Radio Bits

Radio has changed very little since AM broadcasts began around the turn of the 20th century. FM was introduced in the '30s, and stereo FM broadcasts began

Radio has changed very little since AM broadcasts began around the turn of the 20th century. FM was introduced in the '30s, and stereo FM broadcasts began in '61. But both types of transmission are analog in nature and are thus prone to problems such as noise, multipath interference (intermittent cancellation caused by reflected radio waves arriving at the receiver at different times), and crosstalk with signals in adjacent frequency bands.

On October 10, 2002, the Federal Communications Commission (FCC) announced its approval of the largest overhaul of terrestrial radio broadcasting ever to occur. The underlying technology, developed by a company called iBiquity Digital (, is called In-Band On-Channel (IBOC), which piggybacks digital data on top of conventional analog AM and FM transmissions. As a result, radio stations can send digital and analog signals simultaneously without changing their broadcast frequency, providing full backward compatibility with existing receivers while adding significant capabilities for new digital radios.

Both AM and FM broadcasts include sideband frequencies above and below the primary transmission frequency, and IBOC uses these sidebands to send digital data. In fact, the data is redundantly transmitted in the upper and lower sidebands to help ensure that it arrives at the receiver intact. Another safety measure in the FM domain is iBiquity's First Adjacent Canceller (FAC) technology, which cancels out strong adjacent FM channels that can wipe out the digital sidebands.

IBOC overcomes noise and multipath interference with a proprietary encoding scheme that is applied to the sidebands and used in conjunction with custom error-correction algorithms. The power in each redundant sideband is then combined within the receiver to maximize signal gain. Digital bandwidth is conserved by using a perceptual audio coder (PAC) to remove certain elements of the audio data based on psychoacoustic models, much like other audio coders, such as MP3.

All digital-transmission systems face a trade-off between signal robustness and the time it takes for a receiver to acquire the signal. This trade-off is determined by a portion of the control software called an interleaver, which scrambles the timing of the encoded data, redistributing transmission errors to increase the fault tolerance of the system. Short interleavers use small data packets and allow rapid acquisition, but the signal is prone to dropouts. Conversely, long interleavers use large data packets to provide more robust signals, but they take longer to acquire the signal.

IBOC solves this dilemma by first tuning in the analog signal and then crossfading to the digital signal once it has been acquired by a long interleaver, which ensures robust reception. At the boundary of the station's coverage area, the receiver crossfades back to analog as needed to prevent abrupt digital dropouts.

The new capabilities offered by IBOC include the Main Program Service (MPS), which preserves the primary programming in analog and digital forms and adds other data related to the program, such as artist name and song title. Other new elements include Personal Data Service (PDS), which lets users specify the types of on-demand data they wish to receive; Station Identification Service (SIS); and Auxiliary Application Service (AAS), which allows virtually any type of application to be added in the future. Text information is displayed on the radio's front panel.

Under the commercial name HD Radio, IBOC will soon be available in car radios from Kenwood, Delphi, and others, using chipsets from Texas Instruments that can be retrofitted into existing radio architectures. This technology could change forever the way that your music is broadcast to millions of fans, so watch for its introduction in 2003.