Science Fair Project Encyclopedia
iBiquity is a company formed by the merger of USA Digital Radio and Lucent Digital Radio , with the goal of creating an in-band on-channel (IBOC) digital radio system for the United States. It can operate on both AM band and FM band broadcasts either in a digital-only mode, or in a "hybrid" digital+analog mode. Most broadcasts for the foreseeable future will use the hybrid method, reportedly giving AM stations "FM quality" sound, while allowing FM stations to achieve "CD quality" audio or carry multiple audio programs. The technology is marketed under the term HD Radio (or expanded unnecessarily, High Definition Radio).
Digital information is modulated using COFDM, a broadcasting method that has been used in a number of different digital radio and television systems. The audio compression algorithm was initially set to be PAC when iBiquity's standard was first approved by the Federal Communications Commission (FCC) in 2002, but the system was changed to the HDC format in 2003. The change was made because the low-bitrate audio for AM stations was described by some as being "underwater," plus the fact that the partially in-house HDC format has newer patents that can be exploited for longer periods. In hybrid mode, the AM version can carry 36 kilobits per second of data, while FM stations may carry information at 96 kbit/s.
AM stations are usually considered to have 5 kHz of bandwidth. With double sidebands that are standard for most radio broadcasts, this results in a channel 10 kHz wide (in the Americas, at least). However, the AM version of HD Radio might be more appropriately classified as in-band adjacent-channel (IBAC) for AM stations, since the encoded signal adds 10 kHz to each side of the center frequency, meaning that the signal extends out from the center frequency by 15 kHz. Again, with double sidebands, this results in an entire signal that is 30 kHz wide. This extra information is sent at fairly low power, but this is still a reason why iBiquity's technology has only been tested on AM band stations that have 10 kHz channel spacing (no adjacent channels).
Most analog AM radios have filters to remove anything more than 5 kHz away from the center frequency, but some "wide band" receivers don't filter this, making the encoded signal audible. Even on radios that do have such a filter, it is possible to hear the digital "hashes" of the sidebands by tuning up or down from the desired frequency by 10 kHz. Use of the system for AM stations has been highly controversial because of possible interference problems. This is nothing new for the AM band, though, as AM stereo has produced similar controversies. Because of the limited bandwidth on AM stations, iBiquity's standard is incompatible with AM stereo broadcasts. To reduce interference problems with other stations, HD Radio can only be used during daytime broadcast periods on AM at present.
On the wider bandwidth of FM band stations, HD Radio can be used to carry multiple distinct audio services. Some have also proposed using the system to carry surround sound broadcasts with 5.1 channel audio (this may not be very effective on "hybrid" analog+digital broadcasts, though). National Public Radio in particular hopes to be able to carry several different streams through the transmitters of member stations. FM stations may have to drop pre-existing sideband services in order to carry HD Radio, though such services can often be restored through the digital subchannels that are then made available. There have been some concerns that HD Radio on FM will increase interference between different stations, though it is unlikely to make much of a difference. Many stations have carried other sideband programs for decades with few ill effects.
As of February 2005, receivers are still quite expensive, starting at around US$300. Manufacturers have initially focused on making car stereos. Home listening equipment apparently won't roll out until 2006.
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