Digital radio broadcasting technology delivers digital audio and data services to mobile, portable, and fixed receivers. One type of digital radio broadcasting, referred to as in-band on-channel (IBOC) digital audio broadcasting (DAB), uses terrestrial transmitters in the existing Medium Frequency (MF) and Very High Frequency (VHF) radio bands. HD Radio™ Technology, developed by iBiquity Digital Corporation, is one example of an IBOC implementation for digital radio broadcasting and reception.
IBOC DAB signals can be transmitted in a hybrid format including an analog modulated carrier in combination with a plurality of digitally modulated carriers, or in an all-digital format wherein the analog modulated carrier is not used. Using the hybrid mode, broadcasters may continue to transmit analog AM and FM simultaneously with higher-quality and more robust digital signals, allowing themselves and their listeners to convert from analog to digital radio while maintaining their current frequency allocations. IBOC DAB hybrid and all-digital waveforms are described in U.S. Pat. No. 7,933,368, which is hereby incorporated by reference.
IBOC DAB technology can provide digital quality audio, superior to existing analog broadcasting formats. Because each IBOC DAB signal is transmitted within the spectral mask of an existing AM or FM channel allocation, it requires no new spectral allocations. IBOC DAB promotes economy of spectrum while enabling broadcasters to supply digital quality audio to the present base of listeners.
The National Radio Systems Committee, a standard-setting organization sponsored by the National Association of Broadcasters and the Consumer Electronics Association, adopted an IBOC standard, designated NRSC-5, in September 2005. NRSC-5, the disclosure of which is incorporated herein by reference, sets forth the requirements for broadcasting digital audio and ancillary data over AM and FM broadcast channels. The standard and its reference documents contain detailed explanations of the RF/transmission subsystem and the transport and service multiplex subsystems. iBiquity's HD Radio Technology is an implementation of the NRSC-5 IBOC standard.
FM analog receivers implemented with digital signal processor (DSP) algorithms offer high performance, and are common for car radio receivers. They generally offer low distortion, good stereo separation, and often high sensitivity and selectivity. However, IBOC signals may create some new digital interference conditions that were not anticipated by the designers of the FM analog demodulators. Although this digital interference is generally limited to localized reception areas, or occurs infrequently under special signal conditions, there may still be concern about the impact on analog service. Furthermore, the FCC has authorized an increase in digital signal injection power from presently 20 dB below the analog FM host, to 10 dB, potentially increasing the interference.
It would be desirable to have an FM demodulator that is effective in eliminating the effects of interference from IBOC signals.