IBOC

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 in one channel, or one high-fidelity signal and 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 make additional bandwidth available in the modulation baseband. On FM for instance, this might mean removing stereo from the analog signal, relying on the digital version of that signal to provide stereo where available, and making room for another digital channel. Due to the lack of available bandwidth in AM, IBOC is incompatible with analog stereo, although this is rarely used today, 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 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 allotted ±100 kHz FM channel. This creates 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 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 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 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 vdv quality audio to be broadcast alongside other multimedia content - images, video or HTML content are typical examples.

While it is not backwardly compatible with existing FM receiver equipment, with broadcasts digitally encoded using HE-AAC or xHE-AAC, this ability to operate within the internationally agreed FM spectrum of 88-108 MHz makes DRM a viable candidate for future adoption when countries begin to switch off their analogue 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