Information AboutHd Radio |
| CATEGORIES ABOUT HD RADIO | |
| broadcast engineering | |
| digital radio | |
| SHOPPER'S DELIGHT | |
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OVERVIEW Digital information is Transmit ted using COFDM , a Modulation method that has been used in a number of different Digital Television and radio systems, including DVB-T . 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 Patent s that can be exploited for longer periods. HD Radio(TM) stations must pay Royalties each year to iBiquity, plus the costs paid by the Manufacturer s of the transmitters which are then passed along to the stations that buy them. In Hybrid mode, the AM version can carry 36 kilobits per second of data for the main audio channel, while FM stations can carry information at 96 kbit/s. HD Radio(TM) can also be used to carry multiple distinct audio services, called Multicasting but actually more like Multiplexing . Secondary channels, such as for Weather , Traffic , or a Radio Reading Service , can be added this way, though it may reduce the audio quality of all channels on a station. Datacasting is also possible, and RDS -like Metadata about the program and station are included in the standard. Stations may eventually go all-digital, meaning they could no longer be heard on a regular radio. Also notable is that in hybrid mode, a radio will lock onto an Analog Signal first, then try to find a solid Digital one. If the digital signal is lost, it will blend to analog, the same way a car radio will blend from stereo to mono given a weak signal. Much of the success of this relies on proper Synchronization of the analog and Digital Audio signals by the broadcaster at the Transmitter . This Fallback may also be impeded by the use of multiple channels. While iBiquity is responsible for the development of these standards, and the FCC for its Regulation , the National Radio Systems Committee (NRSC) is the Standards Body for HD Radio(TM). When completed, it will be known as NRSC-5. AM AM stations in ITC Region 2, covering the Americas, are usually considered to have 5 kHz of audio Bandwidth . With double Sideband s that are standard for most radio broadcasts, this results in a channel 10 kHz wide. In the other ITC regions of the world, the sidebands are 4.5 kHz, with a 9-kHz channel spacing. However, the AM version of HD Radio(TM) 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 no Adjacent Channel s. For this reason, some consider HD Radio(TM) on AM to be an In-band Adjacent-channel (IBAC) system. Still, power level of the outer signals is quite low compared to the main signal, and the COFDM subcarriers fit within a standard AM Spectral Mask . Most analog AM radios have Electronic Filter s to remove anything more than 5 kHz away from the center frequency, but some " Wideband " 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 C-QUAM AM stereo broadcasts. To reduce Night time Skywave interference problems with other stations, HD Radio(TM) can only be used during daytime broadcast periods on AM at present.-- FM On FM, HD Radio(TM) works on subcarriers rather than sidebands. Most FM stations have a Bandwidth of about 100 kHz or 0.1 MHz, only 15 kHz of which is used by the Baseband (analog Monophonic ) audio. Analog stereo uses up to 53 kHz, with RBDS at 57 kHz. The remainder is available for other services, including Rental for Paging and Datacasting , or as a Transmitter-studio Link for in-house Telemetry . As an IBOC system, HD Radio(TM) uses this extra space for digital broadcasting. On the wider bandwidth of FM stations, it can carry multiple Stream s of FM and/or AM quality. National Public Radio in particular hopes to be able to carry several different streams through the Transmitter s of member stations, calling its proposed addition to the FM standard "Tomorrow Radio". Some have also proposed using the system to carry Surround Sound broadcasts with 5.1 channel audio, though this or other multichannel setups reportedly may prevent the fade-to-analog fallback on "hybrid" analog+digital broadcasts. Also, the FCC is still only authorizing multichannel use Experimental ly to individual stations who ask permission. FM stations may have to drop pre-existing subcarrier services in order to carry HD Radio(TM), though such services can often be restored through the digital subchannels that are then made available. The analog stereo subcarrier will eventually be dropped to make more room for digital (called "extended-hybrid mode"), and eventually stations can elect to drop the analog Baseband ( Monophonic audio) completely and go all-digital. There have been some concerns that HD Radio(TM) on FM will increase interference between different stations, though it is unlikely to make much of a difference as it still fits within the existing spectral mask. Many stations have carried other sideband programs for decades with few ill effects. RECEIVERS As Of May 2005 , receivers are still quite expensive, starting at around US$ 300. Manufacturers have initially focused on making Car Stereo s, BMW being the first with their announcement of HD Radio(TM) being an option for their 2006 7-series, 6-series, and 5-series models. Home listening equipment is currently available from several companies, in both a home tuner, and a table top model. CRITICISMS The HD Radio system has not been immune to criticisms. FM DX'ers who live in markets with several equipped stations have found the system highly detrimental to their hobby due to the strong digital hash generated on adjacent channels. This might also extend to those who live near market boundaries and wish to hear a station on a channel next to one used by a local station. Many find the claims of higher audio quality to be exaggerated, or even untrue when multicasting is used, as individual channel rates are reduced. Some report hearing a decrease in audio quality on the analog signal of stations that have taken up the system. Another common criticism is that the use of the digital transmission format on the Mediumwave (AM) band to be unreasonable, due to the high levels of interference and noise on the band, caused by electrical sources and distant Skywave -propagated stations, which might simply generate an audible buzz or hum on the analog channel but severely limit the range of the "HD" channel, due to the nature of digital audio. In addition, those that tune into distant programs or DX the band might find their desired station or target frequency unusable due to hash, which may carry for very long distances (but not necessarily produce listenable audio over them, as is possible with analog). There are also various marketing-related challenges {Link without Title} that HD must overcome. EXTERNAL LINKS
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