Digital Signal Processing Reference
In-Depth Information
and actually developed for DAB as part of the DAB project. In MPEG-1
and -2 it is possible to transmit audio in mono, stereo, dual sound and joint
stereo modes. The frame length is 24 ms in MPEG-1 and 48 ms in MPEG-
2. These frame lengths are also found in the DAB Standard and also affect
the length of the COFDM frames. The same applies as before: DAB is a
completely synchronous transmission system where all processes are syn-
chronized with one another.
MPEG1, 2
compatible
part
Bit
allocation
Scale
factors
Header
Subband samples
XPAD
SCFCRC
FPAD
Fig. 26.7.
DAB audio frame
Fig. 26.7. shows the structure of a DAB audio frame. An MPEG-1-
compatible frame has a length of 24 ms. The frame begins with a header
containing 32 bits of system information. The header is protected by a 16
bit long CRC checksum. This is followed by the block with the bit alloca-
tion in the individual sub-bands, followed by the scale factors and sub-
band samples. In addition, ancillary data can be optionally transmitted.
The sampling rate of the audio signal is 48 kHz in MPEG-1 and thus does
not correspond to the 44.1 kHz of the audio CD. The data rates are be-
tween 32 and 192 kbit/s for a single channel or between 64 and 384 kbit/s
for stereo, joint stereo or dual sound. The data rates are multiples of 8
kbit/s. In MPEG-2, the MPEG-1 frame is supplemented by an MPEG-2 ex-
tension. In MPEG-2 Layer II, the frame length is 48 ms and the sampling
rate of the audio signal is 24 kHz.
This audio frame structure of the MPEG-1 and -2 Standards is repeated
in DAB. The MPEG-1- and MPEG-2-compatible part is supplemented by
a DAB extension in which program-associated data (PAD) are transmit-
ted. Between these, stuffing bytes (padding) are used, if necessary. In the
PAD, a distinction is made between the extended PAD "X-PAD" and the
fixed PAD "F-PAD". Among other things, the PAD include an identifier
for music/voice, program-related text and additional error protection.
DAB audio data rates normally used in practice are:
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