The engineering fraternity now knows the NRSC's opinion concerning
the use of AM IBOC at night. This was a decision that surprised a
number of engineers. However, nighttime AM IBOC is not dead, it is
merely delayed for further investigation.
The NRSC's optimistic report on FM IBOC has been out for several
months. Some transmitter manufacturers have already announced the
availability of FM IBOC transmitting equipment, and offered actual
equipment at the NAB show. FM IBOC requires considerably more equipment
than the AM version, and it is more expensive.
In general, vacuum-tube operated AM transmitters do not appear to
have sufficient linearity to pass the IBOC waveform satisfactorily,
while AM transmitters using PDM and solid-state AM transmitters using
digital modulation appear to require only minor modifications to pass
the IBOC waveform. This means that many smaller and older stations
using tube transmitters may have to purchase new transmitters if they
are going to radiate an acceptable signal. The exclusion of AM IBOC
from nighttime operation at present may cause a delay in getting a
number of these smaller stations on the air. The area in which many of
these AM stations need signal improvement is at night and the added
cost of introducing IBOC, which would produce no improvement in night
service, will probably not be attractive to licensees.
The Shively IBOC filter-injector
balances the coupling losses to the analog and digital signals.
AM problems and FM considerations
The average small market AM station may fail to meet the IBOC
requirements of wide bandwidth and minimum phase distortion. Many AM
stations, especially directional stations, have bandwidth and phase
distortion that are less than perfect. If the frequency response falls
off at higher frequencies, a transmitter will have difficulty handling
the IBOC waveform.
IBOC hybrid FM signals can be produced by three different methods:
high-level combining, low-level combining and the use of separate
Low-level combining is probably the simplest and least expensive of
the three FM IBOC methods. To accomplish this, an existing FM analog
exciter has its output combined with the output from a separate IBOC
exciter. These signals are then fed into a low-level combiner followed
by a broadband power amplifier. This method reduces the equipment
required, as well as saving on power consumption, but it requires a
highly linear RF amplifier.
Existing high-powered FM stations may prefer to use the high-level
system, which takes advantage of an operating high-powered FM
transmitter and also requires a high-power combiner, filter and IBOC
With high-level combining, there is a loss of about 10 percent of
analog power and about 90 percent of the digital power to the reject
load. The combiner with its reject load leg appears to be an
unfortunate feature of IBOC operation. It is the required 10dB coupling
figure that necessitates what seems to be an unfortunate inefficiency
The third method of obtaining FM IBOC operation uses two
transmitting antennas. This method of operation is still being
examined, but early tests appear to show that at least 40dB of
isolation between the two antennas is required. If this method can be
used commercially it could make IBOC attractive to stations that have
room for another antenna on their towers and additional equipment in
the transmitter building. Separate antenna operation would not require
the cost of a combiner, or the generation of power that is ultimately
lost in the reject load.
Several manufacturers have paid a great deal of attention to the
development of the hybrid combiners specifically for high-power
broadband FM IBOC operation. At this stage in the development of FM
IBOC, it appears that a 7dB or 10dB coupling level will be used for
commercial operation. However, most manufacturers of combiners offer
various coupling levels covering suitable ranges and power levels. The
physical size of combiners depends on the individual manufacturer's
design, but they seem to be smaller than combiners used with regular FM
Electronics Research Inc. (ERI) offers several versions of its IBOX
hybrid combiner. The high-power version handles an analog input of 55kW
with 40dB of isolation between the analog and digital inputs and only
-0.46dB nominal coupling between the analog input and the antenna
output. Its size is surprisingly small — only about 42 inches by
15 inches overall.
For medium-power installations, the IBOX medium-power hybrid
combiner handles 30kW of analog power with -10dB of coupling between
the digital input and antenna output. Other operating parameters are
similar to those for the high-power model, and customized versions with
various coupling conditions and power levels are available.
Shively Labs has developed an Injector Filter IBOC combiner. This
device combines the low-level digital signal with the high-level analog
signal and dumps the excess signal in the reject load. A one-eighth
wavelength (λ/8) coupling loop is used in this device, and the
overall size is only about 20 inches by six inches.
The Shively injector filter comes with a standard 3⅛-inch
coaxial connector and ⅞-inch coaxial input for the digital
signal. The standard 3-inch unit will handle 35kW analog input and 5kW
digital input. The reject load is not supplied by Shively, but a good
rule of thumb is to use a reject dummy load capable of handling about
⅓ of the input analog power. Thus, a 30kW analog transmitter
would require about a 10kW reject load.
Dielectric Communications has also spent a lot of time working on
IBOC. Dielectric offers a 10dB hybrid injector/combiner that allows
high-level combining of the analog and digital transmissions.
Dielectric also offers a low-level combiner for use in IBOC systems
that uses a common linear amplifier and that does not need to separate
Dielectric's experience in the development of HDTV transmission has
led to the development of another type of shaping device in the formal,
off-elliptic function filters. These filters, a combination of combiner
and mask filter, form supplemental masking filters that may help to
provide isolation at band edges where there is insufficient bandwidth
E-mail Battison at firstname.lastname@example.org.
Find links to the two NRSC reports at www.beradio.com under