Digital Audio Broadcasting - DAB - Digital Video and Audio Broadcasting Technology - page 467

There are 4 different selectable modes in DAB. These modes are pro-
vided for different applications and frequency bands. Mode I is used in the
VHF band and Mode II to IV are used in the L band, depending on fre-
quency and application. The number of carriers is between 192 and 1536
and the bandwidth of the DAB signal is always 1.536 MHz. The difference
between the modes is simply the symbol length and the number of subcar-
riers used.
Mode I has the longest symbol and the most subcarriers and thus the
smallest subcarrier spacing. This is followed by Mode IV, Mode II and fi-
nally Mode III with the shortest symbol period and the least carriers and
thus the largest subcarrier spacing. In principle, however, it holds true that
the longer the COFDM symbol, the better the echo tolerance and the
smaller the subcarrier spacing, the poorer the suitability for mobile appli-
cations.
The modes actually used in practice are Mode I for the VHF band and
Mode II for the L band

Table 26.2. DAB modes

Mode Frequency
range

Subcar-
rier
spacing
[kHz]

No. of
COFDM
carriers

Used for Symbol

duration
[Μs]

Guard
interval
duration
[Μs]

Frame
length

I Band III

VHF

1 1536 single-

frequency
network
(SFN)

1000 246 96 ms

76 symbols

II L band

(<1.5
GHz)

4 384 multi-

frequency
network
(MFN)

250 62 24 ms

76
symbols

III L band

(<3 GHz)

8 192 satellite 125 31 24 ms

152 symbols

IV L band

(<1.5
GHz)

2 768 small

single-
frequency
network
(SFN)

500 123 48 ms

76 symbols

The audio signals in DAB are coded to MPEG-1 or MPEG-2 (Layer II),
i.e. compressed from about 1.5 Mbit/s to 64 ... 384 kbit/s. During this
process, the audio signal is divided into 24 or 48 ms long sections which
are then individually compressed, using a type of perceptual coding in
which audio signal components inaudible to the human ear are omitted.
These methods are based on the MUSICAM (Masking pattern adapted
Universal Subband Integrated Coding And Multiplexing) principle de-
scribed in ISO/IEC Standards 11172-3 (MPEG-1) and 13818-3 (MPEG-2)