a simulcast IBOC system, the

existence of the "backup" analogue signal for purposes of time diversity has the

added effect of eliminating the cliff effect failure mode, since in those cases the

receiver will blend to analogue and the audio program, while degraded, will not go

away all together, and is likely to remain with the listener throughout the

impairment.

These developments are at a relatively early stage and their viability has to be

assessed, but the work carried out to date is encouraging. The audio quality

achievable with simulcasting remains to be established.

(2)

DRM (Digital Radio Mondiale)

Two types of simulcast are present in the DRM design. The first is confined to a 9

or 10 kHz channel. Half the channel is used for an analogue signal capable of

envelope detection (in order that a conventional AM radio receiver can demodulate 

the signal).  The other half is a DRM digital signal that requires digital demodulation. 

The second technique requires 18 or 20 kHz of 2 adjacent channels where one

channel contains standard AM and the other contains either a 4.5/5 or 9/10 kHz

DRM signal.

For Regions 2 and 3 the simulcast solution is potentially much simpler as the Long 

and Medium Wave bands have been allocated 18/20 KHz channels.  In Region 3

the 18 kHZ allocation is also protection against night time sky wave interference.

4.5.3

Coverage

The move from analogue to digital transmission raises important questions under the

heading ”coverage.”

One of the main differences between analogue and digital broadcasts is the mode of

failure when the received signal starts to fail. It happens at the edge of the service area

and at locations within the coverage footprint where the signal strength is affected by

shadowing or interference. When the signal strength reduces, analogue reception is often 

described as degrading “gracefully.”  By contrast, a digital signal will at some point fail

suddenly and completely. Whilst usually robust in areas of generally poor analogue

reception, the digital signal gives little indication as it approaches a point of failure.

Within a defined coverage area, the service availability from analogue and digital services 

will be affected by the type of receiver (fixed, mobile or portable), by the type of

environment (urban, rural), and by the topography.  It is also a function of the

transmission frequency and the system performance.

COFDM signals (such as those used in the Eureka 147, DRM and AM and FM IBOC

schemes) have characteristics which facilitate the planning of single frequency networks