HD Radio

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The HD Radio Logo
The HD Radio Logo

HD Radio is the brand name of a method of digital transmission of audio on the FM and AM radio bands. The technology was developed by iBiquity Digital Corporation. 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 radio stations. Supporters claim CD quality sound and reduced interference and static. Although the acronym HD has come to mean "high-definition" in reference to HDTV, the "HD" in HD Radio is a trademark with no meaning (although a hybrid digital technology is used).

Contents

[edit] Overview

Digital information is transmitted using COFDM, a modulation method that has been used in 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 sounding as if it were "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 (by splitting up the 96 kbit/s), 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 (legacy analog-only) radio receivers.

While in hybrid mode, an HD Radio will lock onto an analog signal first in mono, then stereo, then try to find a digital one. If digital signal reception is lost, the radio will revert to analog, the same way a car radio will go into mono operation from stereo when signal strength is insufficient for stereo. Much of the success of this system property relies on proper synchronization of the analog and digital audio signals by broadcast engineers at the transmitter.

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.

[edit] 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. Even so, the power level of the outer band signals is low compared to the main signal, and the COFDM subcarriers fit within a standard AM spectral mask (although the mask was never designed with digital in mind).

Most analog AM radios have electronic filters to remove all signals more than 5 kHz away from the center frequency, but some "wideband" receivers do not filter this way, 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. Proposals for AM stereo have produced similar controversies. Because of the limited bandwidth assigned to AM stations, iBiquity's standard is largely incompatible with C-QUAM AM stereo broadcasts. To reduce nighttime interference problems with other stations, HD Radio can only be used during the daytime on AM at present.

[edit] 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 are created, and so the actual occupied bandwidth of the signal extends well past the highest modulating frequency (usually ±100 kHz), because of the non-linear nature of frequency modulation. In order to prevent harmful interference to other stations, the carrier frequencies of stations within individual markets are thus assigned on alternate channels; thus they are 400 kHz apart. In addition, the output of the FM transmitter is carefully filtered to limit the RF energy in the sidebands more than 120kHz from the center frequency. 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 currently available for other services, including rental for secondary broadcast services, 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 demand) of the FM signal, it is important to consider two factors. First, 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 second, any filtering of the modulated signal causes a certain (acceptable according to regulatory bodies as embedded in their frequency and bandwidth assignments) amount of distortion in the received signal, resulting in the formation of intermodulation components from the various modulating signals.

In regular hybrid mode, an 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 about 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 discontinue 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. This will require new receiving equiopment for subscribers, however. The current 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, if ever.

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 since HD Radio 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 because of the very poor signal-to-noise ratio created. An HD Radio station will not generally cause interference to any analog station within its 1mV/m² signal strength contour, the lowest limit above which the FCC protects most stations. In short, some interfernce of this type is already permitted by FCC assignment rules, and the amount caused by HD Radio is expected to be accepted as well.

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. The ratio of power from the analog to digital signal is 100:1. This means that the digital signal only has 1/100th of the power of the analog.

IBOC Spectrum (subcarriers): markers are on channel center frequencies

Spectrum of FM broadcast station without HD Radio

Spectrum of FM broadcast station with HD Radio


[edit] Receivers, automotive

As of June 2006, receivers are becoming less expensive, starting at around US$125. 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. There are currently two generations of HD Radios available; HD and HD2. HD radio is capable of picking up only the main HD station, while HD2 radios are capable of picking up the main HD station and the HD stations that are being broadcast on the side bands. This is also called multicasting. iBiquity's website has an online guide to the radios currently available. DICE Electronics is the only HD radio that connects to a car's OEM stereo, and includes iPod integration as well. The JVC KD-HDR1 and the Kenwood KTC-HR100 tuner box are for after-market car stereo head units, though the Kenwood is really a tuner box and can be plugged into multiple "head units." of Kenwood make. All 3 are capable of receiving multicasting (HD2).

[edit] Receivers, non-automotive

Home listening equipment is currently available from a few companies, in both a component tuner and a tabletop models. Polk Audio's I-Sonic™ Entertainment System receives HD Radio in addition to several other features, including DVD and XM Satellite Radio. The Boston Acoustics Receptor HD table top radio is a basic, two piece stereo table top radio with an AUX input. Radio Shack has an HD tabletop radio. DaySequerra produces a line of high-end equipment designed primarily for use by broadcast engineers. Sangean makes two HD Radio receivers: a one piece, stereo tabletop radio with an AUX input(HDR-1) and a component tuner (HDT-1).

Portable HD receivers are currently unavailable becasuse the chipset draws excessive current eliminating batteries as an option as a power source. As more chipsets come onto the marketplace more receivers will be built probably by the end of 2007.

[edit] 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.

The HD Digital Radio Alliance is also acting as a liaison for stations in a market to choose unduplicated formats for the multicast (HD2, HD3, etc.) signals. This is designed to provide additional choices for listeners instead of several stations all independently deciding to create the same format.

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 simulcast in high-quality surround stereo on WSRV FM 97.1's HD2 channel. It is common practice to broadcast a former FM station's format on its HD2 channel, such as WPGB (104.7 FM) in Pittsburgh, which carries the smooth jazz format on its HD2 band. Said station was once known as WJJJ.

Other recent additions includes the introduction of airstaffers on their HD2 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.

[edit] Major contributors

As of December 12, 2006, there are 1,146 stations in the United States broadcasting 1,674 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 for the most recent lists of HD Radio broadcast stations, HD2 FM listings and formats for both.

[edit] Criticisms

HD Radio is a proprietary radio system that is currently not in use anywhere outside the United States, although it (and other systems) is being evaluated by other countries such as Canada, France, Mexico, New Zealand, Philippines, Switzerland and Thailand. [1][2] Worldwide, however, Digital Audio Broadcasting and Digital Radio Mondiale have become the most widely evaluated, and licensed, digital radio systems by international governments and their broadcasters.

For broadcasting in frequencies below VHF (including Shortwave and AM/Medium Wave), most countries (and the standards organizations ITU, IEC and ETSI) have adopted the Digital Radio Mondiale, abrev "DRM" (not related to Digital Rights Management).

For VHF and higher frequencies, a majority of countries have adopted or are evaluating the Digital Audio Broadcasting, abrev "DAB" system (see "Regional implementations of DAB" in Digital Audio Broadcasting).

HD Radio has been criticized for being incompatible with the standards selected by most other countries; hence overseas travel with an HD Radio, or the sale of radios, to or from countries that don't use HD Radio is not possible. Manufacturers presently must design and build separate radios only for the U.S. market. (The analog systems already in place differ between North America and other parts of the world. The U.S. uses a 200kHz spacing for FM and 10kHz spacing for AM, while Europe uses 100kHz spacing for FM and 9kHz spacing for AM. A radio from one continent can be taken to another and it will work to some degree, but not to its ideal specification.)

Other criticisms, particularly from the television broadcast industry, cite HD radio’s rather colloquial usage of the term HD, which has historically been used and reserved for television. Some have argued this usage may be a form of trademark infringement. However, most arguments are unsubstantiated—the HD networks and media conglomerates (Fox, ABC, etc) failed to secure the trademark initially, and hence lost the HD branding to the public domain (similar to “Hoover” for vacuums, “Kleenex” for tissues, “Ziplock” for plastic bags, among others).

While the other systems (like Eureka 147) use internationally standardized MPEG family audio codecs, HD Radio uses a proprietary one. Although HD Radio has modified its design specification in a way that does not specify its primary proprietary codec, much of the technical specification is viewed as proprietary. It is thought that the codec is a slightly modified version of AAC.

Whereas DRM and DAB are controlled by non-profit consortiums with members from more than 30 countries; iBiquity ultimately has control over HD Radio receiver-manufacturer licensing[1], and broadcaster licensing.[2]

Unlike subscription-based satellite radio, the content of HD Radio stations are subject to FCC censorship.

The term "HD Radio" is deceiving because it is specifically designed to make one percieve that they are receiving audio of higher quality than analog radio, and this is not the case. While the bandwidth of HD radio extends the upper end from 15 kHz to 20kHz, there are two details that are not widely divulged. First, the audio undergoes very stiff data compression and it is easy to tell if listening to the high end of the sound, the very area that HD radio has more of.[citation needed] Listen to an HD radio station and in the very high end, you will hear a swimming" sound that is very annoying once you know what it is and it's cause, and you won't hear it on analog radio unless the radio station airs MP-3 files.[citation needed] Almost every station is being forced by the advertising community to accept commercials as MP-3s so they will have this artifact on a regular basis.[citation needed]

As the multicast channels are added, the available data bandwidth (96kBps) is divided down, you don't get more bandwidth. So if you have an HD-1 and an HD-2 channel, to make them both of equal quality, they are both 48kBps channels. A standard MP3, which is not the highest fidelity sound, is 128k.[citation needed] There are also severe compromises in how the audio can be shaped and processed[citation needed] by the station's program director and engineer which limits the creative freedom once enjoyed by these broadcasters. HD Radio can't handle aggressive processing, which gives radio that "punch" that jumps out of the radio. Modern high quality audio processors like those made by Omnia Audio try to compensate for these limitations. If you add an HD-3 chanel, you have to take the bandwidth away from one or both of the existing channels. Since multicasting is the "killer app" as the industry puts it, this must be changed before "HD Radio" is really "high definition".[citation needed]

HD radio signals have a problem with building penetration.[citation needed] The HD Radio signal is injected into the FM carrier 20dB below the analog modulation, to there is 100 times less power. In other words, a 50,000 watt FM station only puts out 500 watts average HD radio power. While a digital radio receiver needs less signal to work since it only needs to tell the difference between a 1 and a 0, the building penetration is a problem that is yet unsolved.

Analog radio "degrades gracefully", meaning that it gradually gets noisier until it's unuseable. FM radio stations can actually be heard far from their intended service areas, allowing listening across a very large area. Digital signals are either on or off, once the signal goes below a certain level, the receiver just quits working[citation needed] and you see "acquiring" or a similar message on the radio, with no sound on the digital channel.

It is a common misconception that "digital" is superior to analog.[citation needed]


[edit] External links

[edit] References

  1. ^ a b "HD Rollout", 2006-04-11. Retrieved on 2007-01-29.
  2. ^ a b "Brazil Hosts HD Radio Rollout", 2006-11-01. Retrieved on 2007-01-29.
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