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IBOC TECHNOLOGY FOR DIGITAL RADIO Digital is the future of radio. Let’s see how the IBOC system for digital radio delivers CD-quality audio and data services on the existing AM and FM bands
iBiquity Digital’s HD Radio D. PRABAKARAN
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igital radio, also called digital audio broadcasting (DAB), is transmission and reception of radio signals in the digital domain, as opposed to the traditional analogue transmission/reception by AM and FM systems. While digital television signals are now routinely received over cable and satellite systems, reception of digital radio broadcasts is still fairly uncommon. Digital radio is similar to hooking up the digital output from a CD player directly to a radio transmitter. At the other end is a digital-to-analogue converter (DAC), which converts the digital signal back into analogue mode so that it can be heard on the audio system as it was recorded. While this is not exactly what happens, the result is essentially the same. In practice, the CD player is hooked up to a control board, which, in turn, routes the signal as a part of the feed to the radio station’s transmitter. The signal is either impressed onto a signal carrier and transmitted via uplink to a satellite (which, through a transponder onboard, retransELEC EL ECTR TRON ONIC ICS S FOR FOR YO YOU U
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mits the signal earthward) or transmitted across the land (terrestrial). In both the cases, the signal is received and converted into analogue by a specialised DAC within the radio’s tuner circuit. Currently, digital radio broadcasts are available in select countries, including the UK, Germany and Canada. These are the most prevalent in Europe, particularly the UK, where British Broadcasting Corporation (BBC) simulcasts most of its programming in digital. The requirement of a new hardware (digital radio tuner) is impeding the adoption of digital radio. Digital radio works like satellite TV, except that it is normally broadcast from transmission towers (not the satellite) just like standard radio. Sirius Satellite Radio and XM Satellite Radio Holdings are planning to offer satellite-based subscription radio services on the lines of digital TV.
Why digital radio? Radio gets a new lease of life with the move to digital. The main advantage of digital radio is that it doesn’t have the usual distortion associated with analogue radio such as hissing, popping and phasing. It is immune to distortion from multipath, adjacent stations, overly weak or overly strong signals, etc. Also, digital radio signals can carry text information. Users get a new array of data-rich services including traffic information, sports score and weather updates, stock prices, etc. This data is displayed on the liquid
crystal display (LCD) in the form of text, images and/or video. Thus multimedia radio becomes a reality. Audio features such as time-shift recording/digital recording of playlists using transmitted electronic programme guides (EPGs) can be implemented. The digital radio will converge with different types of compelling end products including MP3 players, cell phones and personal digital assistants (PDAs). This economical, futuristic digital communication pipe provides new advertising and electronic commerce opportunities to radio broadcaster broadcasters. s. Digital signals are not subject to interference. Therefore the signals arriving at the receiver would be free of the usual noise associated with typical analogue, AM- or FM-generated signals. These can coexist with other digital and analogue signals within the same space. To explain, in analogue radio, a large number of radio signals can exist in the atmosphere at any time. However, each of them is being transmitted on a different radio frequency. Stations that share the same radio frequency are usually far apart so that they don't interfere with each other. In order to receive a station within its transmission range, all we have to do is tune the receiver to its frequency. The drawback to this analogue mode is that the signals in close proximity in terms of frequency can interfere with each other to some extent, depending on the modulation pattern of the radio station and the ability of the radio receiver to reject interfering adjacent signals. Therefore two signals cannot be transmitted on the same frequency from two sources that are within a certain distance of each other. These will
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usually interfere at a radio receiver that is within the reception range of both the stations. A small screen attached to the digital receivers displays text and pictures. Text, data and images can be diffused at the scale of a country or a region. The protocols used in the compression and broadcasting techniques are standardised, facilitating connection with existing networks such as the Internet. Multiplexing makes it pos- Visteon's MACH DSP combined with iBiquity sible to broadcast several radio technology brings HD Radio to reality programmes and data for a given area on one frequency, whatever be the (IBOC) that is compatible with the cursize of the area. So a vehicle driver can rent analogue radio formats. The IBOC syslisten to his favourite radio programme tem will be compatible with existing tunwithout having to change the frequency ers as it utilises the existing AM and FM and experiencing distortion. bands by attaching a digital side-band signal to the standard analogue signal. So if a station is currently located at 99.9 on Eureka 147: the digital radio the FM band, it will remain at 99.9 FM system in use whether in analogue or IBOC digital radio. For digital compression, the IBOC uses The Eureka 147 digital radio system is a perceptual audio coder (PAC) developed currently being used around the world. by Lucent Technology. This system broadcasts at much higher However, utilising the existing AM and frequencies than standard radio transmisand FM bands has disadvantages as well: sions over the 88-108MHz FM band and Due to the bandwidth limitations, digital 0.525-1.705MHz AM band. The actual freAM broadcasts will have FM-quality quencies used for digital radio depend on sound, while digital FM broadcasts will the availability for that particular country. have near-CD-quality sound. Also, AM and For example, the UK utilises Band III (rangFM frequencies are not suitable for sateling from 174 to 240 MHz), while Canada lite transmission if extraterrestrial transuses part of the L-Band (1452-1492 MHz mission is desired in the future. of 300-1500 MHz). In comparison, satellite television utilises 4-20GHz frequencies. The Eureka system broadcasts IBOC technologies from multiple stations and services over a single USADR and iBiquity Digital frequency in something called a multiplex. The bandwidth within the multiplex is alThe IBOC technology allows digital audio lotted to stations as needed. For instance, broadcasting without the need for new a high-quality stereo station is given more spectrum allocations for the digital signal. bandwidth than a news or talk radio staIt provides for enhanced sound fidelity, tion which may be broadcast in mono. improved reception and new wireless data Stereo programming is typically broadcast services. This enhancement to AM and at 192 kilobits. Same as in the case of FM will offer tremendous growth opporMP3, digital compression is used to combine many services onto one signal for Benefits of Digital Audio broadcast over a single frequency. Broadcasting With a bandwidth of 192 kbits for ste1. High-quality digital audio reo broadcast, the sound quality of digital 2. Rugged, reliable delivery to fixed, porradio, which utilises Musicam digital comtable and mobile receivers for interferpression, is quite good but not CD-quality. ence-free reception
IBOC: a new system The US has decided not to adopt the Eureka system and is developing a digital radio system called in-band on-channel
3. Efficient use of the limited radio frequency spectrum available 4. Easy-to-use receivers 5. Flexibility and choice of programmes for listeners 6. Added-value system features such as RDS and multimedia
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In-band systems for DAB The two main in-band systems under development are in-band on-channel (IBOC) and in-band adjacent channel (IBAC). The IBOC system transmits a digital signal simultaneously within the spectrum of an existing analogue AM or FM signal. The IBAC system transmits a digital signal within the unused spectrum adjacent to that used by existing FM signals. Another system called the in-band reserve channel (IBRC) uses the ‘spare’ spectrum that is not suitable for other services. Because in-band systems operate within or close to existing frequency allocations, these systems (particularly IBOC) should allow for a smooth transition from existing services. For example, it should not be necessary to allocate a new spectrum for digital services. This is one fundamental difference between in-band and Eureka 147 systems.
tunities for broadcasters, manufacturers, retailers and automotive manufacturers. USA Digital Radio (USADR) is engaged in the design of AM and FM IBOC DAB systems. The FM hybrid mode developed by it allows the station to simultaneously broadcast the same programming in analogue and digital. Although the level of the digital signal in the hybrid mode must be limited to accommodate the analogue broadcast, the hybrid system still has an edge over the existing analogue service due to its enhanced audio fidelity, improved signal robustness and expanded auxiliary services. The USADR AM IBOC DAB system basically comprises the codec, forward errorcorrection (FEC) coding and interleaving section, modem and blender. The codec encodes and decodes the audio signal. FEC coding and interleaving provide robustness through redundancy and diversity. The modem modulates and demodulates the signal. Blending provides a smooth transition from the digital to analogue signal. iBiquity Digital Corp. has developed a fully digital IBOC system that permits a smooth evolution from the current analogue FM. The system delivers digital audio and data services to mobile, portable and fixed receivers from terrestrial transmitters in the existing VHF radio band. The broadcasters may continue to transmit analogue FM simultaneously with the new, higher-quality and more robust digital signals. This approach allows broadcasters to shift from analogue to digital radio while maintaining their current frequency allocations. iBiquity Digital’s IBOC technology will allow radios to be backward- and forwardcompatible, so they can receive traditional analogue broadcasts from stations that are ELECTRONICS FOR YOU
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yet to convert and digital broadcasts from stations that have converted. The existing analogue radios will continue to receive the analogue portion of the broadcast. Specific features of IBOC improve the existing analogue reception during the hybrid mode.
Benefits to broadcasters Although each station will require a new digital broadcast exciter, some stations could be upgraded to the IBOC technology without replacing or with minimal modification of the existing transmitters. Depending on the existing infrastructure of the station, the broadcaster will need to spend between $30,000 and $200,000 to upgrade the station to digital. The broadcasters will be able to preserve the existing listener base of stations and retain all brand equity associated with their dial positions, typically developed at great expense over long periods of time.
ence to the ‘host’ analogue station and stations on adjacent channels. The system employs multiple digital signalling techniques, such as redundant sidebands, blend, first adjacent cancellation and code and power combining. To compress the audio data and increase transmission without losing sound quality, it uses the PAC audio-compression technology. An integral part of iBiquity Digital’s AM and FM digital broadcast technology, the PAC audio-compression technology utilises advanced signal processing and psycho-acoustic modelling to interpret human hearing and eliminate redundancies and irrelevancies in the audio signal. iBiquity Digital has designed the PAC technology for Internet broadcasting as well. Companies will use the PAC technology to encode and decode music, talk and information programming that will be delivered over the Internet. iBiquity Digital’s PAC technology will also be used in satellite digital radio. Audio quality at a given compression rate is a crucial factor for satellite digital radio. With PAC technology, a high audio quality can be obtained at various bitrates.
FM hybrid IBOC waveform Fig. 1 shows the FM hybrid IBOC spectrum. Low-level digital sidebands are added to each side of the analogue signal. The bandwidth is limited to 200 kHz from the centre frequency. Restricting the digital sub-carriers to the 70kHz Fig. 1: FM hybrid IBOC power spectral density region between 129 and 199 kHz from the centre frequency on either side of The wireless data capacity associated the analogue spectrum minimises interferwith an all-digital broadcast will allow ence to the analogue host and adjacent broadcasters to offer enhanced services such channels without exceeding the existing as surround sound or other multicasting FCC spectral mask. This bandwidth is wide solutions, opening new sources of revenue enough to support a robust, hybrid IBOC for them. service with virtual CD-quality audio that mirrors the coverage of existing analogue radio stations. Basics of IBOC The dual-sideband structure enables the use of frequency diversity to further The IBOC technology is used to transmit combat the effects of multipath fading and CD-quality audio signals to radio receivinterference. The baseline hybrid system ers along with such data as station, song and artist identification, stock and news simultaneously transmits 96 kbps of error-protected digital audio information, information and local traffic and weather plus auxiliary services, on each DAB sideupdates. The broadcasters can use the exband. Each sideband has all the informaisting radio spectrum to transmit AM and tion and thus can stand alone. However, FM analogue simultaneously with higherwhen neither sideband is corrupted, adquality digital signals. iBiquity Digital’s IBOC system brings vanced FEC coding techniques allow the combination of both sidebands to provide the benefits of digital audio broadcasting to today’s radio while preventing interfer- additional signal power and coding gain.
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Because adjacent channel interference occurs on over half of all radio stations, the IBOC technology employs redundant information placed in the upper and lower sidebands to ensure that the system performs in all radio environments. It further combats interference through first adjacent canceller (FAC) technology. FAC cancels the effects of a strong analogue adjacent channel station that can easily wipe out a digital sideband. This patent-pending technology greatly increases the chances of a sideband survival and, when combined with redundant sideband implementation, provides robust digital reception under all interference conditions. The IBOC technology overcomes multipath and noise through the use of
Field test platform assembled in a test van by iBiquity Digital Corporation
proprietary coding and power combining techniques. It uses proprietary error-correction techniques that apply complementary coding to the upper and lower sidebands. The use of these complementary codes allows for a more powerful errorcorrection algorithm by code-combining the sidebands, ensuring near dropout-free digital reception. The sidebands are also power-combined in the receiver to deliver an additional power gain. Station acquisition or tuning time is a critical system requirement. Listeners are accustomed to tuning in a station without the delays associated with digital acquisition. In digital transmission systems, signal robustness is traded off versus tuning time. Interleavers, which are portions of control code integral to all digital audio broadcast ELECTRONICS FOR YOU
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systems, drive this trade-off. Short interleavers lead to rapid tuning, but result in higher rates of audio dropouts. Long interleavers provide more robust performance, but result in longer tuning times. The IBOC technology meets both the objectives by incorporating a long interleaver to ensure the required signal robustness while achieving instant tuning by using the available analogue signal and seamlessly blending to digital after the digital signal is acquired. At the edge of a station’s coverage area, blending to analogue prevents abrupt digital dropouts (the dreaded digital ‘cliff effect’) by using the host analogue audio as a backup channel. The quadrature phase shift keying (QPSK) modulation technique ensures robust performance while providing sufficient throughput for virtual CD-quality digital audio. It permits the use of advanced FEC coding techniques that exploit knowledge of the non-uniform interference environment. QPSK is also simpler and more robust than higher-order forms of modulation, especially in a multipath environment. Since QPSK has a bandwidth efficiency of two bits per second per hertz, it supports an information bitrate that is sufficient for transmission of virtual CD-quality audio in the bandwidth available. Orthogonal frequency-division multiplexing (OFDM) allows frequency-division multiplexing of many QPSK-modulated sub-carriers in an orthogonal fashion such that each sub-carrier does not interfere with its adjacent sub-carriers. This offers a high level of robustness in a multipath channel. When combined with FEC coding and interleaving, the digital signal’s robustness is enhanced further. The OFDM structure supports FEC coding techniques that maximise performance in the non-uniform interference environment. The most important coded bits can modulate OFDM carriers that are located in the most protected regions of the channel.
Source coding CD digital audio has a data rate of 1.4112 Mbps (44,100 16-bit samples per second, for left and right channels). The FM channel bandwidth does not have the capacity to support a sufficiently high data rate to provide uncompressed CD-quality audio. Therefore an audio codec (coder-decoder) compression technique must be employed. The audio codec is a source-encoding device that removes redundant information
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wide range of applications from Internet audio to multichannel surround sound.
FEC coding and interleaving Forward error correction and interleaving greatly improve the reliability of the transmitted Fig. 2: Multipath interference from multiple sources information. Advanced FEC coding techniques exploit the non-uniform nature of the interference. Special interleaving techniques spread burst errors over time and frequency to assist the FEC decoder in its decision-making process. The combination of advanced FEC coding and interleaving techniques, together with superior modem perforFig. 3: Block diagram of the IBOC DAB transmitter mance, allows the IBOC system to defrom a digital audio signal in order to reliver CD-quality audio with coverage comduce the bitrate and hence the bandwidth parable to the existing analogue service in required to transmit the signal. The codec a mobile environment. must perform this information rate comThe functional block diagram of an pression while preventing the generation FM hybrid IBOC transmitter is shown in of perceptible artifacts. Fig. 3. The sampled stereo audio source The IBOC system uses the AAC codec. feeds both the analogue and digital signal The AAC codec compresses the CD generation paths. A diversity delay is inbitstream to 96 kbps, delivering audio troduced in the analogue path for blendthat the listener will perceive to be CDing pupose. In the power combiner, the quality. Use of the AAC codec meets the analogue audio is processed within the raw throughput requirements of the moduexciter just as an existing analogue FM lation and FEC coding techniques. Also, signal would be, prior to amplification by special error concealment techniques emthe high-power amplifier. ployed by the codec ensure graceful degThe DAB path first encodes the audio radation of the received digital signal for signal in the audio encoder. The audio operation in an impaired channel. encoder removes redundant information The AAC offers the advantage of befrom the audio signal to reduce the bitrate ing an open system based on the MPEG and hence the bandwidth required to transfamily of ISO standards. It is a very flexmit the signal. To ensure that the commuible coding scheme that supports data rates nication of information through the fadabove 8 kbps. It can encode mono and ing channel is robust, the compressed stereo input data, as well as multichannel bitstream is then passed through the FEC data (up to 48 channels). It is used for a coding and interleaving section. The reELECTRONICS FOR YOU
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sulting bitstream is packaged into a modem frame and QPSK- and OFDM-modulated to produce the DAB base-band signal. The base-band signal is up-converted and amplified before being power combined with the analogue signal.
Service flexibility Typically, the IBOC DAB system provides two types of auxiliary services: ancillary services and opportunistic data. Ancillary services. Data can be transmitted at up to 120 kbps by reducing the audio quality and FEC parity accordingly. The audio rate could vary over time, based on the programming or the desired audio quality. For example, speech may be encoded at a lower rate than music; news and talk formats require less throughput that could be dedicated to data. If data throughput is not required, the extra capacity could be allocated to FEC parity to increase the robustness of the digital audio. Opportunistic data. Up to 32 kbps can be intermittently multiplexed with the audio on a priority basis or when spare bandwidth is available, as determined by an entropy analysis from the audio encoder. For example, the opportunistic data rate might be increased during a less complex passage of music or during a news report following a musical programme. Programme-associated data, which is used to display audio and station information on the receiver, takes a small fraction of the capacity since it does not need to be transmitted in real time.
Comparison with other digital radio systems Digital satellite radio. The digital satellite radio (DSR) transmission system provides CD-quality audio without audio data reduction and compression. DSR is broadcast via satellite and is thus receivable only on fixed basis, i.e. with dish antenna or with cable connection. DSR reception is limited to the line of sight and is not possible in moving vehicles. ‘C’ and ‘K’ band frequencies are used for DSR transmission. DSR systems include WorldSpace Sat Radio, Sirius and xl. Astra digital radio. The Astra digital radio (ADR) transmission system provides CD-quality audio with some audio data reduction, just like Musicam in DAB. It offers limited services and has less flexibility with regard to future multimedia appli-
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cations. ADR is broadcast via Astra series satellites such as Astra 1A and Astra 1B, covering most parts of Europe. It cannot be received in car and by portable receivers. Radio data system. The radio data system (RDS) is an add-on to the FM radio. It has simplified operation and improved reception quality, but the disturbances due to multipath propagation in in-car FM reception are still present. DAB offers all the functions of RDS but in a better and more extended form. The transmission rate of RDS is very low at 730 bits/second. Digital television. The digital video broadcasting (DVB) system was developed mainly for use in television, although it can also transmit audio and data. There are a variety of standards for satellite, cable and terrestrial broadcasting. Terrestrial DVB, like DAB, uses the OFDM system at a bandwidth of 8 MHz. Its mobile reception is possible at the cost of reception quality or service area. Digital AM—DRM. This system transmits below the 30MHz shortwave band (AM). It uses the COFDM transmission technique, which means that the digital signal is shared across a large number of closely spaced carriers that are contained
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Visteon’s satellite digital radio system
within the allotted transmission channel. The DRM system works well over long paths as long as the signal-to-noise ratio is high enough.
Why delay in adoption? The IBOC technology is not free from problems: Low-power FM stations are prone to interference. Receivers used for radio-reading services for the blind may also be impacted. IBOC licensing and service rules have not been adopted yet. Because iBiquity is the only IBOC vendor, stations willing to use the IBOC technology will have no choice but to make arrangements with iBiquity.
However, the transition to digital radio will not be subject to the paralysis that has characterised the transition to digital TV. Fraught with delay and a lukewarm response from the public unwilling to buy costly new sets, digital TV has faced an uphill battle. Where consumers are currently asked to pay thousands of dollars for digital television receivers, radio equipment capable of receiving digital IBOC transmissions will cost about $100 more than the equivalent analogue equipment now on the market. But even though broadcasts on the Eureka system are available in many parts of the world, penetration of digital radio tuners into homes has been minimal. It remains to be seen how the IBOC system fares in the US or whether the two digital radio formats will continue to coexist. Either way, even if digital radio becomes a success, it will probably take five to ten years—possibly even longer—before conventional analogue radio broadcasts are ❑ suspended, if ever! The author is a lecturer at N.L. Polytechnic College, Mettupalayam, Tamil Nadu
