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HD Radio is a trademarked term for iBiquity's in-band on-channel (IBOC) digital radio technology used by AM and FM radio stations to transmit audio and data by using a digital signal embedded “on-frequency” immediately above and below a station's standard analog signal, providing the means to listen to the same program in either HD (digital radio with less noise) or as a standard broadcast (analog radio with standard sound quality). The HD format also provides the means for a single radio station to simultaneously broadcast one or more different programs in addition to the program being transmitted on the radio station's analog channel. It was selected by the U.S. Federal Communications Commission (FCC) in 2002 as a digital audio broadcasting method for the United States,〔 〕 and is the only digital system approved by the FCC for digital AM/FM broadcasts in the United States. It is officially known as NRSC-5, with the latest version being NRSC-5-C.〔(【引用サイトリンク】work=National Radio Systems Committee )〕 Other digital radio systems include FMeXtra, Digital Audio Broadcasting (DAB) (Eureka 147), Digital Radio Mondiale (DRM30 and DRM+ configurations), and Compatible AM-Digital (CAM-D). While HD Radio does allow for an all-digital mode, this system currently is used by some AM and FM radio stations to simulcast both digital and analog audio within the same channel (a hybridized digital-analog signal) as well as to add new FM channels and text information. Although HD Radio broadcasting's content is currently subscription-free, listeners must purchase new receivers in order to receive the digital portion of the signal. As of May 2009, there were more stations in the world on the air with HD Radio technology than any other digital radio technology. More than 1,900 stations covering approximately 84% of the United States〔(【引用サイトリンク】 title=iBiquity Digital Corporation )〕 are broadcasting with this technology, and more than 1,000 HD2 and HD3 multicast channels are on the air.〔(【引用サイトリンク】 title=iBiquity Digital Corporation )〕 According to iBiquity's website, the "HD" is simply a brand name and has no meaning. There is no connection with high-definition television (HDTV), although like HDTV the HD Radio specification provides enhanced capabilities over the old analog format, such as 5.1 surround sound. The FCC has not indicated any intent to force off analog radio broadcasts as it has with analog television broadcasts,〔 as it would not result in the recovery of any radio spectrum rights which could be sold. Thus, there is no deadline by which consumers must buy an HD Radio receiver. In addition, there are many more analog AM/FM radio receivers than there were analog televisions, and many of these are car stereos or portable units that cannot be upgraded. ==Overview== Digital information is transmitted using OFDM with an audio compression algorithm called HDC (High-Definition Coding). (HDC is a proprietary codec based upon, but incompatible with, the MPEG-4 standard HE-AAC). HD Radio equipped stations pay a one-time licensing fee for converting their primary audio channel to iBiquity's HD Radio technology, and 3% of incremental net revenues for any additional digital subchannels.〔(iBiquity Licensing Fact Sheet 2009 (PDF) ).〕 The cost of converting a radio station can run between $100,000 and $200,000.〔(【引用サイトリンク】title=Terrestrial Digital Radio )〕 Receiver manufacturers pay a royalty.〔(【引用サイトリンク】work=iBiquity Digital Corporation )〕 If the primary digital signal (HD-1) is lost the HD Radio receiver will revert to the analog signal, thereby providing seamless operation between the newer and older transmission methods. The extra HD-2 and HD-3 streams are not simulcast on analog, causing the sound to drop-out or "skip" when digital reception degrades (similar to HDTV drop-outs). Alternatively the HD Radio signal can revert to a more-robust 20 kilobit per second stream, though the sound is reduced to AM-like quality. Datacasting is also possible, with metadata providing song titles or artist information. iBiquity Digital claims that the system approaches CD quality audio and offers reduction of both interference and static;〔(【引用サイトリンク】work=HD Digital Radio )〕 however, some listeners have complained of increased interference on the analog AM band (see AM, below). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「HD Radio」の詳細全文を読む スポンサード リンク
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