NAB 2002 has come and gone, and we can now assess equipment availability for use in a DAB system. The introduction of radios that
are capable of receiving the Ibiquity system is slated for the January
2003 CES, so it's time to get started. Before deriving budget numbers,
review the information at hand so that you know how IBOC works.
AM and FM IBOC systems have much in common. They carry the same
audio as the analog portion as well as ancillary data services. The AM
and the FM IBOC DAB systems use the spectrum already allocated to the
particular broadcast station, hence the descriptor in-band
Instead of transmitting all of the data serially, on one carrier,
the data is transmitted in a parallel fashion, diluted over multiple
groups of subcarriers that are spread about symmetrically with respect
to the center carrier frequency. In the case of AM IBOC, there are
three sets of subcarrier groups: primary, secondary and tertiary.
Block diagram of an AM path.
The primary group is transmitted from about 10kHz to 15kHz from the
center frequency. The secondary group is transmitted from about 5kHz to
10kHz from the center frequency, and the tertiary group is transmitted
from about 360Hz to 4.7kHz from the center frequency. The power of each
subcarrier varies according to its group. The strongest subcarriers are
in the primary group. Each subcarrier in the primary group is modulated
at 30dB below the carrier level (i.e., about 5W per sub-carrier for a
5kW station). With FM IBOC the data is transmitted via two groups of
subcarriers. The lower digital side-band group spans the spectrum from
almost 200kHz to about 130kHz below the center frequency, and the upper
from 130kHz to just under 200kHz above the center frequency. Each
subcarrier in these groups has a power of -48dB below the unmodulated
carrier level, and the total average power for each of the groups
(upper and lower) is 23dB below the unmodulated carrier level.
Transmitting the IBOC signal
The AM and FM schemes use exciters that are
similar. Both have two AES inputs (44.1kHz sampling frequency). The
first AES input feeds the Ibiquity Perceptual Audio Coder (PAC)
encoder, which is used in generating the data stream that ultimately
carries the program through the system. The second AES input is delayed
in time and then processed for the analog transmission.
The delay on the analog portion is needed to synchronize it with the
digital signal. This way the receiver in the field can accomplish a
graceful blend from digital to analog and vice versa.
Block diagram of an FM high-level combining
The AM IBOC exciter interfaces with the AM transmitter via two
signals: one representing magnitude, and one representing frequency and
phase. To transmit an AM IBOC signal, a station will need a transmitter
capable of transmitting the IBOC subcarriers while faithfully
generating and transmitting the backwards-compatible AM signal. The FM
IBOC exciter output signal consists of just the digital side-band
groups centered around the carrier frequency. The Ibiquity system
specification is for the transmission of the digital side bands (added
together) 20dB below the unmodulated carrier level. There are three
methods currently being discussed for accomplishing this: low-level
combining, high-level combining and the use of separate antennas for
the two signals.
Low-level combining is accomplished by summing the output of the
digital exciter and the analog exciter, and amplifying the resulting
signal with a linear amplifier. A linear amp is necessary because the
IBOC carrier levels vary in amplitude by as much as 5.5dB with respect
to their average levels. The advantage to this method is simplicity;
however, there is a major disadvantage.
Block diagram of an FM low-level combining
Only recently have transmitters been created to accommodate the
IBOC+analog signal. A station planning to transmit IBOC in this way
will be compelled to buy a brand new transmitter-or even two. The
station will reuse its current antenna.
High-level combining makes use of more readily available technology.
The analog portion of the broadcast comes from the transmitter the
station already has. The digital portion comes from a separate linear
amplifier with the IBOC carrier. The signals are then added at high
power levels in a four-port combiner. The output of this combiner feeds
the antenna the station already has.
The major advantage of this method is that the analog transmitter
can be reused. However, due to combiner losses, the analog transmitter
will need to produce about 10 percent more output power. Additionally,
due to the -10dB coupling on the digital port, the digital power amp
will actually need to generate 10dB more power than would otherwise be
needed. Ninety percent of the output of the digital transmitter is
wasted in heat.
Block diagram of an FM system with separate
The final method is that of the separate antenna. The station adds a
second antenna and transmits the IBOC-only signal from it. The
advantage is that the IBOC transmitter can be a much smaller unit, and
that the station can continue to use its older analog transmitter and
antenna. The disadvantage is that the station will need to use
additional tower space.
IBOC DAB is on the verge of becoming reality. Now is the time to
start planning ahead for it.
Irwin is director of engineering for Clear Channel, San
Francisco. Diagrams courtesy of Ibiquity Digital.