In-Band On-Channel (IBOC) is a proprietary system for digital radio broadcasting from iBiquity. The system is also known as HD Radio.
In IBOC the digital radio signals are broadcasted on the same channel and at the same time as the conventional AM or FM signal. Eventually, as analogue receivers are replaced with digital receivers, the analogue signal can be replaced with extra digital channels. This makes a smooth transition from analogue to digital radio possible without the need for additional frequency bands for digital radio.
IBOC supports program associated data, such as the name of the radio station, the playlist and enhanced data services. It is also possible to use the digital radio channel for data transfer.
iBiquity has developed a solution for both AM and FM. FM IBOC is an OFDM system which creates a set of sidebands at each side of the analogue FM signal. The combined analogue FM and IBOC signal fits in the same spectral mask as is specified for conventional FM. This is possible due to the fact that the digital sidebands are transmitted at a much lower power level and that part of a normal analogue FM signal is used for datacasting. Transmission of the datacasting signal has to be ceased.
AM IBOC works in about the same way as FM IBOC. The digital channel is transmitted alongside the conventional analogue AM signal. The digital sidebands are transmitted on a much lower power level to fit in the spectrum mask of the analogue AM. There will be a small performance penalty to the analogue side to accomodate the IBOC digital signal. The digital sidebands have a bandwidth of 10 kHz each. The total bandwidth of the combined signal is 30 kHz. Not all analogue AM receivers will filter out the digital sidebands. In that case the sidebands will be audible.