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Digital radio has made the incremental step of improving quality by increasing audio bandwidth and adding ancillary services, but this is primarily still focused on delivering two-channel content. To make a significant step, the next generation of digital radio services needs to offer the surround sound experience. Several surround sound radio pilot projects have been completed and the war of standards is raging for a suitable transmission protocol. ?killer app? for digital radio Stereo digital broadcasting was introduced to Europe a few years ago. Listener response has remained lukewarm. The lesson to be learned is that mere "improved digital sound" is not enough to cause listeners to buy new and more expensive radios. It's the same with the FM mono vs. stereo story: not a lot of FM radios were sold in the USA before stereo broadcasts, even though FM offered much improved fidelity compared to AM. History often repeats with regard to consumer adoption of new technologies. The technology to transmit 5.1 surround over Digital Radio has just recently been perfected and is now being introduced. Just a few years ago, it seemed we didn't have enough bandwidth even for quality stereo in digital radio. But multichannel audio coding technology has advanced quite amazingly, and surround is a real here-and-now possibility for radio broadcasting. Multi-channel surround is becoming an increasingly hot consumer desire, mostly driven by the DVD Video format, which delivers the experience to consumers at home. Almost all DVD releases have a 5.1 surround option and many high-end television receivers, home theatre and audio systems are being installed with multi-loudspeaker surround capability. Surround is coming to automobiles as well, with a number of manufacturers already having announced 5.1 car audio systems. DVD Audio and SACD Multi-channel disks offer high-quality surround music to consumers. The term "surround sound" is not strictly defined. There are several systems available that provide multiple audio channels of information for playback over a system with speakers distributed around a listening sound field. All the surround systems use a low-frequency channel to provide the deep bass of the listening experience. Because of the omnidirectional properties of low frequencies, this deep bass channel feeds a single subwoofer that is usually placed near the front of the sound field. From there, five, six or seven speakers can be placed around the listener to provide the surround soundfield. The generic term surround sound is often used to describe anything with more than two reproduction channels. Most recent progress in this area has taken place in the cinematic and television sound industries, but a growing body of work in pure-audio applications now exists, primarily in the form of commercial music releases, concert recordings and radio drama productions. With a few exceptions, this content is produced today in a five-channel form, with a configuration intended for reproduction with speakers? placement relative to the primary listening position as left-front (LF), center-front (CF), right-front (RF), left-back (LB) and rightback (RB). Despite this placement description, however, it is conventional to refer to the two back channels in this configuration as left surround (LS) and right surround (RS), while the center-front channel is typically called simply center (C). A narrowband sixth channel is sometimes added for low frequency effects (LFE), with its content fed to a subwoofer that can be placed anywhere in the listening area. Given that the LFE channel?s bandwidth is about 1/10th that of the other five full-range channels, this format has come to be known as 5.1 channel surround. It is the standard format for most multichannel music content produced today. Different Forms of multichannel audio Three general terms to describe the different forms in which multichannel audio may appear in Digital Radio ? Discrete multichannel audio ? the production format of multichannel content, in which all five (or more) channels are kept wholly separate from one another, and all signals are fed to and from audio devices via isolated audio paths. ? Composite surround format ? a system that converges a discrete multichannel audio signal into a smaller number of channels (no less than two), via an encode/decode approach that allows downstream re-extraction of a multichannel signal. This approach allows the storage and transport of surround audio on existing stereo architectures, either analog or digital. ? Component surround format ? a digital-only approach wherein audio data and multichannel spatial bit stream data are maintained as separate signals. Downstream processing can apply the spatial data to the audio data to re-extract a multichannel signal. This approach allows the audio data to be managed independently from its spatial reproduction format. Early surround Surround sound is a term that has been thrown around loosely since the appearance of the early multi-channel "Cinerama Sound" format of the 1950s. In current technology, surround sound typically refers to an audio system with a left, center and right speaker in front of the listener and one or more rear speakers behind the listener. This setup often is accompanied by a separate subwoofer to handle the thumpy, low-frequency energy. You might start to see how this speaker configuration would lend itself to car audio, with speakers in the front, speakers in the rear and a subwoofer. In an ideal world, the listener's surround audio source would be able to address each channel of sound discretely, putting separate audio information into each speaker. Until recently, most audio sources were stereo, providing only two channels of discrete sound information to the listener. Assuming the listener was in a "perfect" listening environment, "perfectly" centered in front of two matched speakers, sound images could be placed anywhere around that listener by playing with phase relationships between the left and right audio channels. To make the audio appear in front of the listener, the recording engineer would place the audio with equal volume, in phase in both speakers. To make the audio appear behind the listener, the audio is placed audio 180 degrees out of phase in one speaker as compared to the other speaker. An impressive effect, but listeners rarely found themselves in a perfect listening position, so the multi-directional sound was rarely heard. To overcome this, matrix audio decoders were developed. Matrix audio decoders watch for these phase and intensity differences between stereo inputs and decode them into multiple outputs. Probably the most recognized example of the matrix decoder was Dolby's surround sound decoder appearing in the early 1980s. Suddenly stereo TV and videotapes had a "center" and "rear" audio channel. The major downfall of the matrix audio decoder was that audio appeared in unpredictable areas. For example, imagine a piece of audio with a race car driving in circles around the listener. This probably travels quite nicely around the four speakers, until you add an announcer to the mix. When the announcer starts speaking loudly in the center channel, the listener would also hear the race car quickly bleed to the center channel, because the dominant intensity and phase was the announcer's voice. This problem compounded itself with more-complex sources such as music; the listener would hear various sounds out of all speakers, but it would not necessarily be an accurate representation of the original source. Another drawback, according to listeners of these decoders, was noise in the surround channels. Often interference from poor reception, misaligned audio, etc. would appear as out-of-phase noise resulting in a loud presence in the surround channels. This problem has only become worse with the invention of compressed audio codecs. Surround sound approaches There are two general approaches to providing surround sound audio. The multiple channels of audio can be carried through a system as discrete elements. A 5.1 audio system would need six discrete audio paths. The other approach is to use an encoded signal to carry the surround information. One example of this is matrix encoding, which takes into account the amplitude and phase differences of the various audio channels. The matrix approach reduces the number of discrete paths needed to transmit a signal and has been tried several times in the past with mixed results. Surround sound in HD Radio Moving ahead with surround sound technology for HD digital radio, the four companies that are developing systems for IBOC HD Digital radio are SRS Labs, Neural Audio, Fraunhofer and Coding Technologies. All four systems are different, and accomplish the surround effect in different ways. Neural Audio surround sound Neural Surround, a breakthrough in audio technology, was developed by Neural Audio in cooperation with leading electronics manufacturers to bring the excitement of surround music to a new listening audience. Neural Audio has partnered with Harris to produce products based on Neural's technology. The product that applies to surround sound for IBOC is the Harris Neustar 5225 processor. This processor accepts a 5.1 audio stream and creates a stereo signal with a watermark that is used to reconstruct the surround signal when it is decoded. Neural uses a watermark technology and its 2-D stereo downmix of the original 5.1 material allow seamless and artifact-free cross fades from mono/stereo to surround, surround to mono/stereo and surround to surround. Neural?s spatial compression and rendering methodology allows the distributor or broadcaster the ability to capture original source 5.1 content and ?downmix? it to a 2.0 channel format while allowing the consumer to render the content in any spatial format they choose; mono, stereo, 3 channel stereo, quad, 5.1 or whatever. In the absence of spatial rendering the content is received and perceived as ?stereo? on existing receivers. 5.1 content is downmixed into two fine structures. The 5.1 image envelope is transformed to a two dimensional version of the original 5.1 image envelope. The azimuth of the original image envelope is represented by inter-channel intensity differences. The front-to-back, or ?depth?, of the original image envelope is represented by average inter-channel coherence (normalized crosscorrelation). The 2-D image envelope of the 5.1 content is imbedded in the two downmixed audio channels in the form of watermarking. Intensity/coherence watermarking is an excellent choice because of its similarity to the image construct of naturally occurring 2-D stereo and compatibility with already prevalent Lt/Rt content. Upon decoding, the image envelope of the original 5.1 content is re-synthesized based on the intensity/coherence information contained in the watermark of the two fine structures. Using this methodology, an impression of the original source 5.1 content is rendered from the two downmixed audio channels with a high degree of merit. The 5.1 rendering is accomplished by a programmable, transform based spatial rendering system. SEE (Spatial Environment Engine) can render any two dimensional audio source (both 5.1 and stereo are 2-D) into as few as 2 to as many as 256 outputs with a high degree of perceived separation. The spatial elements of 2.0 stereo (or Lt/Rt) are segregated based on the 2-D image envelope naturally residing in the content, nothing is either created or destroyed. ?Re-downmixing? of the 5.1 rendering of stereo back to 2.0 stereo (which happens quite often in the HDTV industry) results in ?near perfect reconstruction? of the original stereo content with the stereo image completely intact. Neural surround technology has been successfully employed on KUVO-FM's live broadcast of three-time Grammy winner Dianne Reeves in September of 2004, HD Radio's first 5.1 live event. In addition, Neural Audio was the first to broadcast 5.1 surround sound using the HD Radio system. Neural Surround capable receivers launched in the US market in spring of ?06. With the support of leading broadcasters such as XM Satellite Radio and others ? US consumers now can enjoy surround music via analog and digital radio broadcast. Fraunhofer surround sound This multi-channel system invented by Fraunhofer Institute (FhG) and Agere Systems. The Fraunhofer technology differs from the other methods of encoding/decoding surround sound in the way it encodes surround information. Still taking shape, the Fraunhofer/Telos system works with HDC codec of HD radio. This method divides the stream of 96 kbps by devoting 80 kbps of data to the existing stereo signal and 16 kbps to an ancillary data channel. FhG has been busy pushing the frontiers of audio perceptual research. The latest result is a powerful spatial audio coding system, taking advantage of the most up-to-date knowledge in aural perception. It is accomplished using a technique called coded-discrete which prepares the audio for transmission over HD Radio system. Psychoacoustics studies prove the level difference, time difference, and coherence between channels creates the perception of spatial image. The key to FhG?s multi-channel system is representing these difference values with very compact coding, rather than transmitting all of the individual audio channels. The encoder estimates the values as a function of frequency (that is, within each sub-band) and transmits them to the decoder in an ancillary stream accompanying the main coded audio stream. The Figures given illustrate how an encoder/ decoder pair would work within a broadcast channel in HD Radio. The first step is to create the compatible stereo downmix from the multi-channel material. The resulting stereo signal is coded using any perceptual codec. Since there are no changes to the basic codec, this signal can be received by stereo radios. The spatial encoder extracts the various spatial cue parameters from the multi-channel input, which are transmitted in an ancillary data channel. The decoder, if present in the receiver, recreates the original multi-channel audio. The FhG system allows a producer to make a manual downmix, thus preserving maximum artistic freedom and allowing maximum flexibility to adapt to different kinds of audio material. Since almost all music released in surround format also has a stereo version on the same disk that could be used as input to the encoder, this stereo version is what would be heard by listeners with non-surround radios ? with no modification or compromise of any kind. Advanced automated downmixing is also an option when manual mixes are not available. A processor could dynamically modify the scaling values and relative phase during mixdown. Such a processor would use advanced algorithms that can take into consideration absolute source positioning, panning laws, the way sources were mixed into the multichannel signals, and original inter-channel phase relationships, so it would have the potential to achieve a quality comparable to manual downmixes. The FhG spatial encoding system is fully compatible with HD Radio?s current codec for the stereo channels. (The side-channel for spatial information is less than 20 kbps, a rate possible in HD Radio?s ancillary data channel.) Because FhG's spatial encoding uses an independent digital side-channel and a modern perceptual approach to spatial cue encoding, it offers high separation that does not depend on the nature of the audio or that needs to be compromised for stereo compatibility. Still in its infancy, the Fraunhofer method does not have production models of its encoders or decoders in the marketplace. Fraunhofer says it has products in development, including a 5.1 version of the Omnia processor, that will be available soon. Because this method would require a broadcast facility to store, route, mix and broadcast discrete 5.1 audio or carry the ancillary data that is time-aligned beside its stereo audio paths, it would likely require a major overhaul of a facility looking to adopt this method. Stations looking to adopt this method also would need to consider that most source material is stereo at present; listeners would only hear sound out of two of their five speakers unless their library was up-mixed prior to broadcast. HDC Surround sound Having debuted at the NAB Radio Show 2005, Coding Technologies has entered into the digital radio surround with its HDC Surround sound Technology. Coding Technologies company is specialized in enhancing perceptual audio encoders with its Spectral Band Replication technology, has developed the AAC Plus and MP3 Pro. Coding Technologies also worked with Ibiquity compnay to develop the HDC audio codec used in HD Digital Radio. Because this system is based on HDC, the surround system is fully compatible with the existing codec, eliminating the need to add hardware or software elements to decode the surround information. Any successful surround sound technology for HD Radio needs to have the following features: backward compatibility with existing radios, good for both single-program and multi-program stations and scalable from "pseudo surround" to true surround, impact on radio stations. It must also be built with industry collaboration. HDC Surround, fits all of these requirements. HDC Surround is the combination of the existing HDC codec and the forthcoming MPEG Parametric Surround. The latter is an enabling technology that allows the encoding of multichannel audio based on the normally coded stereo signal with low additional bit rate. Instead of coding each channel discretely, MPEG Parametric Surround extracts information at encode time on the difference between the stereo mix and the 5.1 channel signal. This extra information is sent along with the encoded stereo to be interpreted by the decoder. The more bits that are allocated to the Parametric Surround, the more accurate is the end result to the original 5.1 mix. This technique reduces the 5.1 channel overhead from 150 percent to 15 percent. Similar in approach to the Fraunhofer method, HDC Technology uses a stereo downmix to provide compatibility with stereo receivers, and adds a data stream that contains the information needed to create the surround channels when the decoder is present. The main difference is that while Fraunhofer locks the rate of the stereo and surround streams, Coding Technologies allows the streams to be dynamic in their bit allocation without exceeding the 96kb/s limit. This allows for greater flexibility in the datastream application.Also, HDC surround can be deployed within a modern digital radio station without a complete overhaul of the existing stereo infrastructure. The surround information in HDC Surround can be scaled up as high as 24 kbps or as low as 1kbps. This low-bit-rate capability means that stations today can start with full surround on a single program and retain the pseudo-surround effects similar to the other solutions if they move to multiple program broadcasts. Due to the stereo-mixdown nature of HDC Surround, stations can be HDC Surround-enabled through additions to the existing stereo chain, not a replacement of the entire system. SRS Circle Surround SRS Lab has developed a technology called Circle Surround, which can encode 6.1 channels of discrete audio for distribution over two-channel carriers. Circle Surround encoding is compatible with mono and stereo playback. This represents a substantial improvement over previous Lt/Rt based encoding systems, which are not as well suited to handle modern, multichannel media with more than four channels of audio. This means radio can be delivered for playback and broadcast in surround sound through existing stereo infrastructures. Circle Surround encoding is also backward compatible with mono, stereo and all matrix decoders. Circle Surround is similar to the Dolby Surround encoding/Dolby Pro Logic decoding process that was prevalent in the 1980s and 1990s, before the advent of 5.1-channel discrete digital systems such as Dolby Digital and DTS. To broadcast in Circle Surround, a station routes up to 6.1 channels of sound into a Circle Surround encoder, from which two channels of sound emerge and are then broadcast through HD Radio. Circle Surround is a matrix encoder, using a summing and difference network to downmix the surround signal into a stereo format, which includes a bias signal that is used to reconstruct the surround information when decoded. Additional stereo information can also be added to the encoded stream. Circle Surround encoding can be distributed via a bit-rate as low as 48kb/s, however 128kb/s will provide maximum separation. At 64kb/s, a station could transmit a surround signal with some separation in the main signal. Advanced matrix systems like SRS Labs' Circle Surround significantly improve decode performance over older systems by using multi-band and variable time-constant steering to stabilize the sound field. With systems of this type, full 5.1 or even 6.1 encode/decode capability is supported, while stereo and mono compatibility as well as functional cross capability between matrix encode/decode systems is retained. On playback, the decoder detects the surround-encoded information in the stereo signal by analyzing the ratio between correlated information and anti-phasic surround material inserted by the encoder. This ratio is generally much higher in an encoded stereo signal than in a nonencoded signal. Implementation of the Circle Surround encoding solution for the HD Radio platform allows radio stations to encode any multichannel content into two-channel output for broadcast over the HD Radio system, which can then be decoded into full-bandwidth surround sound with any decoder found in millions of home theater and automotive systems. Surround sound for EUREKA 147 DAB DAB Surround DAB Surround is a new surround sound technology developed by Fraunhofer Institute for EUREKA 147 DAB. DAB Surround is fully backward compatible to all the DAB receivers already in the market. DAB Surround combines the existing MPEG Layer-2 audio codec with MPEG Surround. This brings 5.1 Surround Sound to Digital Audio Broadcasting EUREKA 147 DAB still including the full range of Programme Associated Data (PAD) services. MPEG Surround is a generic surround extension, which can be associated with almost any perceptual audio codec while remaining fully backward compatible to stereo or even mono. MPEG Surround is a compromise between the discrete and the matrix methods. It provides the accuracy of discrete encoding while preserving the stereo compatibility and bandwidth efficiency of the matrix encoding. The transmission channel requirements for an MPEG Surround coded signal remain similar to those required for conventional stereo, and the existing stereo transmission infrastructure can stay in use. At the highest level, MPEG Surround adds digital information to a stereo mix, enabling reconstruction of an original 5.1 surround audio with little bandwidth overhead. The nature of the MPEG Surround technology allows it to scale in quality proportional to the amount of bandwidth allocated to the surround signal. Quality in this case is measured by the ability of the encoded signal to accurately reproduce the original 5.1 channel audio signal sent into the encoder. While the nominal operating bit rate for MPEG Surround on HD Radio would be around 6kb/s, MPEG Surround can scale all the way up to full transparency. Key Features of MPEG Surround Very low bitrate representation of high-quality multi-channel signals Full backward compatibility to monoor stereo Surround enhancement data rate scalable from 3 kbit/s up to 32 kbit/s A multi-program Digital Radio station would likely allocate 64kb/s out of the total 96kb/s to the main stereo program, then add 6kb/s MPEG Surround to achieve a full 5.1 surround sound broadcast. By tuning the encoder even further the stereo bit-rate could even be pulled back a bit thereby reducing the surround sound overhead to be almost negligible. MPEG Surround provides surround sound to a stereo broadcast with room for extra audio programs or other digital broadcast services. The achieved audio quality is very close to a fully discrete surround system despite the fact, that the surround image is represented by a very low additional bit rate down to 5 kbit/s. Currently, the Moving Pictures Expert Group (MPEG) standardizes MPEG Surround as an ISO standard. Thus, MPEG Surround guarantees a smooth, compatible and cost-effective introduction of multi-channel sound to DAB: To upgrade to surround sound broadcasting stations may just use a DAB Surround encoder instead of the conventional MPEG Layer-2 Stereo encoder. On the receiver side a conventional DAB receiver would simply ignore the surround information and play high quality stereo whereas DAB Surround receivers reproduce the full multi-channel sound. For DAB Surround transmission, Fraunhofer Institute for Integrated Circuits IIS offers a DAB Surround encoder. For playback on the receiver side a PC-based solution is available as a WinAmp Plug-in. For surround sound on portable DAB devices, Ensonido is the ideal supplement. This recent development of Fraunhofer IIS enables playback of surround sound on common stereo headphones. That way, listeners can just plug-in their headphones to the DAB Surround receiver and enjoy full 5.1 surround sound. Challenges ahead Dolby Labs and SRS Labs both have established bases of existing surround sound playback systems in the marketplace that decode their modified matrix formats. Franhofer/Telos and Coding Technologies/Orban are proposing new high-performing parametric discrete systems. Neural Audio appears to have the most easily manipulated and transmitted discrete method using their watermark technique. With four companies developing potential systems, there could be a conflict in deploying them in consumer devices. Stations can only implement one system for transmission. If all four prove to be viable, it is possible that consumer receivers will only be able to decode one or two of the formats. One possible solution to this problem would be a software-defined receiver that would automatically sense and decode the proper system. For broadcasters, backward compatibility is ideal for existing facilities with a stereo infrastructure because tripling the audio channel capability for discrete 5.1 is not an easy task. The potential challenge ahead is all about standards. Without a clear industry-wide technical standard, stations are not compelled to implement the necessary production and transmission infrastructure to deliver 5.1 surround sound. Unless radio stations encode and receivers decode the same 5.1 format, nobody will hear it. The another challenge is related to multicasting, refers to the broadcasting of multiple audio programs within a single IBOC service. The HD radio system accommodates multicasting programs in two ways, utilizing either the Supplemental Program Service (SPS) or Advanced Application Services (AAS) features of the system. The reduction in audio program bit rate required for IBOC multicasting have more subjective impact on a surround audio signal than on a stereo or mono signal.20 This impact may also vary with the surround encoding format used. Surround sound radio is new technology, which has just been standardized, and will require time to be integrated into production and transmission equipment and decoder chips. Conclusion In conclusion, surround has many benefits, but has to fit within the many restrictions imposed by the digital radio system, existing broadcast plants and the new service offerings being planned. The future looks promising for surround sound multichannel music. Digital Radio technology and subscription satellite radio services look like potential sources for surround sound music in the near future. The future is moving towards surround sound in the car ? maybe sooner than you think!
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