HISTORY OF SPATIAL CODING Mark F. Davis, AES Member Dolby Laboratories, Inc., San Francisco, CA, USA INTRODUCTION The goal of spatial audio coding and reproduction is to convey some measure of the dimensional aspects of a sound event to the end listener, preferably in a practical and cost-effective manner. The importance of the spatial aspects of sound has been clear since before the dawn of audio reproduction, as evidenced, for example, in the design of concert halls. So it is not surprising that concern for the accurate capture, transmission, and reproduction of the spatial components of a sound event goes back almost to the birth of the audio industry as we know it today. Yet even now, completely accurate spatial reproduction remains elusive, and is perhaps the largest single imperfection in modern audio technology. This article is a brief overview of some of the notable efforts that have been made in the name of spatial coding. Although some may tend to conceive of spatial coding as a recent technology—part of the digital audio age—many of the concerns around spatial coding, and even some of the fundamental techniques employed, have existed for many decades, and in some cases have had to be recycled multiple times before reaching viability. The basic approaches to spatial audio systems tend to fall into two groups: the brute-force solution of trying to reproduce an approximation of the original soundfield and the psychoacoustic approach of conveying the essential spatial cues without actually recreating the soundfield itself. Not all spatial audio systems fall cleanly into one of these categories, and some are crude enough to leave the question of categorization open to debate. Certain elements do tend to be common to almost any spatial audio system. 554 These include means for capturing the sound (for example, some sort of microphone array); means for combining and/or encoding the raw signals into a net composite signal; means for decoding at the reproduction site; and presentation means, such as a loudspeaker array or headphones. A problem underlying most spatial audio systems is that of trying to capture and reproduce a three-dimensional wavefield using essentially zero-dimensional transducers because microphones and loudspeakers are basically point-in-space devices. That is in contrast to, for example, video processing, in which cameras and display screens can capture and display two-dimensional images. Happily, the audio problem is made somewhat more tractable by the ability of the human auditory system to create a three-dimensional sonic perception from just the two ear signals. Of course, any viable sound system must be practical and cost effective, with realistic hardware requirements (for example, low MIPS for digital systems) and low data rate. Ideal content preparation requirements should be reasonable without placing undue time or equipment constraints on the producer. To the extent possible, the system should be tolerant of suboptimal playback configurations or listener positions. A number of sometimes interdependent threads run through the history of spatial audio. Much of the design effort to date has been devoted to squeezing as many channels as possible into the available storage and transmission media. A lot of work has also gone into exploring the most effective uses, orientations, and positions of the available microphones and loudspeakers. Another issue has been differing, market-dependent focus. In the consumer domain, spatial sound has large- ly meant stereo until recent years, while for the most part the cinema industry went straight from mono to full surround systems. One final thread is the need to develop infrastructures to support a particular spatial audio configuration as a prerequisite for the viability of that technology. In addition to content producers and end users, such infrastructures may include a multiplicity of wired and wireless transmission media and storage formats that have included mechanical, magnetic and/or optical disks, tape, and film. THE DAWN OF AUDIO To provide some initial perspective, it is worth noting a few of the signatory events that marked the dawn of audio technology. The desire to be able to capture and reproduce sound at will is not a recent development. References to audio systems of one sort or another can be found in literature long before such systems were possible. However, the theoretical foundations for practical audio systems were not established until the 1800s when pioneers such as Faraday, Henry, and Ohm in the electrical sciences and Helmholtz, Tyndall, Lissajous, and others in acoustics demystified and quantified these phenomena, providing a basis for physical instrumentation. Work done by these pioneers in turn led to the first great audio invention in 1867, the string and tin can “telephone.” The identity of the inventor appears to be lost to history. Although this invention was primitive, it clearly demonstrated a strong intuitive understanding of the nature and workings of sound. A much more practical and farreaching invention followed nine years later when Alexander Graham Bell inaugurated the telephone on March 10, J. Audio Eng. Soc., Vol. 51, No. 6, 2003 June History of Spatial Coding J. Audio Eng. Soc., Vol. 51, No. 6, 2003 June A. G. Bell First telephone transmitter T. Watson Photo courtesy of Oberlin Archives 1876 by speaking the immortal words to his assistant, “Mr. Watson, come here. I want you.” Bell actually filed his patent application on February 14, 1876, just two hours before electrician and inventor Elisha Gray filed a caveat (intent to patent) for a similar device. The invention of the telephone was significant not only for its intrinsic value, but also for establishing the principles of electroacoustic transduction in both directions. A long succession of refinements and improvements in microphones, headphones, loudspeakers, and other transducers would follow. With the transmission of sound established, it was only a little more than a year later that Thomas Edison demonstrated the ability to record and reproduce sound with his recording on December 8, 1877, of “Mary Had a Little Lamb.” Edison’s invention is elegant and almost starkly simple, employing only mechanical transduction; in hindsight it is surprising that it took mankind so long to devise it. Despite its simplicity, Edison’s initial phonograph employed separate record and reproduce heads, so “offthe-tinfoil” monitoring has presumably been a feature of recording devices since day one. In 1889 Edison followed up with the invention of motion pictures, using some new 35-mm film stock prepared by George Eastman. The first film showed Edison’s assistant, Fred Ott, sneezing. Although initially not strictly an audio invention, Edison always intended motion pictures to be accompanied by sound. It would be some years before this became commercially viable. Edison was but one of several people pursuing the idea of motion pictures during this period, and it appears that the first motion picture on a strip of film was produced a year earlier, in 1888, by Auguste Louis Le Prince, who mysteriously disappeared shortly before his invention could be commercialized. A number of other soon-to-be important formats got their start in this period. In 1894 Guglielmo Marconi, at the ripe old age of 20, invented wireless transmission. It would be some years before full continuous-wave transmission of audio was possible, ➥ Elisha Gray Frame from Le Prince 1888 Film Three photos of Thomas Edison Edison’s first phonograph Fred Ott sneezes in Edison’s first film. 555 History of Spatial Coding Marconi but Marconi established the basic principles of wireless transmission. On December 1, 1898, Valdemar Poulsen became the first to patent a magnetic recording device, using steel wire as the recording medium. Poulson was partly motivated by a desire to avoid the wear of grooved media played with a stylus. deForest Audion Leon Gaumont, in 1901, began experimenting with optical sound on film. This, too, was intended in part to avoid the wear of grooved media, as well as to provide reliably synchronized sound with video. Rather amazingly, all the inventions of this time period were realized without the aid of electronic amplification, as it was not until 1906 that Lee deForest invented the “Audion” triode vacuum tube. Indeed, it would be another 10 years before the vacuum tube became a commercially viable mass-produced item and another decade or so before it permeated various designs. It was not until the late 1920s or early 556 Drawing of Clement Ader’s 1881 demonstration in Paris 1930s that basic monaural audio technology was refined to the point that it made sense to start considering multichannel systems capable of conveying spatial audio information. THE DAWN OF SPATIAL AUDIO Regardless of the time it would take for single-channel audio to achieve initial maturity, early experiments into spatial audio began soon after the development of the telephone. In 1880 Bell did some tentative binaural experiments using a pair of telephone transmitters connected to a pair of receivers. The first widely noted public demonstration of spatial audio followed a year later in 1881 when Clement Ader set up a series of microphones across the stage of the Paris Opera and fed their outputs via wires to headphones in nearby hotel rooms. Listeners perceived a crude but effective binaural rendering of the performance, noted for its compellingly natural quality. Unfortunately, there appears to have been little immediate follow-up interest. The first crude system for presenting spatial sound to an audience was patented on January 12, 1915 (Patent Number 1,124,580) by Edward H. Amet. This system employed a monaural record synchronized to a movie projector. A mechanical commutator attached to the turntable allowed the sound to be panned to any of a number of loudspeakers arrayed across the screen, presumably allowing spoken dialog to follow the position of actors as they moved about. The system could also direct sounds to loudspeakers in the audience. This was a remarkably far-sighted invention, as it would be another dozen years before even mono-synchronized sound was commercially employed in the cinema. At this point some additional formative events bear mentioning, although none formally employed spatial sound reproduction. On April 28, 1916, Edison conducted a convincing live-versus-recorded demonstration of his Diamond Disk acoustical phonograph in Carnegie Hall, New York. Although the phonograph was monophonic, the hall imparted its exceptional spatial acoustics to both the live performer and the reproduced version. Perhaps if the average living room had similar ➥ J. Audio Eng. Soc., Vol. 51, No. 6, 2003 June Stereo Mono SRS Labs is a recognized leader in developing audio solutions for any application. Its diverse portfolio of proprietary technologies includes mono and stereo enhancement, voice processing, multichannel audio, headphones, and speaker design. • With over seventy patents, established platform partnerships with analog and digital implementations, and hardware or software solutions, SRS Labs is the perfect partner for companies reliant upon audio performance. Multichannel Product Applications Technologies. • Circle Surround II • TruBass • Home Theater/Entertainment • FOCUS • TruSurround XT • Wireless + Portable • SRS 3D • VIP • Telecom + Voice • SRS Headphone • WOW • Gaming • Internet + Broadcast World Wide Partners Aiwa, AKM, Analog Devices, Broadcom, Cirrus Logic, ESS, Fujitsu, Funai, Hitachi, Hughes Network Systems, Kenwood, Marantz, Microsoft, Mitsubishi, Motorola, NJRC, Olympus, Philips, Pioneer, RCA, Samsung, Sanyo, Sherwood, Sony, STMicroelectronics, Texas Instruments, Toshiba The Future of Audio. C 2002 SRS Labs, Inc. All rights reserved. The SRS logo is a registered trademark of SRS Labs, Inc. Technical information and online demos at www.srslabs.com History of Spatial Coding quality acoustics, the goal of viable spatial sound reproduction would have been reached much sooner. On November 2, 1920, the first commercial radio station (KDKA in Pittsburgh) began broadcasting. In the 1920s, Fox Newsreels used optical sound on film to provide narration, and on October 6, 1927, synchronized dialog with film was commercially employed for the first time with the release of The Jazz Singer by Warner Bros. Although the soundtrack was monophonic (a record), the stage was set for the eventual advance to surround sound with film. With the arrival of the 1930s the foundation and infrastructure of single-channel audio was now established enough to support more aggressive experiments in spatial audio. During this period Bell Labs took a pioneering role in advancing the state of the art, something they would eventually be deterred from by the U.S. Securities and Exchange Commission, which was afraid the company would extend its monopoly on telephony to cover all of commercial audio. In many of its experiments during this time, Bell Labs was materially aided by noted conductor Leopold Stokowski, who became something of an evangelist for spatial sound reproduction. In 1931 Stokowski and Harvey Fletcher of Bell Labs took up where Clement Ader had left off, using im- proved recording equipment to convey monaural and binaural sound from the Academy of Music in Philadelphia. The following year Stokowski would cut a crude stereo record using two groove bands. Before that occurred, however, something extraordinary happened. On December 14, 1931, British engineer and inventor Alan Blumlein patented stereo. Blumlein was a prolific inventor with 128 patents to his credit. His 1931 stereo patent didn’t just focus on one aspect of stereo sound but examined the entire audio infrastructure of the time and determined in detail what it would take to convert it to stereo. Blumlein’s patent, number 394325, contains 70 claims addressing such refinements as: • Microphone design • Microphone techniques, such as the use of crossed figure-8 microphones • Disk cutting, particularly the 45/45 stereo disk-cutting technique, in which the left channel is recorded on one groove wall and the right channel is recorded on the other groove wall. This system would be “invented” at least two more times before becoming a commercial reality. • Stereo broadcast techniques including the use of AM and FM on a common carrier to convey the two channels • Sum/difference matrixing, in part to support backward compatibility with mono equipment • A stereo “shuffler” circuit. There is some question about the purpose of this circuit. At one point it is described as a preprocessor of stereo signals, intended to make the acoustic waves reaching the listener more nearly like a live situation, perhaps foreshadowing Michael Gerzon’s Ambisonics system. But at another point, a shuffler is described as separately using interchannel amplitude and phase to independently pan a movie soundtrack left/right and up/down behind a movie screen. If this configuration had just been flipped horizontally, so that its outputs were distributed around the room instead of around the screen, it might have comprised the first matrix surround system. One of the few areas not significantly addressed in Blumlein’s 1931 patent was stereo sound on film, an omission corrected four years later, in 1935, when Blumlein produced the first films with a stereo optical soundtrack, most notably short films of trains at Hayes Station. Such was the nature of Blumlein’s foresight that these films can still be played on current 35-mm projectors with stereo sound heads. Unfortunately, Blumlein’s work was at least 20 years ahead of its time. His Used by permission of Robert Alexander Illustration of 45/45 recording heads from Blumlein’s 1931 patent. Alan Blumlein, as shown on cover of Robert Alexander’s biography (recommended reading) 558 Detail from Patent No. 394,325 (1931) demonstrating recorder assembly whereby both channels may be cut by single tool on same groove. This results in recording at 45o to the wax (or other) surface giving sum and difference. Detail from Patent No. 394,325 (1931) similar to figure on left except here the driving force is generated from electromagnet J. Audio Eng. Soc., Vol. 51, No. 6, 2003 June History of Spatial Coding colleagues apparently regarded his work as fascinating but with uncertain commercial potential. EMI, his employer, rewarded him by transferring him out of the audio group to pursue more pressing projects like television and radar. He died shortly before his 39th birthday in an airplane crash during a test of an experimental radar system. Sadly, during an earlier attempt to produce a biography, Blumlein’s notebooks were loaned out and apparently lost. EMI never attempted to capitalize on Blumlein’s stereo patent before it expired. With Blumlein’s work remaining largely unknown, work by other groups continued as before. In 1933 Bell Labs’ researchers Fletcher, Snow, and Steinberg attempted to widen the sweet spot of stereo by adding a third channel in the center. They conducted a well-received demonstration with a three-channel wire transmission from the National Academy of Sciences to Constitution Hall in Washington, D.C. That same year binaural recordings made using a dummy head were demonstrated at the Chicago Century of Progress Exhibition. In 1936 Arthur Keller of Bell Labs reinvented the 45/45 stereo disk recording system, patent number 2,114,471. The filing was delayed because Bell Labs felt there was no commercial ap- Philips-Miller Recording 2-Channel PhilipsMiller Recording J. Audio Eng. Soc., Vol. 51, No. 6, 2003 June Leopold Stokowski plication. Keller was unaware of Blumlein’s work until the 1950s, when Westrex Corporation invented 45/45 recording for the third time. 1936 was also the year the PhilipsMiller optical film sound system was introduced. This system used an opaque strip on film that was selectively sliced away by a vibrating stylus, leaving a variable-area soundtrack that could be immediately played with a conventional photocell arrangement. The system was capable of recording two-channel sound and had very high quality for its day. 1940 was a watershed year for spatial audio. On April 9 and 10, Fletcher and Stokowski demonstrated a novel threechannel recording system at Carnegie Hall. This system used optical sound on a film track that was running at high enough speed to support a frequency response of 30 to 15000 Hz. A fourth optical track contained a gain control signal, effectively implementing a wideband compander, resulting in an impressive system dynamic range of 120 dB. Without question, however, the big news in the world of spatial audio in 1940 was the release of the film Fantasia, with music conducted by Leopold Stokowski. Road-show presentations of this film featured a full surround sound system called “Fantasound,” developed by Stokowski, Disney, and RCA. The Fantasound system also used a four-channel optical soundtrack on a separate strip of film, synchronized with the projected film. There were three audio channels plus a control track. The control system allowed the audio tracks to be panned to any of 10 loudspeakers: Front Left/Center/Right; Front Left/Right corners; two side loudspeakers; two rear loudspeakers; and one on the ceiling. The side and rear loudspeakers were used sparingly, except in the “Ave Maria” segment, where they provided an enhanced sense of envelopment. Unfortunately there was little subsequent interest in Fantasound, and the equipment was donated to a Russian company. Tragically, it was lost at sea during shipping. Audio progress in the next few years was understandably slowed, as the world took time out to fight a war. German researchers made significant refinements to magnetic recording during this period, and in 1942 the first stereo tape recordings were produced by Helmut Kruger at German Radio in Berlin. As the war wound down, British record company Decca began experimenting with lateral/vertical stereo records, but the heavy-tracking pickups of the era made the records difficult to play, and it would be another dozen years before stereo records became a commercial reality. In 1948 three milestones occurred that, while not directly related to spatial audio, helped provide the foundation for further progress. One of these was the development of the 33-1/3 RPM LP record by Peter Goldmark of CBS Laboratories. Similar formats had been used in broadcast applications (and Muzak) for some time, but its use as a consumer format was revolutionary and led to the introduction of the stereo LP nine years later. The second event of note to occur in 1948 was the formation of the Audio Engineering Society in New York. On June 30, 1948, the transistor was invented by William Shockley at Bell Labs. In the following years transistors would almost entirely replace vacuum tubes in analog circuits, perhaps to the consternation of listeners enamored of “tube sound,” and some 20 years later latter-day descendents of the transistor would be a cornerstone of the digital revolution in audio technology. THE 1950S: STEREO COMES OF AGE Probably the biggest audio news of the 1950s was that stereo finally came of age as a consumer medium. On the ➥ 559 History of Spatial Coding cinema side, there were some notable experimental forays into multichannel sound systems, although mainstream cinema surround sound was still a couple of decades away. The arrival of stereo disk recordings may have seemed imminent in 1951, when Emory Cook made a series of stereo recordings of railroad trains, realized as a dual-band LP titled Rail Dynamics, which was demonstrated at the Audio Fair in New York. However, the dual-band technique required a pair of precisely mounted mono pickups and had limited playing time, thus it did not achieve commercial acceptance. In the meantime the cinema industry experimented with some truly memorable surround sound systems, although none would enjoy an extended life span. On September 30, 1952, the Cinerama process was introduced with the release of the film, This Is Cinerama. This amazing system, developed by Fred Waller, employed three synchronized projectors, each covering one-third of the screen. The sound system, developed by Hazard E. Reeves, was based around a synchronized dubber running a sevenchannel magnetic soundtrack (six audio tracks, one control track). The loudspeaker system included five screen loudspeakers (predating SDDS) and an array of surround loudspeakers that could be fed a mixture of the source channels. The effect produced by this system was almost visceral, particularly during the roller coaster sequence of This Is Cinerama. It is worth the effort to seek one of the few remaining theaters capable of showing this film in its original format. Unfortunately, the expense of the Cinerama process was prohibitive, and the system was abandoned in 1963. Cinemascope, a system somewhat less complicated than Cinerama, was introduced on September 16, 1953, with the release of the film The Robe. The system used an anamorphic (horizontally magnifying) lens to project a wide image with a single 35-mm projector. The sound system, developed 560 Diagram of Cinerama presentation by Ampex, used four magnetic tracks striped onto the print, intended for L, C, and R front, plus a mono effects track. The Robe is credited as the first film to use off-screen voices. The high quality of the Cinemascope sound system allowed films to be presented without the dreaded Academy Mono equalization, a high-frequency rolloff used in theaters to minimize the hiss associated with optical soundtracks of the period. Although several dozen films were made with the full Cinemascope system, it also proved too expensive in practice, and its use of magnetic surround tracks was eventually discontinued. Magnetic tracks were also used on 70-mm prints, usually comprising five screen channels and a mono effects channel, beginning in 1955 with the film Oklahoma and 1956 with Around the World in Eighty Days. One more cinema surround system notable for its practicality was the Perspecta Sound system, developed in 1954 by recording engineer C. Robert Fine, also remembered for his “PingPong” stereo recordings on Command Records some years later. Perspecta Sound used the existing mono optical sound track, to which was added three low-frequency control tones used by an external controller to pan the mono sound to three screen loudspeakers. Somewhat reminiscent of Amet’s 1915 switched system, Perspecta Sound allowed dialog tracks to follow the actors on the screen using a single inventory print, and improved on Amet’s switched system by allowing different levels to be fed simultaneously to different loudspeakers. The slow time constants of the system resulted in a type of pretransient artifact, as the gain of a channel had to be ramped up before the transient arrived. Although the prenoise and the lack of true multichannel sound led to the system’s eventual retirement, it was used on many Merry Melodies cartoons. For Todd-AO 35-mm magneticstripe versions of Around the World in Eighty Days, Perspecta Sound was used to expand the mono effects track to three surround loudspeakers. Most of the multichannel cinema systems of this era were used primarily for spot sound effects, like airplane flyovers, rather than for true surround sound. Ultimately the film industry would require a high-quality, low-cost, optical surround soundtrack to replace mono as the standard sound format, and that would take at least another couple of decades. While film sound engineers scratched their collective heads over the lessons learned from these pioneering systems, stereo sound was gearing up to make its grand entrance as a consumer format. The next volley was delivered by Murray Crosby, who demonstrated a compatible stereo FM system in 1954. The performance of this system so impressed the engineers at RCA that, although they did not commit to using it, they did initiate the release of stereo material on 30-ips two-track tapes. The first of these was a recording of Also Sprach Zarathustra recorded by Fritz Reiner and the Chicago Symphony, which sold for $18.95. 1957 was to be the breakthrough year for consumer stereo. By this time Arthur Haddy and associates at British J. Audio Eng. Soc., Vol. 51, No. 6, 2003 June History of Spatial Coding Decca Records had significantly refined their vertical/lateral stereo disk system and demonstrated it to RCA. But on September 5, 1957, Westrex started giving demonstrations of its newly reinvented 45/45 stereo disk, some 26 years A.B. (after Blumlein). On October 11, 1957, Westrex demonstrated this system to some acclaim at the New York AES Convention. And in November 1957 Audio Fidelity Records copped the honor of releasing the first disk in this format, a curious pairing of The Dukes of Dixieland with Railroad Sounds, produced by Sidney Frey. The story, perhaps apocryphal, is that Frey sent the master tapes to Westrex to create a pair of evaluation disks, and to discourage their release Westrex cut one side of each, but Frey went ahead and released the record anyway. In short order the RIAA gave its official stamp of approval to the system, including the use of a 0.7 to 1.0 mil stylus and 6 grams vertical tracking force, and the rest was history. THE 1960S: STEREO INFRASTRUCTURE AND RESEARCH With the rapid acceptance of the stereo LP disk, the decade of the 1960s saw the development of complementary technology that greatly expanded the use of stereo in consumer audio. In 1961 stereo radio was born (at least in the U.S.) when the FCC selected the GE/Zenith FM multiplex system, largely designed by Carl Eilers, to support national stereo broadcasts. Stereo broadcasts began on June 1, 1961, by WEFM in Chicago and WGFM in Schenectady. The system chosen employs sum/difference matrixing for compatibility with mono receivers and conveys the difference information on an amplitude-modulated 38-kHz subcarrier. The FCC’s decision was somewhat controversial, as the system chosen exhibits 20 dB less dynamic range than mono transmission, or that of the competing Crosby system, but allows spectral space for background music services. The FCC also withheld stereo certification of AM broadcast at the time in order to promote the struggling FM medium. The fate of AM stereo was ultimately J. Audio Eng. Soc., Vol. 51, No. 6, 2003 June Arthur Haddy left to the marketplace, where confusion and apathy have largely relegated it to limbo. The stereo infrastructure was extended with the introduction of two new consumer tape formats principally configured for stereo. In 1963 Philips introduced the compact cassette, followed in 1966 by the introduction of the eight-track cartridge format. Acceptance of the cassette as a high-fidelity medium was aided in 1969 with the advent of the Dolby B-type noise reduction system. The establishment of these formats motivated numerous investigations of preferred recording and reproduction arrangements, with the sometimes-conflicting goals of preserving both image position and ambience. In 1959 Goldmark and Hollywood of CBS Labs found that using two small loudspeakers with a subwoofer produced imaging little different from that produced by a pair of full-range loudspeakers. Thus was born the satellite loudspeaker system, although it would be a while before it became popular. The following year Ben Bauer of CBS Labs demonstrated that crossfiring the loudspeakers somewhat could widen the sweet spot in stereo audition. This principle has since been used in a number of commercial designs. That same year Beaubien and Moore investigated stereo virtual imaging as a function of frequency and concluded that the fusion of such images could be improved by panning low-frequency and high-frequency information along separate trajectories. This notion has since been investigated for surround systems as well. However, the conventional wisdom regarding widening the sweet spot while preserving image fu- sion is that the most robust solution is to use additional loudspeakers, especially a center loudspeaker for stereo. In a somewhat different vein, in 1963, Schroeder and Atal developed the acoustic crossfeed cancellation circuit. Although very sweet-spot dependent, this system eliminates the infamous 2kHz notch encountered with centered virtual images and allows the rendering of images outside the span of the loudspeakers. Although initially intended as a tool for studying concert hall acoustics, it has become the basis of many modern “virtual surround” systems. QUAD: THE BIG NEWS OF THE 1970S The 1970s saw the start of two significant trends in audio. Unquestionably, the higher public profile of these at the time was the rise, fall, and eventual rebirth of four-channel audio, otherwise known as quadraphonic sound, or “quad.” Quad began innocently enough as a series of test recordings produced and demonstrated by Robert Berkovitz at Acoustic Research Corporation. The stated intent was to simply show what happens when a pair of rear channels was added to a front stereo pair, but the idea rapidly spiraled into a massive commercial enterprise. To some extent this was technically justified, because quad avoided the compromise in stereo between precise imaging and enveloping ambience, with the front channels providing the imaging and the surround loudspeakers providing the ambience. But part of the underlying motivation was almost certainly commercial one-upmanship, arguably resulting in products and systems being rushed into the marketplace prematurely. In any case, audio engineers who had worked so long to push two channels through a variety of broadcast and storage media were suddenly faced with trying to squeeze another two channels through the same media. The proposed systems included discrete tape systems, a discrete multiplexed LP, and a discrete quad FM system, Quadcast, proposed by Lou Dorren. But the primary entries were matrix systems designed to extract four channels from two. The godfather of quad matrix systems was Peter ➥ 561 History of Spatial Coding Scheiber, who established most of the basic principles of matrix systems then and since. Other matrix systems introduced on the heels of Scheiber’s included those by David Hafler, ElectroVoice, the CBS SQ system, and the Sansui QS system. These systems all used a square loudspeaker arrangement. One notable variant was a system described in 1971 by Shiga, Okamoto, and Cooper that used a diamond-pattern matrix: center front, left, right, and a mono surround channel, presaging the later Dolby MP matrix system. Each of these matrix systems employed an encoder to reduce four source channels to two. Usually the encoder was a passive downmixer employing signed weighted sums of the source channels, sometimes including phase shift networks. The decoders, most of which were originally passive, then employed complementary equations to upmix the two encoded channels back to four. Unfortunately, matrix systems involved some serious compromises that limited their effectiveness, most notably minimal adjacent channel separation, which was usually only about 3 dB. In an attempt to overcome this limitation, designers added gain-riding logic subsystems to better isolate dominant signals, but these were only partially effective and sometimes introduced objectionable gain-pumping artifacts. The matrix systems also tended to be unduly sensitive to interchannel phase and amplitude differences, such as might be encountered in a slightly misaligned tape deck. With some tweaking some of the surviving quad recordings of the 1970s can be played to good effect with current-generation matrix decoders. One matrix system that stood apart from the rest was the Ambisonics systems originally invented by Michael Gerzon in 1969. Rather than trying to recover discrete loudspeaker signals, the goal of Ambisonics is to recreate a valid approximation of the original soundfield in the neighborhood of the listener’s ears. Within limits, the system is compatible with both upmixing and downmixing, and normally does not use any active components, like gain riding. Properly set up, Ambison562 1970s CBS SQ Matrix Decoder ics is one of the most compelling surround systems ever devised, but as it uses wavefront synthesis with a small number of loudspeakers, it tends to be sweet-spot dependent. Recognizing the limitations of matrix systems, several companies chose to develop more discrete systems. The highest profile of these was the RCA/JVC CD-4 Quadradisc system, which used sum/difference matrixing to carry composite stereo baseband signals plus stereo difference signals conveyed by a subcarrier centered at 30 kHz. Unfortunately, tracking such high-frequency signals without significant groove wear was largely beyond the capabilities of phono pickups— even those equipped with the Shibata stylus developed for this application— and the system was not widely adopted. From a technical perspective, probably the most effective of the quad formats were the discrete tape systems. Quad versions of eight-track cartridges and reel-to-reel tape decks were commercialized, and there was at least one quad cassette format proposed. Given the weaknesses in the rest of the quad infrastructure and the general unpopu- larity of the eight-track format for high-fidelity applications, it is not surprising that these formats failed to achieve lasting consumer acceptance. Just as the wide deployment of stereo had motivated research into preferred modes of recording and reproduction, so did the quad era boost audio exploration. During the 1970s and 1980s the role of technical evangelist that had been occupied earlier by Leopold Stokowski was effectively assumed by musician Quincy Jones. One of the more compelling demonstrations of surround sound ever mounted was implemented in 1970 by Tom Horrall of Bolt, Beranek, and Newman (now Acentech). Horrall constructed a model of Boston Symphony Hall and used it to calculate an approximation of the spatial impulse response of the hall. He then recreated that response using an elaborate tape delay system with multiple playback heads, standard stereo material, and a dozen loudspeakers strategically arranged around the listener. The amazing thing about this system was not just that it created a highly satisfactory sense of envelopment, but that it actually sounded identifiably like Boston Symphony Hall. Few commercial spatial sound systems of the era were able to make such a claim. Horrall’s system was subsequently commercialized in 1988 as the Pioneer DSP-3000. Another interesting surround sound investigation was published in October 1971 by Nakayama et al., in which the effectiveness of a number of horizontally arrayed loudspeaker arrangements was examined. Their preferred ar- Discrete stereo groove with frequency distribution J. Audio Eng. Soc., Vol. 51, No. 6, 2003 June History of Spatial Coding max VCR. A year later in 1976 JVC introduced the VHS format VCR, originally with a monophonic linear analog soundtrack, then stereo linear analog tracks in 1980, and in due course (1983), with a separate companded FM stereo track, VHS Hi-Fi. These stereo formats provided two channels of sufficient quality to support Pro Logic surround in the home. The Laserdisc format was developed in 1978 and formally introduced in 1980. As had been the case with audioonly content a couple of decades earlier, the availability of recorded video content with stereo audio tracks resulted in marketplace pressure for broadcast to follow suit, and in 1984 the Horrall Surround Test BTSC stereo sound system was introduced for commercial U.S. broadcast, using sum/difference matrixing for backward compatibility and a subcarrier modulation scheme developed by Carl Eilers at Zenith. Taking a cue from the impaired-SNR lessons learned with FM stereo, a dual- ➥ Photo used by permission of Acentech Inc. with a diamond pattern of output channels, to supply the needed center loudspeaker, feeding the mono surround signal to pairs of loudspeaker arrays along either wall. Nonetheless, the indusQuad 8-track deck try’s initial response was somewhat tentative until the following year, 1977, Quad 8-track cart when the release of Star Wars, followed by the rerangement consisted of four front loudlease some months later of Close Enspeakers plus two at the sides and two counters of the Third Kind, demonstratin the rear. A similar arrangement was ed the system’s potential. In 1978 the espoused by Theile in 1990. Within a basic matrix was refined by Craig Todd four-channel constraint, Nakayama’s of Dolby to improve separation (the preferred arrangement was two medi“motion picture,” or MP um-spaced front loudspeakers and two matrix, which included wide-spaced side loudspeakers. These phase shifters), and in 1982 and other studies consistently support and 1983 the gain riding in the current standard of 5.1 channels as consumer decoders was upan effective surround compromise. dated with the Pro Logic alWhile the consumer domain was gorithm developed by Doustruggling with the difficulties of glas Mandell, which is still adapting stereo media to quad, more in use. This system, along substantive progress was being made with the gain-adjusting cirin cinema sound systems. In 1974 Dolcuitry used in Dolby cineby Laboratories at last made stereo opma matrix systems, uses a tical prints viable by applying its Aconstant-power dynamic type noise reduction, including a matrix, instead of variable derived center channel with gain riding gain applied to a fixed system, to the film process. Lacking matrix, to reduce pumping surround channels, the system met artifacts. with muted response. Two years later The sustained establishment at last in 1976 Dolby corrected this problem of surround sound in cinema would with the addition of a four-channel maprove to be the gateway through which trix system, used initially on the film consumer audio evolved from stereo to A Star Is Born. surround. In 1975 foundation of the This system initially used what was consumer video revolution was begun basically a modified Sansui QS matrix, with the introduction of the Sony Beta- Teac Quad deck J. Audio Eng. Soc., Vol. 51, No. 6, 2003 June 563 History of Spatial Coding band noise-reduction system developed at dbx, Inc. by the author was included in the system. Other countries have used similar analog modulation schemes for stereo TV, or gone directly from analog mono to digital stereo, as Britain and other European countries did in 1986 with the Nicam-728 (Near Instantaneous Companded Audio Multiplex) system, a 14-bit to 10bit block compander. In 1978 Emil Torick attempted to apply the use of noise reduction to U.S. FM stereo in a backward-compatible manner, using a novel companded quadrature subcarrier, but the system was mired in controversy about its susceptibility to multipath conditions. Although it enjoyed some limited use, it was abandoned a few years later. THE DAWN OF DIGITAL AUDIO While quadraphonic sound was getting the lion’s share of attention in audio during the 1970s, another revolution was quietly getting started: digital audio. As usual, the establishment of this technology was predicated on the development of certain other technologies. One of these was the transistor, as the use of large numbers of vacuum tubes for logic circuits would have been impractical on a large scale. But even using large numbers of discrete transistors would not have been sufficient; it took the development of digital integrated circuits, affordable highprecision analog/digital/analog converters, and digital storage media to provide the necessary basis for digital audio. Once these were in place, their effects on the audio engineering community would be far reaching, leading many audio design engineers to learn to do in DSP (digital signal processing) what they already knew how to do with analog circuits. Eventually they would begin to exploit capabilities of DSP that were beyond the practical abilities of analog electronics. The availability of digital audio components began to reach critical mass around 1969, when Thomas Stockham began experimenting with digital recording, using a modified Hewlett-Packard data recorder. Stockham’s demonstration of digital recording at the November 1976 New York AES Convention was one 564 of the signatory moments of the organization. A few years later he produced the first commercial digital recording at the Santa Fe Opera. In 1970 one of the first commercial digital products was released, the Lexicon Delta-T101 delay line, designed by Francis Lee. It used companded 12bit PCM. In 1975 EMT released the first digital reverb unit, designed by Barry Blesser, which dramatically advanced the state of the art of DSP. Its debut at the New York AES Convention was another of the standout moments of the Society. Extension of digital recordings to the consumer domain followed with demonstrations of the audio compact disc in 1981, released commercially the following year. The CD had a profound influence on the audio engineering community, as it effectively eliminated in one stroke a whole raft of traditional problems such as flutter, wow, speed error, clicks, pops, noise, and distortion. 5.1-CHANNEL SURROUND Despite the acceptance of Dolby’s four-channel matrix surround system, there was clearly pressure to advance to a more discrete system with two surround channels and a subwoofer channel, an arrangement first characterized by Tom Holman of Lucasfilm as a 5.1-channel system, and standardized in June 1991 in Ottawa as ITU-R Rec. BS.775-1. In 1977 close on the heels of the Star Wars matrix surround release, John Mosely proposed a hybrid “quintaphonic” sound system, which employed a combination of discrete and matrix-derived channels, qualifying it as another system to anticipate the current Dolby Digital Surround EX system in operation of a matrix-derived channel. In 1978 Superman became the first movie with a 5.1-channel soundtrack, with 70-mm prints using a Dolby sixtrack magnetic format developed by Ioan Allen that combined two surround channels with two front LFE channels. In 1980 Terry Beard and Nuoptix Corporation developed a discrete fourchannel analog optical soundtrack. Since each track was only 11.5 mils wide, about one-third that of a track in a stereo optical print, noise was a major concern, and a wideband compander for each channel was included. The system was used for the movie Popeye. In 1982 the film Return of the Jedi was released and Tom Holman designed his THX surround systems. Separate systems were designed for home and theater use, with common goals. The THX theater system employed constant-directivity horns, a novel fourth-order Linkwitz-Riley crossover, and a new theater alignment program to assure consistency of sound quality from theater to theater. For the home system, a modified X-curve EQ was used with dipole surround loudspeakers and a surround decorrelator to provide surround envelopment with direct frontal imaging. The use of Dolby Pro Logic matrixing with loudspeaker systems consisting of Left/Center/Right/Surround Array/and optical bass extension continued as the standard surround arrangement until the early 1990s, when digital soundtracks were developed for 35-mm theater film systems. The first public presentation of a film with a digital soundtrack was performed by Boston audio engineer John Allen, who in 1989 fittingly ran a print of Fantasia synchronized to a stereo digital soundtrack on videotape, played with a Sony PCM-F1 processor, and expanded to four channels with an MPmatrix decoder. The honor of producing the first optical digital soundtrack system went to the combined forces of Kodak and Optical Radiation Corporation, who in 1990 introduced the Cinema Digital Soundtrack (CDS) system with the release of the film Dick Tracy. This system used 5.1 channels of 12-bit ADPCM, with the digital data recorded optically on the film in place of the normal analog stereo optical track. Unfortunately this substitution required double inventory prints with separate analog or digital soundtracks, and meant that there was no backup analog track if there was a problem with the digital presentation, and the system was eventually withdrawn. In 1992 Dolby introduced its SR-D digital soundtrack system with the films Star Trek VI and Batman Returns. This system retained the anaJ. Audio Eng. Soc., Vol. 51, No. 6, 2003 June History of Spatial Coding log soundtrack, with Dolby SR noise reduction and placed the digital information in the unused space between the sprocket holes on one edge of the film, thereby avoiding the double-inventory restrictions of the CDS system. In order to stay within the data-rate restrictions imposed by the sprocket-hole placement, this system employed a more aggressive multichannel coder that processed the channels as an ensemble, rather than coding each channel individually. Later that year DTS introduced its digital soundtrack system with the films Dr. Giggles and Jurassic Park (1993), which featured audio data recorded on external CDROMs that were played in sync with the film. In 1993 Sony introduced the SDDS 7.1-channel soundtrack system with the release of the film The Last Action Hero. The extra two channels were used to feed a total of five screen loudspeakers. This system recorded the data on the two outer edges of the film, outside the sprocket holes, and used the Sony ATRAC coder. As of this writing, all three of these systems are still in use, and in fact all three can coexist with each other and with the analog soundtrack on a single 35-mm movie print. With the desire for more audio channels came the need to improve the storage and transmission bandwidth of available media. The 1990s saw the development of a number of multichannel coders, including Dolby Digital (AC3), Musicam surround, AAC, WMA, MLP, and MPAC. Some of the techniques employed to exploit the presence of multiple channels include use of a common bit pool for all channels, interchannel masking, interchannel prediction, and coupling or intensity stereo. One previously tried-and-true technique that has been problematic in multichannel coders is sum/difference matrixing. The problem with this approach in the context of low bit-rate perceptual audio coders is that the dematrixing operation tends to direct the signal and the quantization noise to different output channels, leading to directional unmasking of the noise. In 1993 the 5.1-channel infrastructure was extended to the domain of television broadcasting with the selection by the ATSC of the Dolby Digital J. Audio Eng. Soc., Vol. 51, No. 6, 2003 June Experimental 65-loudspeaker canopy array at University of Gottingen (AC-3) coder for the audio component of the U.S. HDTV system. This coder was also chosen in 1997 to supply audio on DVD-Video, with the DTS coder as an alternate. In 1999 DVDAudio was introduced with very high quality 24-bit multichannel audio and MLP lossless coding. The past few years have seen the rise of Internet audio, the unprecedented establishment by consumers of MP3 as the preferred format for compressed stereo content, and the introduction of various streaming audio formats by Real Networks, Microsoft, Apple, and others. TAKING STOCK: THE PRESENT At this point in the quest for ultimate spatial audio fidelity, the 5.1-channel format is established as the standard surround sound format, supported by DVD, film, Internet audio, and HDTV. Yet there are already alternatives deployed or proposed, including the SDDS 7.1-channel format, various systems with three or four surround loudspeakers, the IMAX channel configuration, and Tom Holman’s musicoriented 10.2-channel system. Tentative forays into better matching of side and screen loudspeakers, top-ofscreen loudspeakers, and the use of a vertical (“voice of God”) ceiling channel have been made. Yet people still have but two ears, and stereo continues to enjoy a high degree of consumer acceptance, supported by such formats as audio CDs, MP3, much Internet audio, cassettes, and broadcast. (For that matter, AM broadcast is still largely monophonic.) In fact, the continued strength of the stereo format has in recent years given rise to the 2:N channel upmixer or surround synthesizer, which attempts to extract an aestheticallypleasing surround presentation from conventional stereo content. Representative systems include Dolby Pro Logic II, DTS Neo6, Lexicon Logic 7, and the TC6000 Unwrap. Within limits, these systems help bridge the gap between stereo and 5.1-channel discrete surround. To date an astonishing array of systems have been proposed or implemented to approximate spatial sound reproduction, including: • Binaural • 45/45 and vertical/lateral stereo disks • FM/AM and FM multiplex broadcast • Two-band stereo disks • Stereo optical analog soundtracks on film, usually with noise reduction, and sometimes accompanied by auxiliary control tracks • Stereo and multichannel tape recording and magnetic film soundtracks • 4:2:4 and 5:2:5 matrix surround systems • Helican scan FM stereo on videotape • Digital PCM on magnetic tape and optical disk (CD) • Digital low bit-rate coders, including multichannel coders. There is every likelihood that this list will continue to expand. FUTURE DIRECTIONS? In contemplating future advances in spatial sound reproduction, it is im- ➥ 565 History of Spatial Coding portant to be realistic about the strengths and weaknesses of current technology. Existing 5.1-channel surround systems represent the cumulative contribution of an enormous number of innovative engineers and inventors and, properly used, are capable of rendering a highly satisfactory spatial audio experience. To be viable, any candidate for a next-generation system will likely have to provide a significantly enhanced experience in a cost-effective manner. What might be desired in such a system? For one thing, a larger sweet spot might be welcome. 5.1-channel systems can support fairly effective virtual imaging for centered listeners but tend to be perceived as individual discrete channels by off-center listeners. Increasing the number of channels can reduce this problem, but how many are necessary? In this regard, a singular point in the history of spatial audio has now been reached, in that there exist media capable of supporting more channels than we know how to employ. For example, if 60-minute playing time is required and low bit-rate coding at 32 kb/s per channel is used, a standard DVD can hold in excess of 400 channels. There is considerable divergence of opinion on whether this is still hopelessly inadequate or in fact overkill. On the one hand, strict application of the spatial sampling theorem would imply the need for a loudspeaker every 3/16th of an inch, conveying potentially millions of channels. On the other hand is the assumption that, even in a large auditorium or theater, virtual imaging becomes effective once the channel density is great enough, so far fewer channels would be adequate. Jens Blauert, in his renowned book Spatial Hearing, suggests that roughly two dozen channels might suffice. Before ramping up the number of channels, however, some practical considerations may have to be addressed. For example, content producers may be less than enthusiastic about having to mix several hundred discrete channels for a single program. Further, as the number of channels increases, the incremental contribution of any one channel diminishes, to the point where, from an engineering standpoint, it represents an inefficient use of available 566 bandwidth. This raises the question of how side-chain information might potentially be used with a reduced number of conveyed channels to render a larger number of output channels. Another desirable element often mentioned for future spatial audio systems is configuration neutrality: the system automatically makes optimal use of an arbitrary number of output channels from a common program source, chosen by the end user. This would avoid the need to revamp the entire system infrastructure every time an additional channel is added. Perhaps some sort of holographic system conveying spatial transform information would be the preferred basis of a configuration-neutral audio format. Beyond the difficulties of conveying full spherical spatial information, there are practical difficulties in presenting it. As difficult as it may be to place loudspeakers in a canopy above the listeners, it is probably even more challenging to arrange for sounds to come from beneath listeners without a suspended floor. Indeed, in a theater it is generally not even possible to have loudspeakers at ear level, since they will be too loud for near listeners and all but inaudible on the far side of the room. Instead they are placed above the audience, resulting in much more even sound distribution, at the cost of limiting the ability to present sounds that should be truly horizontal. Even a seamless spherical presentation by itself won’t necessarily comprise an ultimate spatial audio system, as a means to project sound into the room would also be desired. Several technologies are currently being explored to accomplish this, including ultrasonic drivers, phased loudspeaker arrays, or multichannel echo-cancellation. Even if a fully immersing presentation scheme can be developed, there will still be the question of how to extend current discrete channel spatial coding techniques to the realm of full 3-D. There may even be philosophical questions to resolve. In cinema presentation, for example, each viewer should receive an equivalent presentation. If, say, a parrot flies out from the screen, should everyone perceive the parrot as landing on one patron’s shoulder, or should each viewer per- ceive the parrot landing on his or her shoulder? Is three-diminsional audio even compatible with a two-dimensional movie screen? One of the ultimate benchmarks of a spatial audio system may be the simple experience of standing out in a rainstorm. A million raindrops fall every second, in every direction, and at every distance from a fraction of an inch to a mile. A proper spatial audio system will reproduce the perception of each splash at its proper distance and orientation for a single listener or an audience of hundreds. The realization of such systems may not be that far off. IN CONCLUSION It seems appropriate to conclude with two quotes about spatial audio. Stereophonic systems do not consist of two, three, or any other fixed number of channels. There must be sufficient of these to give a good illusion of an infinite number. And I know there will be compromises. There wouldn’t be any need for engineers if we didn’t need compromises. It’s the one that does the wise compromising that succeeds best. –HARVEY FLETCHER, 1953 [Resolving the spatial audio quest is] the last great problem in audio. –FRANCIS RUMSEY, 2001 ACKNOWLEDGMENTS This review was made possible by the heroic efforts of Dolby Laboratories’ librarians Tamara Horacek and Elizabeth Azinheira, who dug up a mountain of reference material and to whom I extend my deepest thanks. Thanks are also extended to Ioan Allen, Gilbert Soulodre, Craig Todd, and David Gray for constructive comments and contributions; to Tom Holman for his related talk that inspired this work; to Kate Barrett for polishing the prose; Rick Weldon for clearing the photos; and to Eric Benjamin, Dolby’s ad hoc Ambisonics expert. BIBLIOGRAPHY http://www.acmi.net.au/AIC/ LE_PRINCE_BIO.html. Alexander, Robert Charles, The Life and Works of Alan Dower J. Audio Eng. Soc., Vol. 51, No. 6, 2003 June History of Spatial Coding Blumlein (1999 Focal Press). 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Wood, Irving W., and Fichman, Joel S., “The Sound System at the New York State Theatre,” J. Audio Eng. Soc., vol. 13, pp. 104-110 (1965 Apr.). http://www.xs4all.nl/~rabruil/ phmil.html. THE AUTHOR After receiving his Ph.D. in psychoacoustics and electrical engineering from MIT in 1980, Mark Davis worked at dbx, Inc., where he designed the dbx/MTS stereo television noise-reduction system and the Soundfield One phased array loudspeaker. Since 1985 he has been a senior engineer in the R&D Department of Dolby Laboratories, where he helped design AC-3, Dolby Virtual Surround, and DSP ports of Dolby Surround Pro Logic and Dolby SR noise reduction. J. Audio Eng. Soc., Vol. 51, No. 6, 2003 June THE PROCEEDINGS OF THE AES 18TH INTERNATIONAL CONFERENCE AUDIO FOR INFORMATION APPLIANCES Challenges, Solutions, and Opportunities n le o ilab a v A M CD -RO $40.00 Members $60.00 Nonmembers This conference looked at the new breed of devices, called information appliances, created by the convergence of consumer electronics, computing, and communications that are changing the way audio is created, distributed, and rendered. FOR ORDERING INFORMATION You can purchase the book and CD-ROM online at www.aes.org. For more information email Andy Veloz at aav@aes.org or tel: +1 212 661 8528 ext. 39. 569