The above show a spectrum plot of a strong Mode B DRM signal (Deutsche Welle), in reality the spectrum rarely shows this perfect
flat top shape as the radio signal may suffer selective fading. To create this DRM spectrum plot I used Spectrum Lab freeware
program (written by DL4YHF) with a FFT of 16k and averaged the result over several seconds to enhance the FAC reference pilots. The
spectrum also shows that there is no OFDM carrier broadcast where the carrier would be if it was an analogue AM double side band signal.
FAC
reference carriers
The three reference pilots (or carriers) are used by the DRM receiver software to help detect and synchronise 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
carriers.
Baseband frequency 
Mode A 
Mode
B 
Mode C 
Mode D carrier 

750 Hz 
18 
16 
11 
7 
F1 
2250 Hz 
54 
48 
33 
21 
F2 
3000 Hz 
72 
64 
44 
28 
F3 
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. After this is done then the decoding software needs to decode
the FAC to find out what mode and other parameters of the DRM broadcast.
The DRM specification numbers the OFDM carriers
with 0 being where the AM carrier would be. Carriers are numbered negatively if they are to the left of AM carrier (i.e. lower sideband
if we are still thinking in terms of AM), and positively (upper sideband) if they are to the right.
It is not
possible for the decoding software to directly look for these FAC frequencies in the data as there will be an unknown frequency offset
caused by the radio’s local oscillator not being exactly on frequency. For successful OFDM decoding the transmitter and receiver need
to be exactly on the same frequency.
Although the DRM decoding software doesn’t know exactly where these carriers are in the
frequency domain data, it does know the relative carrier frequency spacing will always be the same and will not change. The recognition
software uses three windows (±150 Hz) appropriately spaced to move through the frequency domain data created by the FFT summing the
value of all the individual carriers.
At a certain point there will be a correlation of the carrier amplitude that should coincide
with the three boosted FAC carriers. From this peak the actual frequency can be determined and the offset can be calculated, this
gives a coarse frequency offset. This is a necessary first part of the DRM synchronisation process.
The above plot shows the FAC correlation for a RTL short wave broadcast when they
were broadcasting on 6095 kHz (49 meter band). The X axis is the frequency offset centred on FFT bin 50 (12 kHz), the correlation
peak is at FFT bin 65, giving a frequency offset of (65 – 50) x FFT resolution = 15 x 2.93 = 43.95 Hz.
Internally a DRM decoding
software will compensate for this frequency offset and will also have a tracking frequency routine for any frequency drift within
the DRM receiver.