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In-band on-channel (IBOC) is a hybrid method of transmitting digital radio and analog radio broadcast signals simultaneously on the same frequency.

By utilizing additional digital subcarriers or sidebands, digital information is "multiplexed" on an AM or FM analog signal, thus avoiding re-allocation of the broadcast bands. However, by putting RF energy outside of the normally-defined channel, interference to adjacent channel stations is increased when using digital sidebands.

IBOC does allow for multiple program channels, though this can entail taking some existing subcarriers off the air to make additional bandwidth available in the modulation baseband. On FM, this could eventually mean removing stereo. On AM, IBOC is incompatible with analog stereo, and any additional channels are limited to highly-compressed voice, such as traffic and weather. Eventually, stations can go from hybrid mode (both analog and digital) to all-digital, by eliminating the baseband monophonic audio.

FM methods
On FM there are currently three methods of IBOC broadcasting in use, mainly in the United States.

HD Radio Broadcasting
The first, and currently only, digital technology approved for use on AM and FM broadcast frequencies by the Federal Communications Commission in the United States, is the proprietary HD Radio system developed by iBiquity Digital Corporation, which transmits energy beyond the alloted ±100 kHz FM channel creating potential interference issues with adjacent channels. This is the most widespread system in use: with approximately 1,556 stations transmitting HD radio in the US, plus over 800 new multicast channels (as of Jan 2010). There is a one-time license fee to iBiquity Digital, for the use of its intellectual property, as well as costs for new equipment which range from $50,000 to $100,000 US (2010) per station.

FMeXtra
The other system is FMeXtra by Digital Radio Express, which instead uses subcarriers within the existing signal. This system was introduced more recently. The system is compatible with HD Radio in hybrid mode, but not in all-digital mode, and with RBDS. The stereo subcarrier can be removed to make more space available for FMeXtra in the modulation baseband. However, the system is not compatible with other existing 67-92 KHz subcarriers which have mostly fallen into disuse. 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 has generally all the user features of HD Radio, including multicast capability; the ability to broadcast several different audio programs simultaneously. It uses the aacPlus (HE-AAC) codec.

FMeXtra can control listening with conditional access and encryption.

DRM+
Digital Radio Mondiale allows for simultaneous transmission of multiple data streams alongside an audio signal. The DRM+ standard provides bandwidths from between 35 kbit/s to 185 kbit/s and up to four simultaneous data streams, allowing near-CD quality audio to be broadcast alongside other multimedia content - images, video or HTML content are typical examples.

DRM+ is a superset of the original DRM standard, allowing for broadcasts at frequencies of up to 174 MHz (whereas DRM only operates at up to 30 MHz). While it is not backwardly compatible with existing FM receiver equipment, with broadcasts digitally encoded using AAC, this ability to operate within the internationally agreed FM spectrum of 88-108MHz makes DRM+ a viable candidate for future adoption when countries begin to switch off their analogue broadcasts.

HD Radio Broadcasting
iBiquity also created a mediumwave HD Radio system for AM, which is the only system approved by the Federal Communications Commission for digital AM broadcasting in the United States. The HD Radio system employs use of injected 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 principle of AM stereo where it puts a digital signal where the C-QUAM system would put the analog stereo decoding information.

DRM
Digital Radio Mondiale has had much more success in creating an AM system, and one that could be much less expensive to implement than any proprietary HD Radio system, although it requires new frequency. It is the only one to have been accepted mediumwave but also shortwave (and possibly longwave) by the International Telecommunication Union (ITU) for use in regions I and III, but not yet in region II, the Americas. The HD Radio system has also been approved by International Telecommunication Union.

CAM-D
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, opting for quality over quantity. Unlike the HD system iBiquity calls "hybrid digital" the CAM-D system truly is a direct hybrid of both analog and digital. Some engineers believe that CAM-D may be compatible with analog AM stereo with the right engineering.

Critics of CAM-D point to several drawbacks:
 * 1) being primarily analog, the system will be just as subject to artificial interference and noise as the current AM system
 * 2) there are virtually no receivers available for the system and at present, no major manufacturer has announced even the intention to begin production of them; and
 * 3) the cost of retrofitting with CAM-D is more than that of simply buying a new, HD-ready solid state transmitter.

IBOC Versus DAB
While the United Kingdom has chosen the Eureka 147 standard of digital audio broadcasting (DAB) for creating a digital radio service, the United States has selected IBOC technology for its digital AM and FM stations. The band used for terrestrial DAB in the UK is VHF band III, which does not suffer from L-band's significant line-of-sight problems. However, it is not available in North America since that span is occupied by TV channels 7 to 13. The stations currently occupying that spectrum did not wish to give up their space, since VHF offers several benefits over UHF: relatively lower power, long distance propagation (up to 100 miles (160 km) with a rooftop antenna), and a longer wavelength that is more robust and less affected by interference. In Canada, the Canadian Radio-television and Telecommunications Commission (CRTC) is continuing to follow the analog standard, so the channels remain unavailable there as well. HD Radio testing has been authorized in Canada, as well as other countries around the world.

There was also concern that AM and FM stations' branding, using their current frequencies, would be lost to new channel numbers, though virtual channels such as on digital television would eliminate this. Also, several competing stations would have to share a transmitter that multiplexes them all into one ensemble with the same coverage area (though many FM stations are already diplexed in large cities such as New York). 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 higher data rates (300 kbit/s versus 60 kbit/s in the HD Radio standard) due to greater bandwidth (100 kHz versus 10 kHz).

The most significant advantage for IBOC is its relative ease of implementation. Existing analog radios are not rendered obsolete and the consumer and industry may transition to digital at a rational pace. In addition, the technology infrastructure is in place: most major broadcast equipment manufacturers are implementing IBOC technology and 60+ receiver manufacturers are selling IBOC reception devices. In the UK, Denmark, Norway and Switzerland, which are the leading countries with regard to implementing DAB, the first-generation MPEG-1 Audio Layer II (MP2) codec stereo radio stations on DAB have a lower sound-quality than FM, prompting a number of complaints. The typical bandwidth for DAB programs is only 128 kbit/s using the first generation CODEC, less-robust MP2 standard which requires at least double that rate to be considered near-CD quality.

Other issues with DAB include "downgrading" stations from stereo to monaural, in order to squeeze even 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).

A new version of the Eureka-147 standard called DAB+ has been implemented. Using the more efficient high quality MPEG-4 CODEC called HE-AAC v2, this compression method will allow the DAB+ system to carry more channels or have better sound quality at the same bit rate as the old DAB system. It is the new DAB+ implementation which will be under consideration for new station designs and not the earlier DAB scheme using the MUSICAM CODEC. This new DAB+ system was coordinated and developed by the World DAB Forum, formed in 1997 from the old organization. It will give the Eureka-147 system a similar quality per bit rate as the IBOC system and hence a better sound quality than FM.

Challenges
AM IBOC in the United States 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 Digital Audio Radio Service ]), but in August 2003 a switch to HDC (based-upon ACC) was made to rectify these problems. 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). The system, however, unlike HD Radio, does not permit the existing analog signal and the digital signal to live together in the same channel. DRM requires an additional channel to maintain both signals.

Both AM and FM IBOC signals cause interference to adjacent-channel stations, but not within the station’s interference-free protected contours designated by the U.S. Federal Communications Commission (FCC). It has led to derogatory terms such as IBAC (In-band adjacent-channel) and IBUZ (since the interference sounds like a buzz.) The range of a station on an HD Radio receiver is somewhat less than its analog signal. However, in June, 2008, a group of US broadcasters and equipment manufacturers requested that the U.S. FCC increase the permissible FM IBOC power from 1% (currently) to a maximum of 10% of the analog power. On January 29, 2010, the FCC approved the request. In addition, tropospheric ducting and e-skip can reduce the range of the digital signal, as well as the analog.

In-band on-channel digital radios using iBiquity's standard are being marketed under the brand "HD Radio" to highlight the purported quality of reception. As of June 2008, over 60 different receiver models have been made, and stations have received blanket (no longer individual and experimental) authorization from the U.S. FCC to transmit in a multiplexed multichannel mode on FM. Originally, the use of HD Radio transmission on AM was limited to daytime only, and not allowed at night due to potential problems with skywave radio propagation. The FCC lifted this restriction in early 2007. DRM, however, is being used across Europe on shortwave, which is entirely AM skywave, without issue. With the proper receiver, many of those stations can be heard in North America as well, sans the analog signal.

Argentina
HD Radio technology was tested in 2004 with initial trials in Buenos Aires. Further testing of the technology began in early 2007.

Bosnia
Trial and tests of HD Radio technology began in Sarajevo in March 2007.

Brazil
HD Radio transmission in Brazil was started on September 26, 2005. The radio stations that use IBOC HD in Brazil are Radio Bandeirantes, Radio Globo, and RBS Group. A total of six stations – one FM and one AM from each group – are now transmitting in HD Radio. KISS FM in São Paulo is the first HD Radio station in Brazil. A total of 24 stations are broadcasting in Brazil as of June, 2008. A joint study by the government (ANATEL) and broadcasters’ associations (ABERT, AESP) is completed and awaiting government action to regulate HD Radio broadcasting in Brazil.

Canada
After having L-band DAB for several years, the Canadian Radio-Television and Telecommunications Commission (CRTC) and Canadian Broadcasting Corporation {CBC) have also looked at the use of HD Radio, given its gradual progress in the neighbouring U.S. The CBC began HD Radio testing in September 2006, focusing on transmissions from Toronto and Peterborough, Ontario. The CRTC has since revised its policy on digital radio to allow HD Radio operations pursuant to Public Notice CRTC 2006-160.

Czech Republic
Initial testing of the HD Radio system commenced in Prague in February 2007.

China
In China, Hunan Broadcasting Company started FMeXtra transmissions in Changsha in April 2007, and plans to put others throughout the Hunan province. [1] SARFT (State Administration for Radio, Film and Television) is currently testing HD Radio in Beijing in contemplation for acceptance in that country.

Colombia
Caracol Radio began testing of the HD Radio technology in both the AM and FM bands in early 2008.

El Salvador
El Salvador will be choosing HD Radio as its digital radio standard.

Europe
In September 2007 the European HD Radio Alliance (EHDRA) was formed by broadcasters and other interested groups to promote the adoption of HD Radio technology by European broadcasters, regulators and standards organizations.

France
France began broadcasting an HD Radio signal in March 2006 and plans to multicast two or more channels. The radio stations that use IBOC HD in France are SIRTI and NRJ Group. The owner of the transmitter is Towercast. The frequency of IBOC HD radio is 88.2 MHz. In May 2006, The Towercast group added a single channel of digital audio on 93.9 MHz.

Germany
Radio Regenbogen began HD Radio operations on 102.8 MHz in Heidelberg on December 3, 2007 pursuant to government testing authority.

Indonesia
Forum Radio Jaringan Indonesia had tested IBOC HD transmission from March 2006 to May 2006. The IBOC HD station in Jakarta was Delta FM (99.1 MHz). In April 2006, Radio Sangkakala (in Surabaya), the first AM HD radio station in Asia, went on the air on 1062 kHz.

Jamaica
Radio Jamaica began operating full time with both HD Radio AM and FM signals in the city of Kingston in 2008.

Mexico
All Mexican radio stations within 320 km of the U.S. border are allowed to transmit their programs on the AM and FM bands utilizing HD Radio technology. Approximately six Mexican AM and FM stations are already operating with HD Radio technology along Mexico’s border area with the US. Groupo Imagen commenced HD Radio transmissions on XHDL and XEDA in Mexico City in June 2008.

New Zealand
HD Radio transmission in Auckland, New Zealand was started on October 19, 2005. The frequency of IBOC HD radio is 106.1 MHz. The transmitter is located at Skytower. Following successful testing, the Radio Broadcasters Association (RBA) initiated a comprehensive trial of HD Radio technology in December 2006. The aim of the trial was to assess the coverage potential of the HD Radio system and to make a recommendation on the suitability of the technology for adoption.

Philippines
The first HD Radio station in the Philippines began broadcasting on November 9, 2005. The Philippines National Telecommunications Commission finalized its rules for FM digital radio operations on November 11, 2007.

Poland
An HD Radio trial began in Warsaw in 2006 in order to demonstrate the technology to local radio stations.

Puerto Rico
WPRM FM is the first station in San Juan, Puerto Rico (part of the US) to adopt HD Radio, in April 2005. WRTU in San Juan has also commenced broadcasting in HD Radio technology in 2007.

Switzerland
FM testing sponsored by Radio Sunshine and Ruoss AG began in Lucerne in April 2006. HD Radio operations in Switzerland continue and are spotlighted each year during “HD Radio Days”, an annual gathering in Lucerne of European broadcasters and manufacturers for the purpose of discussing the rollout of the technology in Europe.

Thailand
HD Radio transmission in Thailand was started in April 2006. Radio of Thailand had created a public IBOC HD radio network targeting mass transit commuters in Thailand's capital of Bangkok. To receive the broadcasts, more than 10,000 HD Radio receivers were installed in buses.

Ukraine
The first FM HD Radio broadcasts in Kiev went on the air in October 2006 on two FM stations operated by the First Ukrainian Radio Group.

Vietnam
Voice of Vietnam (VOV) commenced AM and FM HD Radio transmissions in Hanoi in June, 2008 including multicasting, in anticipation of making HD Radio technology a standard.

United States
As of June 2008, more than 1,700 HD Radio stations were broadcasting 2,432 HD Radio channels. HD Radio technology is the only digital technology approved by the FCC for digital AM and FM broadcasting in the US. Over 60 different HD Radio receivers are on sale in over 12,000 stores nationwide, including Apple, Best Buy, Target, and Wal-Mart.

As of May 2007, FMeXtra is on several dozen stations. Several hundred stations belonging to the Idea Bank consortium will also have FMeXtra installed.