Information AboutHd Radio |
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HD Radio is a brand name of a method of digital transmission of AM and FM Radio Station s. The HD Radio system is unique in that it allows stations to broadcast high quality audio and a variety of text-based services, as well as additional FM channels, without changing to new frequency bands. "HD" originally stood for "'''Hybrid Digital/analog'''", not "high-definition" (as in HDTV ). Presently "HD" is simply part of the Trademark ed name, not officially standing for anything. The technology was developed by IBiquity Digital Corporation. The Federal Communications Commission selected HD Radio (a hybrid system in which digital signals are sent along with the analog carrier) as the standard for AM and FM broadcasting in the United States. FM stations can offer multiple channels (called "Multicast" channels) on the same frequencies allocated to analog Radio Station s. iBiquity Digital notes that the system approaches CD quality sound and offers reduced interference and static. As Of July 25, 2007 , more than 1360 stations are broadcasting with HD Radio technology and more than 600 offering Multicast channels. Like traditional AM and FM stations, HD Radio programming is free, but a new radio is required. Brand name (Sony, Cambridge Sound Works, JVC, etc.) HD Radio receivers are available for home and car at major consumer electronics chains, online and through regional stores. Other In-band On-channel Digital Radio competitors include FMeXtra , Digital Radio Mondiale (DRM+), and Compatible AM-Digital (CAM-D) developed by the inventor of AM Stereo . OVERVIEW Digital information is Transmitted using COFDM , a Modulation method that has been used in 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 Codec in 2003 (based-upon MPEG-4 AAC). HD Radio-equipped 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. While in hybrid digital/analog mode, an HD Radio will lock onto an Analog Signal first in mono, then Stereo , then try to find a digital signal. If digital signal reception is lost, the radio will revert to the analog signal. If the analog signal is non-existent, as would be the case after the analog shutdown, then the digital will revert to a very-low-speed stream (equivalent to telephone quality). Much of the success of this system capability relies on proper time Synchronization of the analog and Digital Audio signals by Broadcast Engineer s maintaining the Transmitter . Datacasting is also possible, and RDS -like Metadata about the program and station are included in the standard. The AM hybrid mode offers two options which can carry approximately 40 or 60 Kilobits Per Second of data for the main audio channel (achieving quality equal to Analog AM Stereo ), but the higher the bit rate, the more vulnerable the signal is to poor reception, so most AM-digital stations default to the more-robust 40kbit/s mode which features redundancy (same signal broadcast twice). In pure digital mode, the AM station lacks an analog signal for "fall back", and instead reverts to a low-quality 20 kbit/s signal during times of poor reception. The FM hybrid digital/analog mode offers four options which can carry approximately 100, 112, 125, or 150 kbit/s of data depending upon the Station Manager's power budget and/or desired range of signal (achieving MP3-like quality). In pure digital mode, broadcasts can be made at 270 or 300 kbit/s maximum, thus enabling extra features like surround sound or near-CD-quality audio (CD==1400+ kbit/s), but these high-speed data rates can not be used until the FCC allows the main analog signal to be dropped. Like AM, pure digital FM also includes a "fall back" condition where it reverts to a low-quality 25 kbit/s signal in the event of interference. {Link without Title} 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. The HD Radio standard is officially known as NRSC-5, with the latest version being '''NRSC-5A'''. As Of Spring 2007 , there are reported to be fewer than half a million HD radios in use in the U.S., but a large marketing campaign by terrestrial broadcasters and falling prices for HD Radio receivers may increase sales. {Link without Title} AM The AM version of HD Radio adds 10 kHz to each side of the Center Frequency , meaning that the signal extends out from the center frequency by 15 kHz. With double sidebands, this results in an entire signal that is 30 kHz (three full channels) wide. Even so, the power level of the outer band signals is 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 all signals more than 5 kHz away from the center frequency, but some " Wideband " Receivers do not filter this way, making the encoded signal audible. Even on radios that do have such a filter, it is possible to hear the digital sidebands by tuning above or below the desired frequency by 10 kHz. Proposals for AM Stereo have produced similar controversies. iBiquity's standard is incompatible with C-QUAM AM stereo broadcasts. After the analog portion is dropped, the pure digital signal will be reduced from 30 kilohertz to fit inside a standard 10 kHz channel (40kbit/s) or an extended 20 kHz channel (60kbit/s), at the discretion of the station manager. The FCC initially allowed AM stations to broadcast HD Radio only during daytime hours. On March 22, 2007 the FCC approved nighttime AM HD broadcasts. The new rules go into effect on September 14, 2007. FM FM stations have the option to subdivide their datastream into sub-channels (example: FM97-HD1, HD2, HD3) of varying audio quality (50, 25, 12, or 5 kbit/s are the values recommended by Ibiquity). 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". The multiple distinct services are similar to the sub-channels found in ATSC -compliant Digital Television and typically called Multicasting but actually more like Multiplexing . Second and third channels, such as for weather, Traffic , or a Radio Reading Service , can be added this way, though it will reduce the audio quality of all channels on a station. Stations may eventually go all-digital, meaning they could no longer be heard on analog-only radio receivers, and thus allowing as many as three primary channels of full-power and four secondary channels of low-power (seven total). As defined by iBiquity, these channels would consist of two 98kbit/s FM-quality channels, four 25kbit/s sub-AM-quality channels, and one 5 kbit/s Talk channel. Currently, FM stations in the United States and Canada are licensed to occupy approximately 200 kHz of RF Spectrum , i.e., the FM band Frequency allocations are 200 kHz apart. When a signal Modulates the carrier, an infinite number of Harmonically-related sidebands are created, thus the actual occupied bandwidth of the signal extends well past the highest modulating frequency (usually ± 100 kHz), because of the Non-linear nature of frequency modulation. In order to prevent harmful interference to other stations, the carrier frequencies of stations within individual markets are seldom authorized to be closer than four channels apart, equal to 800 kHz. Occasional exceptions exist with spacing of three channels (600 kHz). In addition, there are constraints on occupied bandwidth as such. Transmitted RF energy in the sidebands more than 120 kHz from the center frequency is required to be significantly attenuated. In the modulation '' Baseband '', FM stations have a bandwidth of about 100 kHz, only 15 kHz of which is used by analog Monophonic ) audio. Analog stereo uses 53 kHz of baseband space, and RBDS is centered at 57 kHz. The "remainder" is currently available for other services, including rental for secondary broadcast services, Paging and Datacasting , or as a Transmitter-studio Link for in-house Telemetry . While the various baseband signals all contribute in a complex manner to the total occupied bandwidth (and modulation level) of the FM signal, it is important to consider two factors. First, the modulating signal will generate a principal RF component at the fundamental sideband frequency, e.g., a 67 kHz Subcarrier will generate RF components at ± 67 kHz from the FM carrier. Second, any Filtering of the modulated signal, or bandwidth limitation in the transmission system, causes a certain amount of Distortion in the received signal, due to the formation of Intermodulation components from the various modulating signals. In regular hybrid mode, an HD Radio station has its full ± 100 kHz of RF bandwidth, and adds its digital signals into part of the upper and lower adjacent RF channels beyond that, using about 1% of the main FM power level. In extended hybrid mode, the bandwidth of the FM signal is reduced to make way for additional OFDM carriers carrying more data. Because of this, FM stations may have to discontinue existing subcarrier services (usually at 92 kHz and 67 kHz) in order to carry extended HD Radio, though such services can be restored through use of the Digital Subchannel s that are then made available. This will require new receiving equipment for subscribers, however. The current analog stereo subcarrier would, in theory, eventually be dropped to make more room for digital transmission, and eventually stations could elect to drop the analog Baseband ( Monophonic audio) completely and go all-digital. However, considering that there are billions of existing analog-only receivers, this is not expected to happen for a very long time, if ever. There are still some concerns that HD Radio on FM will increase interference between different stations, though it is thought unlikely to make a major difference since HD Radio still fits within the existing spectral mask. An HD Radio station will not generally cause interference to any analog station within its 1 mV/m Signal Strength Contour , the limit above which the FCC protects most stations. Some interference of this type is already permitted by FCC assignment rules, and the amount caused by HD Radio is expected to be accepted as well. A distinct possibility exists of interference between HD stations in neighboring markets, which may be assigned frequencies only one or two channels apart. As with AM, FM stations may use separate Exciter s to modulate the very different signals. A Combiner is often used, either before common Amplification or after separate amplification, though stations are also now allowed to use a separate Transmitting Antenna slightly higher or lower on the Radio Tower . In each case the ratio of power of the analog signal to the digital signal is standardized at 100:1. |
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