HD Radio technology is a system used by AM and FM radio stations to transmit audio and data via a digital signal in conjunction with their analog signals. As a standard practice, the kilohertz signal rate is written next to its corresponding data transfer rate kilobits/s or kbits/s in HD Radio documentation. While HD Radio does allow for an all-digital mode, this system currently is used by some AM and FM radio stations to simulcast both digital and analog audio within the same channel (a hybridized digital-analog signal) as well as add new FM channels and text information. Although HD Radio broadcasting's content is currently subscription-free, listeners must purchase new receivers in order to receive the digital portion of the signal. As of May 2009, there were more stations in the world on the air with HD Radio technology than any other digital radio technology. More than 1,900 stations covering approximately 84% of the United States are broadcasting with this technology, and more than 1,000 HD2/3 multicast channels are on the air.

"HD Radio" is the trademark for iBiquity's in-band on-channel (IBOC) technology, which was selected by the Federal Communications Commission (FCC) in 2002 as a digital audio broadcasting method for the United States. According to iBiquity's website the "HD" is simply a brand name and has no meaning, although in its early whitepapers, iBiquity used the original name "Hybrid Digital". That was later dropped. There is no connection with high-definition television (HDTV), although like HDTV the HD Radio specification provides enhanced capabilities over the old analog format, such as 5.1 surround sound.

The HD Radio system, the only digital system approved by the FCC for digital AM/FM broadcasts in the United States, is officially known as NRSC-5, with the latest version being NRSC-5B. Other digital radio systems include FMeXtra, Digital Audio Broadcasting (Eureka 147), Digital Radio Mondiale (DRM and DRM+), and Compatible AM-Digital (CAM-D).

The FCC has not indicated any intent to force off analog radio broadcasts as it has with analog television broadcasts, as it would not result in the recovery of any radio spectrum rights which could be sold. Thus, there is no deadline by which consumers must buy an HD Radio receiver.

Overview

Digital information is transmitted using COFDM with an audio compression algorithm called HDC with spectral band replication (SBR). (HDC is a proprietary codec based upon but incompatible with the MPEG-4 standard HE-AAC). HD Radio equipped stations pay a one-time licensing fee for converting their primary audio channel to iBiquity's HD Radio technology, and 3% of incremental net revenues for any additional digital subchannels. The cost of converting a radio station can run between $100,000 and $200,000. Receiver manufacturers pay a royalty.

If digital signal reception is lost, the HD Radio receiver will revert to the analog signal, thereby providing seamless operation between the newer and older transmission methods-ONLY for the primary HD(-1) signal (The extra HD-2 and HD-3 streams are not simulcast on analog, thus are totally lost when digital reception is gone). Alternatively the HD Radio signal can revert to a more-robust ~20 kilobit per second stream, provided the broadcaster has that setup as well. Datacasting is also possible, with metadata providing song titles or artist information.

iBiquity Digital claims that the system approaches CD quality sound and offers reduction of both interference and static; however, some listeners have complained of increased interference on the AM band (see AM, below).

AM

Sending pure digital data through the approximately 20 kilohertz AM channel is roughly equivalent to sending data through two 33 kbit/s analog telephone lines, thus limiting the maximum throughput possible. By using spectral band replication the HDC+SBR codec is able to simulate the recreation of sounds up to 15,000 Hz, thus achieving FM quality on the bandwidth-tight AM band. The HD Radio AM hybrid mode offers two options which can carry approximately 40 or 60 kbit/s of data, but most AM digital stations default to the more-robust 40 kbit/s mode which features redundancy (same signal broadcast twice). HD Radio also provides a pure digital mode, which lacks an analog signal for fallback and instead reverts to a 20 kbit/s signal during times of poor reception. The pure digital mode transmissions will stay within the AM station's channel instead of spilling into the channels next to the station transmitting "HD radio" as the hybrid digital stations do.

The AM version of HD Radio technology uses the 20 kHz channel (+/- 10 kHz), and overlaps 5 kHz into the opposite sideband of the adjacent channel on both sides. When operating in pure digital mode, the AM HD Radio signal fits inside a standard 20 kHz channel (20-40 kbit/s) or an extended 30 kHz channel (40-60 kbit/s), at the discretion of the station manager. As AM radio stations are spaced at 9 kHz (Europe) or 10 kHz (Americas) intervals, much of the digital information overlaps adjacent channels when in hybrid mode. Some nighttime listeners have expressed concern this design harms reception of adjacent channels with one formal complaint filed regarding the matter: WYSL owner Bob Savage against WBZ in Boston. The digital radio signal received on a conventional AM receiver tuned to an adjacent channel sounds like a large waterfall or similar white noise-like hiss, as can be heard in the audio soundclip referenced in the previous sentence. Listeners (and some professional broadcasters) call it IBUZ instead of the legal term, IBOC because of the "buzz saw" like noise.

FM

The FM hybrid digital/analog mode offers four options which can carry approximately 100, 112, 125, or 150 kbit/s of lossy data depending upon the station manager's power budget and/or desired range of signal. The HD Radio also provides several pure digital modes with up to 300 kbit/s bitrate, and enabling extra features like surround sound. Like AM, pure digital FM provides a "fallback" condition where it reverts to a more-robust 25 kbit/s signal.

FM stations have the option to subdivide their datastream into sub-channels (e.g., 88.1 HD1, HD2, HD3) of varying audio quality. The multiple services are similar to the digital subchannels found in ATSC-compliant digital television using multiplexed broadcasting. For example, some top 40 stations have added hot AC and classic rock to their digital subchannels to provide more variety to listeners. Stations may eventually go all-digital, thus allowing as many as three full-power channels and four low-power channels (seven total). As defined by iBiquity these channels could be sub-divided into CD-quality (100 kbit/s), FM-quality (25-50 kbit/s), AM-quality (12 kbit/s), or Talk-quality (5 kbit/s) channels. Alternatively, they could broadcast one single channel at 300 kbit/s.

Where the digital signal fails, the analog signal is used as a fallback for the main digital channel (normally HD1), requiring synchronization of the two. Current FCC rules require that one channel be a simulcast of the analog signal. In some cases, particularly during tropospheric ducting events, an HD Radio receiver will lock on to the digital sidebands of a distant station, even though there is a much stronger local analog-only station on the same frequency. Because there is no standard identification or other indication sent on the analog signal, there is no way for the receiver to recognize that there is no correlation between the two. The listener can possibly turn HD reception off (to listen to the local station, or avoid random flipping between the two stations), or enjoy listening to the distant stations and trying to get a station ID.

Stations can transmit HD Radio through their existing antennas using a diplexer as on AM, or are permitted by the FCC to use a separate antenna at the same location, or at a site licensed as an analog auxiliary, provided it is within a certain distance and height referenced to the analog main signal. This limitation assures that the two have about the same broadcast range, and that they maintain the proper ratio of signal strength to each other so as not to cause destructive interference with each other at any given location where they may be received.

Bandwidth

Currently, FM stations in the United States and Canada are licensed to carry 100 kilohertz of bandwidth, requiring approximately 200 kilohertz of spectrum. Only 15 kHz of the modulation bandwidth is used by analog monaural audio (baseband), with the remainder used for stereo, RBDS, paging, radio reading service, rental to other customers, or as a transmitter/studio link for in-house telemetry.

In regular hybrid mode, a station has its full ±100 kHz of analog bandwidth, and adds an extra ±30kHz guard band and ±70 kHz for its digital signals, thus taking a full 400 kHz of width. FM stations also have the option 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 use of the digital subchannels that are then made available. However, this requires the replacement of all related equipment both for the broadcasters and all of the receivers that use the discontinued services.

The ratio of power of the analog signal to the digital signal is standardized at 100:1 (−20 dBc), devoting 1% of the analog carrier power to the digital signal. Unlike with subcarriers, where the total baseband modulation is reduced, there is no reduction to the analog carrier power. The National Association of Broadcasters (NAB) is expected to request a 10 dB (10×) increase in the digital signal from the FCC. This equates to an increase to 10% of the analog carrier power, but no decrease in the analog signal. This was shown to reduce analog coverage, but results in a dramatic improvement in digital coverage. Other levels have also been tested, including a 6dB or four-fold increase to 4% (−14dBc or 25:1). National Public Radio is opposed to any increase because it is likely to increase interference to their member stations, particularly to their broadcast translators which are secondary and therefore left unprotected from such interference. Other broadcasters are also opposed (or indifferent), as increasing power would require expensive changes in equipment for many, and the already-expensive system has so far given them no benefit.

There are still some concerns that HD Radio on FM will increase interference between different stations even though HD Radio at the 10% power level fits within the FCC spectral mask [87803]. North American FM channels are spaced 200 kHz apart. An HD Radio station will not generally cause interference to any analog station within its 1 mV/m service contour, the limit above which the FCC protects most stations. However, the IBOC signal resides within the analog signal of the first-adjacent station. With the proposed power increase of 10dB, the potential exists to cause the degradation of the second-adjacent analog signals within its 1 mV/m service contour. Ironically, the National Association of Broadcasters claims this is not a problem, at the same time using it as justification to keep LPFM stations off the air on second-adjacent channels, excluding NAB members' own translators that are allowed higher power than local community radio LPFMs.

Comparison to EU's Digital Radio

FM HD Radio versus DAB

Some European Union countries have implemented Eureka-147 Digital Audio Broadcasting (DAB), with compatible radios having been available since 1999. This system has also been implemented in a limited number of Canadian markets. DAB broadcasts a single station that is approximately 1500 kilohertz in width (~1000 kilobits per second). That station is then subdivided into multiple digital streams (a multiplex) of between 9 and 12 programs. In order to implement DAB, it was necessary for the European Telecommunications organization to set aside a new range of frequencies, whereas FM HD Radio shares its digital broadcast with the traditional 200 kilohertz-wide channels, mixing incompatible digital and analog signals into the same spectrum.

In the UK, Denmark and Norway, which are the leading countries with regard to implementing DAB, they are using the first generation DAB with the MPEG-1 Audio Layer II (MP2) audio codec (also used with some DVB broadcasts and DVD Video audio tracks). MP2 does not have error correction and has less efficient compression than newer codecs. The typical bitrate for DAB programs is only 128 kbit/s and as a result most radio stations on DAB have a lower sound quality than FM, prompting a number of complaints. In Australia, Italy and Switzerland, the newer DAB+ has been used which uses MPEG-4 HE-AAC audio, which has error correction and better compression. However it is incompatible with first generation DAB devices. HD Radio is similar to DAB+ but uses proprietary iBiquity HDC codec also derived from HE-AAC.

A directly related issue with DAB's original inefficient compression is "downgrading" stations from stereophonic to monaural, in order to include more channels into the limited 1000 kbit/s bandwidth, smaller coverage of markets as compared to analog FM, radios that are overly expensive, poor reception inside vehicles or buildings, and a general lack of interest in DAB (only 5 million units sold in the largest take up area of UK by mid-2007). HD Radio shares most of these same flaws (see criticisms below).

AM HD Radio versus DRM

The European Union states are in the process of rolling-out Digital Radio Mondiale (DRM), with compatible radios already available for sale. DRM is very similar to AM HD Radio in that each station is broadcast via a channel 20 kilohertz wide (+/-10 kHz), and the radio is hand-tuned to each individual station's location on the dial. The two standards also share the same modulation scheme (COFDM), codecs based upon MPEG-4 AAC, and like HD Radio, DRM allows broadcasters multiple options:

• Hybrid mode (digital/analog) — 10 kHz analog plus 5 KHz digital bandwidth allows 5-16 kbit/s — (HDR is 20-40 kbit/s)
• 10 KHz digital-only bandwidth (equal to one-half of one channel) allows 12-35 kbit/s — (HDR is 20-40 kbit/s)
• 20 KHz digital-only bandwidth (equal to one channel with both sidebands) allows 24-72 kbit/s — (HDR is 40-60 kbit/s)

Actual DRM bit rates vary depending on day versus night transmission (groundwave versus skywave) and desired robustness. For AM stations DRM offers a growth path for broadcasters. Unfortunately DRM shares many of the same flaws as DAB and HD Radio technology: Shorter broadcast distance as compared to analog AM signal when in hybrid mode; poor reception inside vehicles and buildings; and interference with adjacent channels (though in all-digital mode the signal fits inside the designated channel mask).

Criticisms

Awareness, coverage

There is low awareness among consumers, and even lower uptake. According to a survey dated August 8, 2007 by Bridge Ratings, when asked the question, "Would you buy an HD radio in the next two months?" only 1.0% responded "yes". Some broadcast engineers have also expressed distrust or dislike of the new system. Also, a survey conducted in September 2008 saw a small percentage still confused HD radio with satellite radio.

Most of the first-generation HD Radio tuners have been noted as being very insensitive, making reception problematic. The HD Radio signal is 20 dBW below (one percent the power of) a station's analog signal. In addition it has been noted that the analog section of some tuners displays poor reception capabilities compared to older non-digital models.

Proprietary and incompatible

Even though DAB and DRM standards are open-standards and pre-date HD-Radio, HD Radio receivers cannot be used to receive these stations when sold or moved overseas (with certain exceptions; there are HD Radio stations in Brazil, Thailand, Taiwan, Japan and a few other countries). DAB and DRM receivers cannot receive HD Radio signals in the US. The HD Radio system, which enables AM and FM stations to upgrade to digital without changing frequencies, is a different digital broadcasting standard. The lack of a common standard means that HD Radios cannot receive DAB format broadcasts of other countries and vice-versa, and that manufacturers must develop separate products for different countries, which typically are not dual-format. Whereas the AAC (Advanced Audio Coding) family of codecs are publicly documented standards, the HDC codec exists only within the HD Radio system, and is an iBiquity trade secret. Similarly DRM and DAB are open specifications, while iBiquity's HD Radio specification is partly open but mostly private. While other FM technologies require new bandwidth and make obsolete all current radios, the analog/digital hybrid solution offered by the HD Radio system allows an elegant transition from analog to digital radio.

Reduced-quality concerns

Promotion for HD Radio does not always make clear that some of its capabilities are mutually incompatible with other of its capabilities. For example, the FM system has been described as "CD quality;" however, the FM system also allows multiplexing the data stream between two or more separate programs. A program utilizing one half or less of the data stream does not attain the higher audio quality of a single program allowed the full data stream. The FCC has declared "one free over-the-air digital stream [must be] of equal or greater quality than the station’s existing analog signal". (If the FCC discontinues analog simulcasting, each station will have over 300 kbit/s bandwidth available, allowing for CD or even Surround Sound-quality audio together with multiple sub-channels.)

The broadcasting industry is seeking FCC approval for conditional access, that is, enabling the extra programs to be available only by paid subscription (on future models of HD Radio). Conditional access will also enable special services for the blind (such as radio reading services) and hearing impaired. NDS, a maker of digital media encryption technology, has a deal with iBiquity to provide HD Radio with an encrypted content-delivery system called RadioGuard. NDS claims that RadioGuard will "provide additional revenue-generating possibilities".

Reduced analog signal

Radio stations are licensed in the United States to broadcast at a specific effective radiated power level. NPR Labs recently did a study of predicted HD radio operation if power levels were increased to 10% of maximum power, and found the digital signal would increase RF interference on FM. However the boosted digital HD signal coverage would then exceed analog coverage, with 17% more population covered.

Costs

The costs of installing the system, including fees, vary from station to station, according to the station's size and existing infrastructure. Large companies in larger media markets (such as Clear Channel or Citadel Broadcasting) can afford to implement the technology for their stations. Current HD Radio receivers cost anywhere from around $50 to several hundred dollars (US), compared to regular FM radios which can even be found at dollar stores. By contrast, all of the R&D work for DAB (and much of DAB+) had already been done using an old codec, and requires no licensing fees. FMeXtra is even less expensive, and requires no installation labor or cost for the broadcaster, other than plugging it into the transmitter. It also requires no FCC approval, coming under a station's subsidiary communications authority. FMeXtra requires no license fees, and does not interfere with adjacent channels. Other digital broadcast technologies have not been approved for use in the United States.

Programming

Currently the HD Digital Radio Alliance, a consortium of major owners such as ABC, CBS, and ClearChannel, is acting as a liaison for stations to choose unduplicated formats for the extra channels (HD2, HD3, etc.). This is designed to provide additional choices for listeners instead of several stations all independently deciding to create the same format. HD1 stations broadcast the same format as the regular FM (and some AM) stations, and most of these stations offer an HD2 subchannel to complement their current programming.

Clear Channel is selling programming of several different music genres to other competing stations, in addition to airing them on its own stations. Some stations are simulcasting their local AM or lower-power FM broadcasts on sister stations' HD2 or HD3 channels, such as KUDL in Kansas City simulcasting 980 KMBZ's programming on 98.1-HD3. It is common practice to broadcast an older, discontinued format on HD2 channels, such as WPGB in Pittsburgh, Pennsylvania which carries its former smooth jazz format on its HD2 band. Other recent additions include introduction of air staff on HD2 stations, like KDWB's Party Zone channel in Minneapolis-Saint Paul. Some HD2 or HD3 stations are even simulcasting sister AM stations KBCO in Boulder, Colorado uses its HD2 channel to broadcast exclusive live recordings from their private recording studio. CBS Radio recently announced plans to introduce its more popular superstations into distant markets (KROQ into New York City, WFAN into Florida, and KFRG and KSCF into Los Angeles) via HD2 and HD3 channels.

Translators

Although broadcast translators are prohibited from originating their own programming, the FCC has allowed stations to ignore this by having their programming simulcast on an HD Radio channel of the primary station the translator is assigned to (see, for instance, W237CS, which is licensed as a translator to WXMT but originates its own schedule). This also allows station owners, who already usually own multiple stations locally and nationally, to circumvent the rulemaking process of changing the table of allotments to get a new station, and to avoid controlling-interest caps intended to prevent the excessive concentration of media ownership. These also block new LPFM stations from going on the air, and are allowed much greater broadcast range (height and power) than locally-originated LPFMs.

For translators used for their original intent (to retransmit the primary station unmodified to distant areas with poor reception), HD Radio may be passed along from the main station via a "bent pipe" setup, where the translator simply makes a frequency shift of the entire channel, often by heterodyning it through the use of an intermediate frequency. This may require an increase in bandwidth. Baseband translators which use a separate receiver and transmitter require an HD Radio transmitter, just as does the main station. Translators are not required to pass through HD Radio.

Receivers

Automotive and Home/Professional

There are as many as 100 different HD Radio receivers to choose from, far more than are available to receive any other digital radio signal. Receivers are becoming less expensive, starting at around US$75. Manufacturers have initially focused on making car stereos. In 2008, Ford Motor Company became the first U.S. automaker to feature HD Radio tuners in its vehicles; and beginning with the 2009 model year, Ford will offer HD Radio tuners as a factory-installed option (Compatible with SYNC). BMW offers Factory-Installed HD Radio on nearly all 2008+ models. Hyundai offers factory-installed HD Radio on the Genesis 2008+ model year. Mercedes-Benz offers factory-installed HD Radio on the R320, ML320 and GL320 models (2009 model year). MINI offers factory-installed HD Radio on the Cooper, Cooper S (2008+ model year). Scion offers HD Radio on select audio systems. Volvo offers factory-installed HD Radio on the S40 T5, V50 T5, C30, C70, S80, V70, XC70 and XC90 models (2009 model year).

The growing list of HD Radio automotive receiver manufacturers includes Alpine Electronics, Directed Electronics, Dual, Insignia, Jensen, JVC, Kenwood, Pioneer, Sony, TEAC and Visteon.

Home and office listening equipment is currently available from many companies, in both component tuner and tabletop models, including Audio Design Associates, Boston Acoustics, Day Sequerra, Denon, DICE Electronics, Directed Electronics, Insignia, Jensen Electronics, LG, Marantz, Onkyo, Polk Audio, Radiosophy, Radio Shack, Rotel, Sangean, Sony, Visteon, Yamaha.

Portable

Previously, portable HD Radio receivers were unavailable due to the early chipsets either being too large for a portable enclosure or needing too much power to be practical for a battery-operated device. However, in January 2008 at the Consumer Electronics Show (CES) in Las Vegas iBiquity unveiled a prototype of a new portable receiver, roughly the size of a cigarette pack. Two companies are currently making low-power chipsets for HD Radio receivers: Samsung and SiPort.

On July 12, 2009, Best Buy started selling a house brand portable unit, the Insignia NS-HD01, which was the first portable HD Radio to come to the general market and features FM-only playback and a non-removable rechargeable battery which charges via mini USB. The device sells for around $50 and is the least expensive receiver currently available.

On September 15, 2009, Microsoft released the Zune HD, which includes an HD Radio receiver within the OLED-screened media device.

References

1. iBiquity Digital Corporation - IBOC White Papers.
2. iBiquity Licensing Fact Sheet 2009 (PDF).
3. MP3 sample of the interference: IBOC Observations Soundclip.
4. (Norwegian).
5. http://www.ibiquity.com/press_room/news_releases/2009/1331
6. http://www.siport.com/

External links

• HD Radio official site
• U.S. Federal Communications Commission IBOC Page