HD Radio
HD Radio is
iBiquity's brand of
digital radio technology for transmission and reception of sound that has been
digitized and
compressed. The
Federal Communications Commission selected HD Radio as the standard for local area broadcast of signals within the United States. It offers multiple programs on one channel and works on the same frequencies allocated to analog (
FM and
AM)
radio stations. Supporters claim
CD quality sound and reduced interference.
Digital information is
transmitted using
COFDM, a
modulation method that has been used in a number of different
digital television and radio systems, including
DVB-T. The audio
compression algorithm was initially set to be
PAC when iBiquity's standard was first approved by the
Federal Communications Commission (FCC) in
2002, but the system was changed to the
HDC codec in
2003. The change was made because the low-
bitrate audio for AM stations was described by some as being "underwater", plus the fact that the partially in-house HDC format has newer
patents that can be exploited for longer periods. HD Radio stations must pay
royalties each year to iBiquity, plus the costs paid by the
manufacturers of the transmitters which are then passed along to the stations that buy them.
In
hybrid mode, the AM version can carry 36 kilobits per second of data for the main audio channel, while FM stations can carry information at 96 kbit/s. HD Radio can also be used to carry multiple distinct audio services, called
multicasting but actually more like
multiplexing. Secondary channels, such as for
weather,
traffic, or a
radio reading service, can be added this way, though it may reduce the audio quality of all channels on a station.
Datacasting is also possible, and
RDS-like
metadata about the program and station are included in the standard. Stations may eventually go all-digital, meaning they could no longer be heard on a regular radio.
Also notable is that in hybrid mode, a radio will lock onto an
analog signal first, then
FM stereo, then try to find a solid
digital one. If the digital signal is lost, it will blend to analog, the same way a car radio will blend from stereo to mono given a weak signal. Much of the success of this relies on proper
synchronization of the analog and
digital audio signals by
broadcast engineers at the
transmitter. This
fallback may also be impeded by the use of multiple channels.
While iBiquity is responsible for the development of these standards, and the
FCC for its
regulation, the
National Radio Systems Committee (NRSC) is the
standards body for HD Radio. It is officially known as
NRSC-5, with the latest version being
NRSC-5-A.
AM stations in
ITU region II are usually considered to have 5 kHz of audio
bandwidth. With double
sidebands that are standard for most radio broadcasts, this results in a channel 10 kHz wide. In the other ITU regions of the world, the sidebands are 4.5 kHz, with a 9-kHz channel spacing. However, the AM version of HD Radio adds 10 kHz to each side of the
center frequency, meaning that the signal extends out from the center frequency by 15 kHz. Again, with double sidebands, this results in an entire signal that is 30 kHz (three full channels) wide. This extra information is sent at fairly low power, but this is still a reason why iBiquity's technology has only been tested on AM band stations that have no
adjacent channels. For this reason, some consider HD Radio on AM to be an
in-band adjacent-channel (IBAC) system. Still, power level of the outer signals is quite low compared to the main signal, and the COFDM subcarriers fit within a standard AM
spectral mask (though the mask was never designed with digital in mind).
Most analog AM radios have
electronic filters to remove anything more than 5 kHz away from the center frequency, but some "
wideband"
receivers don't filter this, making the encoded signal audible. Even on radios that do have such a filter, it is possible to hear the digital "hashes" of the sidebands by tuning up or down from the desired frequency by 10 kHz. Use of the system for AM stations has been highly controversial because of possible
interference problems. This is nothing new for the AM band, though, as
AM stereo has produced similar controversies. Because of the limited bandwidth on AM stations, iBiquity's standard is incompatible with
C-QUAM AM stereo broadcasts. To reduce
nighttime
skywave interference problems with other stations, HD Radio can only be used during daytime broadcast periods on AM at present.
Currently, it appears that
Digital Radio Mondiale's AM IBOC system is much more successful overall, even for use on
shortwave, which is skywave-only.
On the wider
sidebands of FM stations, HD Radio can carry multiple
streams of FM and/or AM quality.
National Public Radio in particular hopes to be able to carry several different streams through the
transmitters of member stations, calling its proposed addition to the FM standard "
Tomorrow Radio". Some have also proposed using the system to carry
surround sound broadcasts with 5.1 channel audio, though this or other multichannel setups reportedly may prevent the fade-to-analog
fallback on "hybrid" analog+digital broadcasts. Also, the FCC is still only authorizing multichannel use
experimentally to individual stations who ask permission, however it seems to be becoming a
de facto standard now that so many stations have adopted it.
Currently, FM stations in the United States are licensed to occupy approximately 200 kHz of
RF spectrum, i.e., the FM band
frequency allocations are 200 kHz apart. When a signal
modulates the carrier, an infinite number of
harmonically-related sidebands is created, i.e., the actual occupied
bandwidth of the signal extends well past the highest modulating frequency (usually ±100 kHz), due to the
non-linear nature of
frequency modulation. In order to prevent harmful
interference to other stations, the carrier frequencies of stations within markets are thus assigned on alternate channels, i.e., they are 400 kHz apart. In addition, the output of the FM
transmitter is carefully filtered to limit the RF energy in the sidebands above 120kHz. In contrast, FM stations have a
baseband bandwidth of about 100 kHz, only 15 kHz of which is used by the (analog
monophonic) audio. Analog stereo uses up to 53 kHz of baseband space, and
RBDS is centered at 57 kHz. The "remainder" is available for other services, including
rental for
paging and
datacasting, or as a
transmitter-studio link for in-house
telemetry.
While the various baseband signals all contribute in a complex manner to the total occupied bandwidth (and
power) of the FM signal, it is important to consider two factors: (1) the modulating signal will generate a principal RF component at the
fundamental sideband, e.g., a 67 kHz
subcarrier will generate RF components at ±67 kHz (and ±134 kHz, etc.) from the FM carrier; and (2) any
filtering of the modulated signal causes a certain (acceptable) amount of
distortion in the received signal, resulting in the formation of
intermodulation components from the various modulating signals.
In regular hybrid mode, the HD Radio station has its full ±100 kHz of RF bandwidth, and adds its digital signals into part of the upper and lower adjacent RF channels beyond that, using 1% of the main FM power level. In extended hybrid mode, the bandwidth of the FM signal is reduced to make way for additional OFDM carriers carrying more data. Because of this, FM stations may have to drop pre-existing subcarrier services (usually at 92 kHz and 67 kHz) in order to carry extended HD Radio, though such services can be restored through the
digital subchannels that are then made available (which requires replacement of receivers). The analog stereo subcarrier would in theory eventually be dropped to make more room for digital, and eventually stations could elect to drop the analog
baseband (
monophonic audio) completely and go all-digital. However, considering that there are already billions of analog-only receivers, this is not expected to happen for a very long time.
There is also another royalty-free system called
FMeXtra, which uses subcarriers to transmit digital radio. This is completely compatible with standard HD Radio, but likewise is reduced in bandwidth and bitrate if used on a station using an extended-hybrid mode. Analog audio is also reduced in volume if a station uses extended-hybrid mode, due to the reduction in
modulation. Both situations are worsened by using successivly higher-bitrate modes of HD Radio.
There are still some concerns that HD Radio on FM will increase interference between different stations, though it is thought unlikely to make a major difference as it still fits within the existing spectral mask. It has been confirmed however that trying to listen to a distant FM station on a channel adjacent to a local HD Radio station can be difficult if not impossible, because of the very poor
signal-to-noise ratio that it creates. It does not generally cause interference to any analog station within its 1mV/m²
signal strength contour, the lowest limit to which the FCC protects most stations.
As with AM, FM stations transmit using a separate
exciter to modulate the very different signals. A
combiner is often used, either before common
amplification or after separate amplification, though stations are also now allowed to use a separate
radio antenna slightly higher or lower on the
radio tower.
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Spectrum of FM broadcast station without HD Radio subcarriers - markers are on channel center frequencies |
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Spectrum of FM broadcast station with HD Radio subcarriers - markers are on channel center frequecies |
As of June 2006, receivers are becoming less expensive, starting at around
US$ 100-$150. The Kenwood Tuner, for use with Kenwood stereos is selling for as little as $98 (Price is from a retail store in
Seattle, Washington). Manufacturers have initially focused on making
car stereos, BMW being the first with their announcement of HD Radio being an option for their 2006 7-series, 6-series, and 5-series models. Home listening equipment is currently available from few companies, in both a home tuner and a tabletop model. The
Boston Acoustics Recepter is one common model cited in listener tests.
Currently the
HD Digital Radio Alliance, a
consortium of major radio group owners implementing HD Radio, has urged its members to broadcast multiple programs, without
radio commercials on the extra digital-only channels for a period of at least 18 months (ending sometime in
2007).
Clear Channel is actually selling programming of several different
music genres to other competing stations, in addition to airing them on its own stations.
Some FM owners are also
simulcasting their local AM stations on HD Radio.
The HD Radio system has not been immune to criticisms.
FM DX'ers who live in markets with several equipped stations have found the system highly detrimental to their hobby due to the strong digital hash generated on adjacent channels. This might also extend to those who live near market boundaries and wish to hear a station on a channel next to one used by a local station. Many find the claims of higher audio quality to be exaggerated, or even untrue when multicasting is used, as individual channel rates are reduced. Some report hearing a decrease in audio quality on the analog signal of stations that have taken up the system.
Another common criticism is that the use of the digital transmission format on the
mediumwave (AM) band is unreasonable, due to the high levels of interference and noise on the band, caused by electrical sources and distant
skywave-propagated stations, which might simply generate an audible buzz or hum on the analog channel but severely limit the range of the "HD" channel, due to the nature of digital audio. In addition, those that tune into distant programs or
DX the band might find their desired station or target frequency unusable due to hash, which may carry for very long distances (but not necessarily produce listenable audio over them, as is possible with analog).
There are also various marketing-related challenges [
1] that HD must overcome; overall there has been very little uptake of HD Radio by consumers as of mid-2006. Reasons cited have included the expense and difficulty of replacing existing radio receivers, a lack of compelling content, and no significant difference in signal quality between standard
FM broadcasting and HD Radio streams. The content issue has been somewhat addressed by the increasing use of secondary ("HD2") streams, but receiver demand is still rather low. In addition, some critics of the format believe that HD Radio's primary benefit is to the equipment manufacturers, not the listeners. Relative to
satellite radio, which broadcasters feel is their primary competition, there are no
subsidies for receivers because there are no
subscription fees, thus no
penetration pricing method of getting new customers.
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HD Radio official site*
iBiquity*
HDradioreviews HD radio news, reviews and commentary]
*
WOR Transmitter Tour, containing information on an early IBOC installation
*
HDRadio Receiver, HD Radio receiver products from Radiosophy
*
hear2.0, containing discussion and debate on HD Radio
*
Radio Magazine covers the technology of radio broadcasting
