12-12-2012, 11:43 AM
IBOC TECHNOLOGY FOR DIGITAL RADIO
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INTRODUCTION
Digital radio, also called digital audio
broadcasting (DAB), is transmission
and reception of radio signals
in the digital domain, as opposed to
the traditional analogue transmission/reception
by AM and FM systems. While
digital television signals are now routinely
received over cable and satellite systems,
reception of digital radio broadcasts is still
fairly uncommon.
Digital radio is similar to hooking up
the digital output from a CD player directly
to a radio transmitter. At the other
end is a digital-to-analogue converter
(DAC), which converts the digital signal
back into analogue mode so that it can be
heard on the audio system as it was recorded.
While this is not exactly what happens,
the result is essentially the same.
In practice, the CD player is hooked up
to a control board, which, in turn, routes
the signal as a part of the feed to the radio
station’s transmitter. The signal is either
impressed onto a signal carrier and transmitted
via uplink to a satellite (which,
through a transponder onboard, retrans-
IBOC TECHNOLOGY FOR
DIGITAL RADIO
mits the signal earthward)
or transmitted across the
land (terrestrial). In both
the cases, the signal is received
and converted into
analogue by a specialised
DAC within the radio’s
tuner circuit.
Why digital radio?
Radio gets a new lease of life with the
move to digital. The main advantage of
digital radio is that it doesn’t have the
usual distortion associated with analogue
radio such as hissing, popping and phasing.
It is immune to distortion from
multipath, adjacent stations, overly weak
or overly strong signals, etc. Also, digital
radio signals can carry text information.
Users get a new array of data-rich services
including traffic information, sports
score and weather updates, stock prices,
etc. This data is displayed on the liquid
crystal display (LCD) in the form of text,
images and/or video. Thus multimedia radio
becomes a reality.
Audio features such as time-shift recording/
digital recording of playlists using
transmitted electronic programme
guides (EPGs) can be implemented.
The digital radio will converge with
different types of compelling end products
including MP3 players, cell phones
and personal digital assistants (PDAs). This
economical, futuristic digital communication
pipe provides new advertising and
electronic commerce opportunities to radio
broadcasters.
FM hybrid IBOC waveform
Fig. 1 shows the FM hybrid IBOC spectrum.
Low-level digital sidebands are
added to each side of the analogue signal.
The bandwidth is limited to 200
kHz from the centre frequency. Restricting
the digital sub-carriers to the 70kHz
region between 129 and 199 kHz from
the centre frequency on either side of
the analogue spectrum minimises interference
to the analogue host and adjacent
channels without exceeding the existing
FCC spectral mask. This bandwidth is wide
enough to support a robust, hybrid IBOC
service with virtual CD-quality audio that
mirrors the coverage of existing analogue
radio stations.
The dual-sideband structure enables
the use of frequency diversity to further
combat the effects of multipath fading and
interference. The baseline hybrid system
simultaneously transmits 96 kbps of error-
protected digital audio information,
plus auxiliary services, on each DAB sideband.
Each sideband has all the information
and thus can stand alone. However,
when neither sideband is corrupted, advanced
FEC coding techniques allow the
combination of both sidebands to provide
additional signal power and coding gain.