Many engineers anxiously awaited the release of the National Radio
Systems Committee's AM IBOC Report, in hopes that the report would
reveal good news about AM IBOC.
The engineering world has been working on the development and
evaluation of IBOC transmission for some time. The NRSC began
evaluation proceedings of general DAB systems in 1995. After the
proponents merged into one, Ibiquity was left in the running for
potential adoption. In the fall of 2001, the NRSC issued a report on
Ibiquity's FM IBOC. This comprehensive report, runs 62 pages of
engineering material plus 13 appendices. All of which covers the
features and important areas of the FM application of the system with
its blend-to-analog from digital-to-analog operation as signal levels
The application of FM IBOC has been studied by the NRSC and appears
to be understood and accepted by radio engineers. AM IBOC has recently
been studied by an NRSC working group as a prelude to its adoption for
general broadcast use. Its was presented during the NAB convention in
AM IBOC carried the most promise of
service improvement, but the initial studies show that it may only be
suitable for daytime service.
Now, after the presentation of the AM IBOC report, the FCC must
decide whether to propose a set of standards for the AM and FM IBOC
services based on these reports or to require more testing. The NRSC
report does not adopt the system as a standard by the FCC; it presents
an overview of an investigation of both systems. The decision to
prescribe a set of standards for IBOC must come from the FCC. It is
hoped that the FCC will not cause IBOC to go the way of AM stereo.
Boom or bust?
IBOC could be a valuable addition to a broadcaster's armor, or it
might end up as merely another avenue of expense that will have to be
entered into without a waiting audience of suitably equipped receivers.
The situation is similar to that faced by TV stations compelled to
install expensive digital equipment without a digitally-equipped
receiving audience, which has not yet embraced digital TV with open
arms. On the other hand, if the NRSC report confirms what many
engineers are hoping and expecting, new transmitting broadcast
facilities could produce additional quality air time for AM and FM
licensees to sell from the same site and revitalize our familiar and
somewhat overloaded radio bands.
The FM Report covers eight areas of vital performance concern to the
broadcaster and listener alike. If all of these concerns can be met as
successfully by AM IBOC, and the receiver manufacturers rally to
develop and produce the necessary receiving equipment, I feel that we
can anticipate a brighter outlook for many AM stations.
The evaluated FM concerns were audio quality, service area,
acquisition performance, durability, auxiliary data capacity, behavior
as signal degrades, stereo separation and flexibility.
One of the features of the Ibiquity IBOC system as tested was noted
as a seamless glide to FM from analog after tuning in an IBOC signal.
The delay is about 4.5 seconds. This transmission delay may cause
problems in off-air monitoring, and this will have to be tackled by
individual stations as necessary. Stations are becoming accustomed to a
system delay from the use of codecs, DSP-based audio processors and
Even if an IBOC standard is adopted, it
will be some time before a significant receiver base exists.
The FM IBOC tests used a software radio whose performance is
presumably not the same as a commercial auto radio receiver. For this
reason, the NRSC tests did not comment on the audio performance
obtained in the tests on a received signal that had blended to analog.
It seems likely that the AM IBOC tests might also have to use a
software radio unless some receiver manufacturer has developed a
prototype auto receiver designed for AM IBOC.
The FM report paid strong attention to the use of SCA services on FM
IBOC. About half of all the operating FM stations employ one or more
SCAs for reading for the blind or similar services. In addition, RBDS
and DARC systems use SCAs for data transmission. Appendix J of the NRSC
Report provided detailed test results.
Regular broadcast station WPOC and experimental WD2XAB were used for
the SCA tests, and the NRSC high-lighted something that has probably
been known by users of SCA receivers, but not previously been broadly
discussed. The laboratory tests showed that there are significant
differences in the performance of various makes of SCA receivers. These
differences persisted despite the presence or absence of IBOC
In the field tests, similar results were observed with and without
IBOC. Some receivers took IBOC in stride and performed normally, while
others completely failed.
Proper antenna orientation for analog SCA signals must be practiced
by SCA-receiver users to obtain a good signal. This is to be expected
in view of the complexity of SCA operation, which is dependent on the
operating characteristics of the transmitter, adjacent channel signals
and field strength.
The NRSC report on AM IBOC recommends to the FCC that AM IBOC be
approved for daytime use. The daytime service is comparable to FM, but
at night, due to first- and second-adjacent channel interference
problems, AM IBOC is not recommended. The proponent, Ibiquity, does not
anticipate that sky-wave reception will be possible, and more AM IBOC
tests will be needed to determine if it will eventually be usable at
night as a local ground-wave service. In the 15kHz bandwidth are three
5kHz subcarriers, which are susceptible to first- and second-adjacent
AM IBOC relies on ground wave propagation and works successfully to
around the 2mV/m contour. It has been successfully received with field
strengths as low as 0.6mV/m.
The daytime signal suffers from noise problems,interference and
infringing AM signals. Stereo AM IBOC degrades gently as signal
strength declines, from good FM stereo to FM mono and eventually to AM
It appears that AM IBOC will produce quality audio within the 2mV/m
contours in the daytime, but it is not recommended for night use. This
seems a dubious advantage over regular AM broadcast transmission. Many
stations suffer impaired night signals, and much hope was placed on the
possibility of a superior night signal. Many stations will think twice
about spending money to improve an already good day signal without
improving the night signal. This AM IBOC report is based on
conversations prior to the report's submission to the full committee
and on information from the final recommendation. Pending further test
data, we can only hope for better news for improved night
E-mail Battison at email@example.com.