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HD Radio

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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 (Hybrid Digital) 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. Although the acronym HD has come to mean "high-definition" in reference to HDTV, the "HD" in HD Radio is a trademark (although Hybrid digital technology is used).

Overview

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

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.

FM

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 and Canada 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.

  • IBOC Spectrum (subcarriers): markers are on channel center frequencies
  • Spectrum of FM broadcast station without HD Radio
    Spectrum of FM broadcast station without HD Radio
  • Spectrum of FM broadcast station with HD Radio
    Spectrum of FM broadcast station with HD Radio


Receivers, Automotive

As of June 2006, receivers are becoming less expensive, starting at around US$ 100-$150. The Kenwood HD Tuner (KTC-HR100TR in the US), for use with Kenwood stereos, is selling for about $100. 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.

Receivers, Non-automotive

Home listening equipment is currently available from a few companies, in both a home tuner and a tabletop model. Polk Audio's I-Sonic™ Entertainment System receives HD Radio in addition to a number of other features, including DVD and XM Satellite Radio. The Boston Acoustics Recepter is one common model cited in listener tests. Radio Shack also has an HD tabletop radio. DaySequerra produces a line of high-end equipment designed primarily for use by broadcast engineers. Sangean has announced it will have two HD Radio receivers by mid-December 2006: a tabletop model (HDR-1) and a component tuner (HDT-1). Sangean's component tuner is available to ship from Amazon.com http://www.amazon.com as of December 14, 2006. Although this unit does not have any digital outputs, due to what Sangean says is a problem with AM broadcasts causing undue static. Therefore, only the standard L/R RCA analog output jacks are used on the HDT-1 model.

Programming

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 station owners are also simulcasting their local AM stations on FM HD Radio sister stations to circumvent the current HD Radio daytime only rule for AM. An example of this is Atlanta's WSB AM 750 being simulcasted in high quality surround stereo on WSRV FM 97.1's HD-2 channel.

In addition, it is a very common practice to broadcast a former FM station's format on its HD-2 channel, such as WPGB (104.7 FM) in Pittsburgh, which carries the smooth jazz format on its HD-2 band. Said station was once known as WJJJ.

Other recent additions includes the introduction of airstaffers on their HD-2 spinoffs, like KDWB's Party Zone channel in Minneapolis-St. Paul. This latest move indicates that once the 18 month grace period ends, the broadcasters will start adding local content, including DJs and advertising, to the specialty stations.

Major Contributors

As of December 12, 2006, there are 1105 stations in the United States broadcasting 1623 HD Radio channels on both AM and FM bands. Almost all are currently broadcasting an FM HD2 channel. Most of these stations are owned and operated by Clear Channel, CBS Radio (formerly Infinity Broadcasting), Entercom and Cox Radio.

As HD is gaining popularity, novel services such as RadioSherpa are beginning to appear. RadioSherpa is a web-based realtime program guide for HD radio stations. See also: HDRadio.com[1] for the most recent lists of HD broadcast stations, HD-2 FM listings and formats for both.


External links

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