In-band on-channel
In-band on-channel (
IBOC) is a method of transmitting
digital radio and
analog radio broadcast signals
simultaneously on the same
frequency.
By utilizing additional digital
sidebands,
digital information is "
piggybacked" on a normal
AM or
FM analog signal, thus avoiding any complicated extra
frequency allocation issues. However, by putting extra
RF energy just outside the edge of the station's FCC defined
channel,
interference with
adjacent channel stations is increased when using digital sidebands.
FM IBOC will allow for multiple channels in addition to the main stereo channel. On AM, additional channels aren't possible due to limited bitstream bandwidth, but is capable of transmitting stereo. Eventually, stations can go from
hybrid mode (both analog and digital) to all-digital, by eliminating the traditional analog audio portion.
On FM there are currently two methods of IBOC broadcasting in use, mainly in the
United States.
The first is
HD Radio by the
iBiquity consortium, which uses sidebands beyond the normal ±100 kHz FM channel. This is the most well-known and widespread system in use, though uptake is still extremely limited. HD Radio is also extremely expensive to implement, in large part because of huge licensing
royalties, which must not only be paid on the equipment purchased, but for every year the equipment is used. The system also uses a separate
transmitter, which either requires a
diplexer into the existing
antenna, or another separate
feedline and antenna, as well as space, weight, and
wind load on the
radio tower.The HD system by Ibiquity is the system most widely in use currently because it has the most support from the big corporations that own most radio stations at the present time. That does not translate into it being the best system performance-wise. In fact it is important to note, that the "HD" in HD-Radio does not represent the same thing as the "HD" stands for in HDTV. For television the "HD" stands for "High Definition" but in the case of HD-Radio the "HD" actually stands for "Hybrid Digital" according to the developing company Ibiquity. While the system is currently being implemented around the US, the system is still a work in progress. If certain bugs like receiver sensitivity and digital signal range can be improved then the system looks quite promising.
The other system is
FMeXtra by
Digital Radio Express, which instead uses subcarriers within the existing signal. This was only recently introduced, and consumer receivers are expected in
2006. The system is completely compatible with HD Radio in hybrid mode (but not in all-digital, which is not expected for a very long time), and with
RBDS. The stereo subcarrier can also be removed for greater digital
bandwidth. However, it is not compatible with other existing subcarriers, which are normally not used by the general public anyhow. The system is far less expensive and less complicated to implement, needing only to be plugged-in to the existing
exciter, and requiring no licensing fees. FMeXtra generally has all of the user features of HD Radio, including multi-cast capability or the ability to broadcast several different audio programs simultaneously.
Digital Radio Mondiale is in the initial stages of creating an open-source system for FM, though it may be too late to make a third system viable with two others already in place.
iBiquity also created a
mediumwave HD Radio system for AM, however it has so far failed in technical trials to perform as superbly as promised mainly due to
interference caused by using sidebands in an environment intended for narrowband
amplitude modulation. The HD-Radio system employs use of injecting digital sidebands above and below the audible portion of the analog audio on the primary carrier. This system also phase modulates the carrier in quadrature and injects more digital information on this phase modulated portion of the carrier. It is based on the principal of AM Stereo where it puts a digital signal where the C-Quam system would put the analog stereo decoding information.
Digital Radio Mondiale has had much more success in creating an AM IBOC system, and one that will be much less expensive to implement than any
proprietary HD Radio system. It has already been accepted for not only mediumwave but also
shortwave (and possibly
longwave) by the
ITU for use in regions I and III, but not yet in region II, the
Americas.
CAM-D is yet another method, though it is more of an extension of the current system. Developed by AM Stereo Pioneer Leonard Kahn, It encodes the
treble on very small digital sidebands which do not cause interference to adjacent channels, and mixes it back with the analog baseband. Unlike the other two, it is not intended to be capable of multichannel. Unlike the HD system by Ibiquity that calls itself "Hybrid Digital" the CAM-D system truly is Hybrid Digital/Analog. Some engineers believe that CAM-D may be compatible with analog AM Stereo with the right engineering.
While
Canada and most countries in
Europe have chosen the
Eureka 147 standard of
digital audio broadcasting (DAB) for creating a digital radio service, the
United States has been holding out for advancements in IBOC technology. Part of the reason is the use of the
L band (1452–1492
MHz) for test-flight
telemetry by the
U.S. military and its
contractors. This band is used in Canada for digital radio, but remains unavailable for reassignment by the
NTIA in the U.S. for broadcasting. It also has somewhat of a disadvantage in that its higher frequency is more
line of sight than
VHF (FM), and far more than
mediumwave (AM).
It is also partly because of concern that stations'
branding, using their current frequency, would be lost to new channel numbers, though
virtual channels such as on
digital television would eliminate this, and stations already swap frequencies sometimes. Also, several competing stations must share a
transmitter which
multiplexes them all into one
ensemble with the same coverage area, though some FM stations are already
diplexed. A further concern to FM stations was that AM stations could suddenly be in competition with the same high audio quality, although FM would still have the advantage of being able to provide a wider range of services due to greater bandwidth (100 kHz versus 10 kHz). The most significant advantage to IBOC is the relative ease of implementation. Existing analog radios are not rendered obsolete and the consumer and industry transition to digital at a rational pace.
AM IBOC in the U.S. still faces some serious technological challenges of its own, including nighttime interference with other stations. iBiquity was previously using
PAC (also used at a higher
bitrate in
Sirius satellite radio [see
DARS ]), but in
August 2003 a switch to
HDC was made to rectify these problems. Prior to this, a change back to
AAC or another
MPEG compression algorithm had been considered; however HDC has been customized for IBOC, and it is also likely that the
patent rights and
royalties for every
transmitter and
receiver can be retained longer by creating a more
proprietary system.
Digital Radio Mondiale is also developing an IBOC system, likely to be used worldwide with AM
shortwave radio, and possibly with
broadcast AM and FM. Neither of those have been approved yet for
ITU region 2 (the
Americas).
Both AM and FM IBOC signals cause interference on adjacent stations. It has led to derogatory terms such as
IBAC and IBUZ (since the interference sounds like a buzz.) The range of a station on an HD radio is much less than its analog signal. In addition,
tropospheric ducting and
e-skip can reduce the range of the digital signal.
In-band on-channel digital radios using iBiquity's standard are being marketed under the brand "HD Radio" to highlight the quality of reception, though this is officially said to stand for "hybrid digital" rather than "high definition". As of
May 2006, a limited number of different receiver models have been made, and stations have received
authorization from the
FCC to transmit in a multiplexed multichannel mode on FM. Use of HD Radio on AM has been limited to daytime use only because it causes noise on adjacent channels, and is not yet allowed at night due to its problems with
skywave radio propagation. DRM however is being used across Europe on shortwave, which is entirely AM skywave, without issue. With the proper
receiver, many of these stations can be heard in
North America as well.
*
IBAC*
digital audio broadcasting*
ISDB*
DARS*
FCC info on IBOC*
NSRC-5A IBOC Standard Specification
