DRMradio

FAC Reference Pilots

Typical 10 kHz mode B DRM spectrum centred on 12 kHz IF.

These three reference pilots (or carriers) are used by the DRM receiver software to help synchronise to the DRM signal. The three FAC reference carriers are transmitted at baseband 750 Hz, 2250 Hz, 3000 Hz (marked as F1, F2, F3).

Unlike all the other DRM carriers, they are dedicated reference carriers whereas all the other carriers constantly switch between either carrying data or used as a pilot (to enable channel estimation).

These FAC reference carriers have their amplitude boosted relative to carriers carrying data by a power gain of 2 so that the DRM software can recognize these FAC reference carriers.

Individual carriers designated for the three FAC carriers.

These carriers are transmitted at the same frequency irrespective of the DRM mode being used. FAC reference carriers are used to calculate the initial coarse frequency offset of the received DRM signal compared to the receiver’s local oscillator. This is the first part of the COFDM synchronisation process required before the received DRM signal can be properly decoded.

FAC data

The FAC block contains 72 bits of information (64 bits of FAC data plus 8 bit CRC) to allow the DRM software to decode the multiplex. FAC parameters include: spectrum occupancy, interleaver, modulation mode, number of services, language, audio or data, programme type. Whatever the spectrum occupancy or transmission mode the DRM decoder always needs this FAC information first to know how to decode the rest of the DRM signal. Consequently the FAC is always transmitted in the same place with a fixed code rate. Comparing DRM to AM then the FAC would occupy the equivalent location of AM upper sideband.

Each transmission frame contains an FAC block (400 mSec). When more than one service is carried in the multiplex, a number of FAC blocks are required to describe all the services.

FAC data 64 bits.