In-band on-channel (IBOC) is a hybrid method of transmitting digital radio and analog radio broadcast signals simultaneously on the same frequency. The name refers to the new digital signals being broadcast in the same AM or FM band (in-band), and associated with an existing radio channel (on-channel). By utilizing additional digital subcarriers or sidebands, digital information is multiplexed on existing signals, thus avoiding re-allocation of the broadcast bands.
IBOC relies on unused areas of the existing spectrum to send its signals. This is particularly useful in North America style FM, where channels are widely spaced at 200 kHz but use only about 50 kHz of that bandwidth for the audio signal. In most countries, FM channel spacing may be as close as 100 kHz, and on AM it is only 10 kHz. While these all offer some room for additional digital broadcasts, most attention on IBOC is in the FM band in North American systems; in Europe and many other countries, entirely new bands were allocated for all-digital systems.
Digital radio standards generally allow multiple program channels to be multiplexed into a single digital stream. In North American FM, this normally allows two or three high-fidelity signals combined in one channel, or one high-fidelity signal plus several additional channels at medium-fidelity levels that are much higher quality than AM. For even greater capacity, some existing subcarriers can be taken off the air to provide additional bandwidth in the modulation baseband. On FM for instance, this might mean removing stereo from the analog signal, relying on the digital signal to provide stereo where desired, thus making room for another digital channel. Due to the reduced bandwidth in AM, IBOC is incompatible with analog stereo, although that is rarely implemented, and additional channels are limited to highly compressed voice such as traffic and weather.
Eventually, stations can go from digital/analog-hybrid mode to all-digital, by eliminating the baseband monophonic audio.
FM methods On FM there are three methods of IBOC broadcasting in use, primarily in the United States.
HD Radio Broadcasting The first, and 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 allotted ±100 kHz FM channel. This creates potential interference issues with adjacent channels. This is the most widely used system, with approximately 1,560 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 FMeXtra 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 into 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 restrict listening with conditional access and encryption.
DRM Digital Radio Mondiale allows for simultaneous transmission of multiple data streams alongside an audio signal. The DRM mode for VHF provides bandwidths from between 35 kbit/s to 185 kbit/s and up to four simultaneous data streams, allowing 5.1 surround DVD quality audio to be broadcast alongside other multimedia content - images, video or HTML content are typical examples.
Like HD Radio, it can be backwards compatible with existing FM receiver equipment receiving the analog signal, with side-band broadcasts digitally encoded using HE-AAC or xHE-AAC received on compatible devices, this ability to operate within the internationally agreed FM spectrum of 88-108 MHz makes DRM a viable candidate for future adoption if countries begin to eliminate their analog broadcasts.
AM methods
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 R. 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: being primarily analog, the system will be just as subject to artificial interference and noise as the current AM system 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 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 and many other countries have 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 commonly used for terrestrial DAB is part of 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 and the amateur radio 1.25 meter (222 MHz) band. 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 station operators 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. (Norwegian) The typical bandwidth for DAB programs is only 128 kbit/s using the first generation CODEC, the less-robust MP2 standard which requires at least double that rate to be considered near-CD quality.
An updated version of the Eureka-147 standard called DAB+ has been implemented. Using the more efficient high quality MPEG-4 CODEC HE-AAC v2, this compression method allows the DAB+ system to carry more channels or have better sound quality at the same bit rate as the original DAB system. It is the DAB+ implementation which will be under consideration for new station designs and not the earlier DAB scheme using the MUSICAM CODEC. The DAB+ system was coordinated and developed by the World DAB Forum, formed in 1997 from the old organization. It gives the Eureka-147 system a similar quality per bit rate as the IBOC system and hence (arguably) a better sound quality than FM.
Challenges AM IBOC in the United States still faces some serious technological challenges, including nighttime interference with other stations. iBiquity initially used an audio compression system known as 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).
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. 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.