inventors — in-band on channel
(IBOC) radio — meaning that the
new digital system works within
the same spectrum and channel
assignments that are now used for
The creator of this system is
Ibiquity Digital Corporation of
Columbia, MD. They have been
working on this terrestrial digital
radio system for years. It was finally
blessed by the Federal
(FCC) back in October, 2002.
It has taken this long to make
commercial equipment available so
that broadcast stations can transmit
digital signals. It has taken time for
radio manufacturers to design and
build compatible receivers. As of this
year, almost everything is in place
for a roll out of this service.
One of the reasons that this
service was approved by the FCC is
that it is fully compatible with the
current analog AM and FM radio
services. Each station will continue
to broadcast its standard analog
signals for listeners with older,
traditional radios, but — for between
$70,000.00 and $100,000.00 — a
station can buy the hardware to
upgrade to the IBOC system. This
system adds the digital transmission
in only slightly more than the same
spectrum space allocated for analog
transmissions. This is called the
hybrid mode. It is expected that, over
the years, the system will evolve into
a fully digital mode, where the
analog signal is dropped completely
and the entire spectrum is digital.
The digital signals are transmitted
as sidebands above and below the
analog sidebands. Figure 1 shows
the arrangement in the AM band.
Normally, AM stations only use 10
kHz of space. With a frequency
response of only up to 5 kHz, this
makes the sidebands extend out 5 kHz
above and below the carrier frequency.
With AM stations usually spaced
10 kHz apart, the FCC usually
makes sure that there are no local
stations adjacent to one another, in
order to prevent interference. This
means that the local station is actually
permitted to use up to 15 kHz of
bandwidth above and below the carrier,
giving it the extra space needed to
accommodate the digital sidebands.
The same is true in the FM band.
Stations are typically spaced 200
kHz apart, but the bandwidth of the
signal — with an upper frequency
limit of 15 kHz and a modulation
index of five — is about 260 kHz. An
additional 70 kHz is added above
and below the analog sidebands to
accommodate the digital sidebands,
Figure 2. HD Radio FM hybrid spectrum.
NUTS & VOLTS
making the total bandwidth about
400 kHz (Figure 2).
As you may know, it takes a
great deal of bandwidth to transmit
digital signals. Even with efficient
modulation methods, digital signals
consume a great deal of bandwidth.
To reduce the amount of bandwidth
needed, audio signals with a frequency
range up to 20 kHz are compressed.
That is, after the audio signals are
digitized by an analog-to-digital
converter (ADC), they are put through
a mathematical process that reduces
the total number of bits needed to
accurately represent voice or music.
MP3, for example, is a type of
digital audio compression that
permits many songs to be stored in
flash memory or on a small hard
drive in an MP3 player. Another type
of compression is used in digital
radio. By reducing the overall number
of bits, the data rate can be lower and
that translates into less bandwidth.
After the data has been
compressed, it is scrambled to
randomize the bits and to minimize
multiple serial 0 or 1 bits that can
degrade reception. Next, the scrambled signal is encoded. The encoding
process is essentially one of adding
extra bits that will be used in a forward error correction (FEC) scheme.
Some form of error detection and correction is needed in most digital radio
systems in order to overcome the
problems that are invariably caused
by noise, fading, and interference.
Following the FEC encoding, the
digital data is interleaved. This
process reorders the serial bits to
disperse errors that develop in a
fading channel. The interleaved
signal is finally sent to the modulator.
As for the modulation method,
both the AM and FM systems use
orthogonal frequency division multiplexing (OFDM). This extremely
complex method spreads the signal
over a relatively wide band, which
helps to minimize the fading and
reflections often encountered in
vehicle radios. (Note to Readers:
Many of you have expressed an
interest in learning how OFDM