In-band on-channel
|
|
In-band on-channel (IBOC) is a method of transmitting digital radio and analog radio broadcast signals simultaneously on the same frequency. By utilizing subcarriers and sidebands, digital information is "piggybacked" on a normal AM or FM analog signal, thus avoiding any complicated extra frequency allocation issues. However, by putting extra RF energy toward the edge of the station's channel, interference with adjacent channel stations is increased.
IBOC does allow for multiple channels, though this means taking more existing subcarriers off the air to make additional bandwidth available. On FM, this will eventually mean removing stereo. On AM, IBOC is generally incompatible with stereo at all. Eventually, stations can go from hybrid mode (both analog and digital) to all-digital, by eliminating the baseband monophonic audio.
| Contents |
IBOC versus DAB
While Canada and most countries in Europe have chosen the Eureka 147 standard of DAB for creating a digital radio service, the United States has been holding out for advancements in IBOC technology, now branded as HD Radio under the direction of iBiquity. Part of the reason is the use of the L band (1452–1492 MHz) for test-flight telemetry by the U.S. military and its contractors. This band is used in Canada for digital radio, but remains unavailable for reassignment by the NTIA in the U.S. for broadcasting. It also has somewhat of a disadvantage in that its higher frequency is more line of sight than VHF (FM), and far more than mediumwave (AM).
It is also partly because of concern that stations' branding, using their current frequency, would be lost to new channel numbers, though virtual channels such as on digital television would eliminate this, and stations already swap frequencies sometimes. Also, several competing stations must share a transmitter which multiplexes them all into one ensemble with the same coverage area, though some FM stations are already diplexed. A further concern to FM stations was that AM stations could suddenly be in competition with the same high audio quality, although FM would still have the advantage of being able to provide a wider range of services due to greater bandwidth (100kHz versus 10kHz). Finally, station owners would be able to keep total control of their own stations and a general monopoly on the broadcast bands.
Challenges
IBOC in the U.S. still faces some serious technological challenges of its own, including interference with other stations on AM, and poor audio quality likened by some to being underwater. iBiquity was previously using PAC (also used at a higher bitrate in Sirius satellite radio [see DARS ]), but in August 2003 a switch to HDC was made to rectify these problems. Prior to this, a change back to AAC or another MPEG compression algorithm had been considered; however HDC has been customized for IBOC, and it is also likely that the patent rights and royalties for every transmitter and receiver can be retained longer by creating a more proprietary system. Digital Radio Mondiale is also developing a patent-free IBOC system, likely to be used worldwide with AM shortwave radio, and possibly with broadcast AM and FM. Neither of those have been approved yet for ITU region 2 (the Americas).
In-band on-channel digital radios using iBiquity's standard are being marketed under the brand "HD Radio" to highlight the quality of reception. As of May 2005, a limited number of different receiver models have been made, and a few stations have received experimental authorization from the FCC to transmit in a multiplexed multichannel mode on FM. Use of HD Radio on AM has been stalled because it causes noise on adjacent channels, and is not yet allowed at night due to its problems with skywave radio propagation. DRM however is being used across Europe on shortwave, which is entirely AM skywave, without issue. With the proper receiver, many of these stations can be heard in North America as well.
See also
External links
- FCC info on IBOC (http://www.fcc.gov/mb/audio/digital/)