In-band on-channel

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In-band on-channel (IBOC) is a method of transmitting digital radio and analog radio broadcast signals simultaneously on the same frequency.

By utilizing additional digital 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 just outside the edge of the station's FCC defined channel, interference with adjacent channel stations is increased when using digital sidebands.

FM IBOC will allow for multiple channels in addition to the main stereo channel. On AM, additional channels aren't possible due to limited bitstream bandwidth, but is capable of transmitting stereo. Eventually, stations can go from hybrid mode (both analog and digital) to all-digital, by eliminating the traditional analog audio portion.

Contents

1 FM methods
2 AM methods
3 IBOC versus DAB
4 Challenges
5 See also
6 External links

FM methods

On FM there are currently two methods of IBOC broadcasting in use, mainly in the United States.

The first is HD Radio by the iBiquity consortium, which uses sidebands beyond the normal ±100kHz FM channel. This is the most well-known and widespread system in use, though uptake is still extremely limited. HD Radio is also extremely expensive to implement, in large part because of huge licensing royalties, which must not only be paid on the equipment purchased, but for every year the equipment is used. The system also uses a separate transmitter, which either requires a diplexer into the existing antenna, or another separate feedline and antenna, as well as space, weight, and wind load on the radio tower.

The other system is FMeXtra by Digital Radio Express, which instead uses subcarriers within the existing signal. This was only recently introduced, and consumer receivers are expected in 2006. The system is completely compatible with HD Radio in hybrid mode (but not in all-digital, which is not expected for a very long time), and with RBDS. The stereo subcarrier can also be removed for greater digital bandwidth. However, it is not compatible with other existing subcarriers, which are normally not used by the general public anyhow. The system is far less expensive and less complicated to implement, needing only to be plugged-in to the existing exciter, and requiring no licensing fees. FMeXtra generally has all of the user features of HD Radio.

Digital Radio Mondiale is in the initial stages of creating an open-source system for FM, though it may be too late to make a third system viable with two others already in place.

AM methods

iBiquity also created a mediumwave HD Radio system for AM, however it has so far failed in technical trials to make it out of the experimental phase, mainly due to interference caused by using sidebands in an environment intended for narrowband amplitude modulation.

Digital Radio Mondiale has had much more success in creating an AM IBOC system, and one that will be much less expensive to implement than any proprietary HD Radio system. It has already been accepted for not only mediumwave but also shortwave (and possibly longwave) by the ITU for use in regions I and III, but not yet in region II, the Americas.

CAM-D is yet another method, though it is more of a extension of the current system. It encodes the treble on very small digital sidebands which do not cause interference to adjacent channels, and mixes it back with the analog baseband. Unlike the other two, it is not intended to be capable of multichannel.

IBOC versus DAB

While Canada and most countries in Europe have chosen the Eureka 147 standard of digital audio broadcasting (DAB) for creating a digital radio service, the United States has been holding out for advancements in IBOC technology. 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). The most significant advantage to IBOC is the relative ease of implementation. Existing analog radios are not rendered obsolete and the consumer and industry transition to digital at a rational pace.

Challenges

AM IBOC in the U.S. still faces some serious technological challenges of its own, including nighttime interference with other stations. 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 an 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).

Both AM and FM IBOC signals cause interference on adjacent stations. It has led to derogatory terms such as IBAC and IBUZ (since the interference sounds like a buzz.) The range of a station on an HD radio is much less than its analog signal. In addition, tropospheric ducting and e-skip can reduce the range of the digital signal.

In-band on-channel digital radios using iBiquity's standard are being marketed under the brand "HD Radio" to highlight the quality of reception, though this is officially said to stand for "hybrid digital" rather than "high definition". As of May 2006, a limited number of different receiver models have been made, and a several 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.

External links

--[[User:BobRadil|BobRadil]] 00:40, 12 July 2006 (UTC)