IBOC transmission


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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 on-channel.

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 IBOC transmission 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 IBOC transmission 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 system.




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 system.




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 antennas.




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.




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