As the sole developer of IBOC technology, Ibiquity Digital fields a
lot of questions from broadcasters about the technical requirements to
convert AM and FM stations to digital broadcasting. The single largest
factor in the cost of such conversion, the transmitter, is the subject
of the majority of inquiries. IBOC conversion, especially on FM
stations, offers several upgrade options. Ibiquity's experience with
field testing on several dozen stations indicates that many stations
will be on the lower end of the projected cost range of $30,000 to
$200,000 for IBOC conversion.
FM stations considering IBOC conversion have three basic
transmission options, each requiring an IBOC exciter: high-level
combining, low-level combining or separate antennas.
High-level combining, the option used most frequently in Ibiquity's
field testing, combines the station's existing transmitter with a new,
but lower-power, digital transmitter to create the hybrid IBOC waveform
(with analog and digital components). This approach requires a power
overhead of about 0.5dB in the existing analog transmitter to account
for combiner loss. It also requires a linearized transmitter with an
IBOC power output of 10dB below the analog transmitter's TPO to
generate the digital signal. High-level combining is possible using any
existing transmitter with the necessary headroom.
Ibiquity also used low-level combining successfully in the field.
With this method, the hybrid signal is formed before amplification and
passed through a linearized transmitter. FM stations with a modern,
solid-state transmitter and about 20% power overhead are candidates for
low-level combining with modifications to the amplifier modules.
Low-level combining using an existing transmitter is the least
expensive option to convert an FM station to IBOC.
Separate antennas for the analog and digital signals may provide
another method for conversion. A separate antenna approach uses
separate transmitters feeding separate transmission lines and antennas.
No overhead is required in the existing analog transmitter, and the
linearized transmitter needs a power rating of 20dB below the analog
transmitter's TPO. While Ibiquity has not tested this method, several
manufacturers are taking steps to field test this approach.
Every FM field test conducted by Ibiquity on an FM station used the
station's existing transmitter for the analog transmission. For most of
Ibiquity's test stations, high-level combining was used. In these
cases, a linearized IBOC transmitter was obtained from one of
Ibiquity's manufacturing partners and installed at the site. An
Ibiquity IBOC test exciter was used to generate the digital component
of the IBOC waveform and in turn fed the linearized IBOC digital
transmitter. The output from the existing analog transmitter was then
combined with the output of the digital transmitter using a custom
injector/combiner supplied by another manufacturer. One station in the
test group is using a single linearized transmitter for analog and
digital combined at a low level.
Stations with analog STLs were upgraded to either a digital RF path
or T1-based Telco channelbank. While not required for the conversion to
IBOC, changing the STL allowed for a better evaluation of the audio
performance without STL-induced limitations.
AM stations considering IBOC conversion have fewer options because
combining the analog and digital components of the AM IBOC waveform is
not an option. However, AM stations that possess modern, solid-state
transmitters will only require an IBOC exciter and minor modifications
to their existing transmitter. Converting these stations may actually
cost less than their FM counterparts.
In most cases, Ibiquity targeted AM stations with existing
IBOC-compatible transmitters for field-testing. In general, the
hardware installations at the AM stations were easier than at the FM
stations. Most of the AM test stations had antenna systems capable of
supporting IBOC bandwidth requirements. Generally, if an antenna will
support C-QUAMAM Stereo, it will support an IBOC transmission.
The test stations that were operating in delay for live talk or
listener call-in segments were already set up to deal with IBOC's audio
delay. However, the stations that needed to interact with live traffic
reports or remote broadcasts had to establish a return audio path via
an IFB. Many of the FM stations were able to use their SCA channel for
this function. All the AM stations, and the FM stations with occupied
SCA spectrum, were able to use bi-directional ISDN phone circuits to
handle the IFB.
Ibiquity's experiences in field-testing underscores the relative
ease with which AM and FM broadcasters can transition to digital.
Replacing a station's main analog transmitter will be unnecessary in
the majority of IBOC station conversions — especially stations
likely to convert in the early years. By modifying the existing
transmitter or adding a smaller digital transmitter, most stations will
be able to transition to IBOC affordably and with relative ease.
Detweiler is technology manager of Ibiquity Digital.