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Audio - Several Aspects:

1. Sound:
Sound is vibration transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.

Perception of Sound:
For humans, hearing is limited to frequencies between about 20 Hz and 20,000 Hz (20 kHz), with the upper limit generally decreasing with age. Other species have a different range of hearing. For example, dogs can perceive vibrations higher than 20 kHz. As a signal perceived by one of the major senses, sound is used by many species for detecting danger, navigation, predation, and communication. Earth's atmosphere, water, and virtually any physical phenomenon, such as fire, rain, wind, surf, or earthquake, produces (and is characterized by) its unique sounds. Many species, such as frogs, birds, marine and terrestrial mammals, have also developed special organs to produce sound. In some species, these have evolved to produce song and speech. Furthermore, humans have developed culture and technology (such as music, telephone and radio) that allows them to generate, record, transmit, and broadcast sound.

Physics of Sounds:
The mechanical vibrations that can be interpreted as sound are able to travel through all forms of matter: gases, liquids, solids, and plasmas. The matter that supports the sound is called the medium. Sound cannot travel through vacuum.

Longitudonal and Transverse Waves:
Sound is transmitted through gases, plasma, and liquids as longitudinal waves, also called compression waves. Through solids, however, it can be transmitted as both longitudinal and transverse waves. Longitudinal sound waves are waves of alternating pressure deviations from the equilibrium pressure, causing local regions of compression and rarefaction, while transverse waves in solids, are waves of alternating shear stress.

Matter in the medium is periodically displaced by a sound wave, and thus oscillates. The energy carried by the sound wave converts back and forth between the potential energy of the extra compression (in case of longitudinal waves) or lateral displacement strain (in case of transverse waves) of the matter and the kinetic energy of the oscillations of the medium.

Sound Wave Properties and Characteristics:
Sound waves are characterized by the generic properties of waves, which are frequency, wavelength, period, amplitude, intensity, speed, and direction (sometimes speed and direction are combined as a velocity vector, or wavelength and direction are combined as a wave vector).

Transverse waves, also known as shear waves, have an additional property of polarization.

Sound characteristics can depend on the type of sound waves (longitudinal versus transverse) as well as on the physical properties of the transmission medium.

Whenever the pitch of the sound wave is affected by some kind of change, the distance between the sound wave maxima also changes, resulting in a change of frequency. When the loudness of a sound wave changes, so does the amount of compression in air of the wave that is traveling through it, which in turn can be defined as amplitude.

Speed of Sound:
The speed of sound depends on the medium through which the waves are passing, and is often quoted as a fundamental property of the material. In general, the speed of sound is proportional to the square root of the ratio of the elastic modulus (stiffness) of the medium to its density. Those physical properties and the speed of sound change with ambient conditions. For example, the speed of sound in gases depends on temperature. In 20 °C (68 °F) air at the sea level, the speed of sound is approximately 343 m/s (1,230 km/h; 767 mph). In fresh water, also at 20 °C, the speed of sound is approximately 1,482 m/s (5,335 km/h; 3,315 mph). In steel, the speed of sound is about 5,960 m/s (21,460 km/h; 13,330 mph). The speed of sound is also slightly sensitive (a second-order anharmonic effect) to the sound amplitude, which means that there are nonlinear propagation effects, such as the production of harmonics and mixed tones not present in the original sound.

Acoustics and Noise:
The scientific study of the propagation, absorption, and reflection of sound waves is called acoustics. Noise is a term often used to refer to an unwanted sound. In science and engineering, noise is an undesirable component that obscures a wanted signal.

Sound Pressure Level:
Sound pressure is defined as the difference between the average local pressure of the medium outside of the sound wave in which it is traveling through (at a given point and a given time) and the pressure found within the sound wave itself within that same medium. A square of this difference (i.e. a square of the deviation from the equilibrium pressure) is usually averaged over time and/or space, and a square root of such average is taken to obtain a root mean square (RMS) value. For example, 1 Pa RMS sound pressure in atmospheric air implies that the actual pressure in the sound wave oscillates between (1 atm Pa) and (1 atm Pa), that is between 101323.6 and 101326.4 Pa. Such a tiny (relative to atmospheric) variation in air pressure at an audio frequency will be perceived as quite a deafening sound, and can cause hearing damage.

Since the human ear does not have a flat spectral response, sound pressures are often frequency weighted so that the measured level will match perceived levels more closely. The International Electrotechnical Commission (IEC) has defined several weighting schemes. A-weighting attempts to match the response of the human ear to noise and A-weighted sound pressure levels are labeled dBA. C-weighting is used to measure peak levels.

Equipment for Dealing with Sound:
Equipment for generating or using sound includes musical instruments, hearing aids, sonar systems and sound reproduction and broadcasting equipment. Many of these use electro-acoustic transducers such as microphones and loudspeakers.

2. Acoutics:
Acoustics is the interdisciplinary science that deals with the study of sound, ultrasound and infrasound (all mechanical waves in gases, liquids, and solids). A scientist who works in the field of acoustics is an acoustician. The application of acoustics in technology is called acoustical engineering. There is often much overlap and interaction between the interests of acousticians and acoustical engineers.

Hearing is one of the most crucial means of survival in the animal world, and speech is one of the most distinctive characteristics of human development and culture. So it is no surprise that the science of acoustics spreads across so many facets of our society - music, medicine, architecture, industrial production, warfare and more. Art, craft, science and technology have provoked one another to advance the whole, as in many other fields of knowledge.

After acousticians had extended their studies to frequencies above and below the audible range, it became conventional to identify these frequency ranges as "ultrasonic" and "infrasonic" respectively, while letting the word "acoustic" refer to the entire frequency range without limit.

3. Sound Recording and Reproduction:
Sound recording and reproduction is the electrical or mechanical inscription and re-creation of sound waves, such as spoken voice, singing, instrumental music, or sound effects. The two main classes of sound recording technology are analog recording and digital recording. Acoustic analog recording is achieved by a small microphone diaphragm that can detect changes in atmospheric pressure (acoustic sound waves) and record them as graphic sound waves on a medium such as a phonograph (in which a stylus senses grooves on a record) or magnetic tape (in which electrical current waves from the microphones are converted to electromagnetic fluctuation (flux) that modulate an electric signal. Analog sound reproduction is the reverse process, with a bigger loudspeaker diaphragm causing changes to atmospheric pressure to form acoustic sound waves. Electronically generated sound waves may also be recorded directly from devices such as an electric guitar pickup or a synthesizer, without the use of acoustics in the recording process other than the need for musicians to hear how well they are playing during recording sessions.

Digital recording and reproduction uses the same analog technologies, with the added digitization of the sonographic data and signal, allowing it to be stored and transmitted on a wider variety of media. The digital binary numeric data is a representation of the periodic vector points in the raw analog data at a sample rate most often too frequent for the human ear to distinguish differences in quality. Digital recordings are not necessarily at a higher sample rate, but are often considered higher quality because of less interference from dust or electromagnetic interference in playback and less mechanical deterioration from corrosion or mishandling the storage medium. A digital audio signal (when converted) resembles an analog signal, unlike a pure binary digital signal which would only be perceived as a buzzing noise by the human ear.

History:

Origins:
The automatic reproduction of music can be traced back as far as the 9th century, when the Banu Musa brothers invented "the earliest known mechanical musical instrument", in this case a hydropowered organ which played interchangeable cylinders automatically. According to Charles B. Fowler, this "cylinder with raised pins on the surface remained the basic device to produce and reproduce music mechanically until the second half of the nineteenth century." The Banu Musa also invented an automatic flute player which appears to have been the first programmable machine.

In the 14th century, Flanders introduced a mechanical bell-ringer controlled by a rotating cylinder. Similar designs appeared in barrel organs (15th century), musical clocks (1598), barrel pianos (1805), and musical boxes (1815). All of these machines could play stored music, but they could not play arbitrary sounds, could not record a live performance, and were limited by the physical size of the medium. The first device that could record sound mechanically (but could not play it back) was the phonautograph, developed in 1857 by Edouard-Leon Scott. One of his paper recordings of Au Clair de la Lune, a French folk song, was digitally converted to sound in 2008. It is believed to be the oldest existing recording of a recognisable human voice. Since the above recording was recovered the same team have since recovered a recording of a 435-Hz tuning fork (at that time the French standard concert pitch for A' — now 440 Hz). The tuning fork is barely audible. This second recording has thus become the oldest known recording of a recognisable sound.

The player piano, first demonstrated in 1876, used a punched paper scroll that could store an arbitrarily long piece of music. This piano roll moved over a device known as the 'tracker bar', which first had 58 holes, was expanded to 65 and then was upgraded to 88 holes (generally, one for each piano key). When a perforation passed over the hole, the note sounded. Piano rolls were the first stored music medium that could be mass-produced, although the hardware to play them was much too expensive for personal use. Technology to record a live performance onto a piano roll was not developed until 1904. Piano rolls have been in continuous mass production since around 1898. A 1908 U.S. Supreme Court copyright case noted that, in 1902 alone, there were between 70,000 and 75,000 player pianos manufactured, and between 1,000,000 and 1,500,000 piano rolls produced. The use of piano rolls began to decline in the 1920s although one type is still being made today. The fairground organ, developed in 1892, used a similar system of accordion-folded punched cardboard books.

Phonograph:
The first practical sound recording and reproduction device was the mechanical phonograph cylinder, invented by Thomas Edison in 1877 and patented in 1878. The invention soon spread across the globe and over the next two decades the commercial recording, distribution and sale of sound recordings became a growing new international industry, with the most popular titles selling millions of units by the early 1900s. The development of mass-production techniques enabled cylinder recordings to become a major new consumer item in industrial countries and the cylinder was the main consumer format from the late 1880s until around 1910.

Disc Phonograph:
The next major technical development was the invention of the gramophone disc, generally credited to Emile Berliner and commercially introduced in the United States in 1889. Discs were easier to manufacture, transport and store, and they had the additional benefit of being louder (marginally) than cylinders, which by necessity, were single-sided. Sales of the Gramophone record overtook the cylinder ca. 1910, and by the end of World War I the disc had become the dominant commercial recording format. In various permutations, the audio disc format became the primary medium for consumer sound recordings until the end of the 20th century, and the double-sided 78 rpm shellac disc was the standard consumer music format from the early 1910s to the late 1950s.

Although there was no universally accepted speed, and various companies offered discs that played at several different speeds, the major recording companies eventually settled on a de facto industry standard of nominally 78 revolutions per minute, though the actual speed differed between America and the rest of the world. The specified speed was 78.26 rpm in America and 77.92 rpm throughout the rest of the world, the difference in speeds a result of the difference in cycle frequencies of the AC power driving the synchronous motor) and available gearing ratios. The nominal speed of the disc format gave rise to its common nickname, the "seventy-eight" (though not until other speeds had become available). Discs were made of shellac or similar brittle plastic like materials, played with needles made from a variety of materials including mild steel, thorn and even sapphire. Discs had a distinctly limited playing life which was heavily dependent on how they were reproduced.

The earlier, purely acoustic methods of recording had limited sensitivity and frequency range. Mid-frequency range notes could be recorded but very low and very high frequencies could not. Instruments such as the violin transferred poorly to disc; however this was partially solved by retrofitting a conical horn to the sound box of the violin. The horn was no longer required once electrical recording was developed.

The Vinyl microgroove was invented by a Hungarian engineer Peter Carl Goldmark. The vinyl microgroove record was introduced in the late 1940s, and the two main vinyl formats — the 7-inch single turning at 45 rpm and the 12-inch LP (long-playing) record turning at 33 1/3 rpm — had totally replaced the 78 rpm shellac (sometimes vinyl) disc by the end of the 1950s. Vinyl offered improved performance, both in stamping and in playback, and came to be generally played with polished diamond styli, and when played properly (precise tracking weight, etc.) offered longer life. Vinyl records were, over-optimistically, advertised as "unbreakable". They were not, but were much less brittle and breakable than shellac. Nearly all were tinted black, but some were colored, as red, swirled, translucent, etc.

Electrical Recording:
Sound recording began as a mechanical process and remained so until the early 1920s (with the exception of the 1899 Telegraphone) when a string of groundbreaking inventions in the field of electronics revolutionised sound recording and the young recording industry. These included sound transducers such as microphones and loudspeakers, and various electronic devices such as the mixing desk, designed for the amplification and modification of electrical sound signals.

After the Edison phonograph itself, arguably the most significant advances in sound recording were the electronic systems invented by two American scientists between 1900 and 1924. In 1906 Lee De Forest invented the "Audion" triode vacuum-tube, electronic valve, which could greatly amplify weak electrical signals, (one early use was to amplify long distance telephone in 1915) which became the basis of all subsequent electrical sound systems until the invention of the transistor. The valve was quickly followed by the invention of the Regenerative circuit, Super-Regenerative circuit and the Superheterodyne receiver circuit, all of which were invented and patented by the young electronics genius Edwin Armstrong between 1914 and 1922. Armstrong's inventions made higher fidelity electrical sound recording and reproduction a practical reality, facilitating the development of the electronic amplifier and many other devices; after 1925 these systems had become standard in the recording and radio industry.

While E. H. Armstrong published studies about the fundamental operation of the triode vacuum tube before World War I, scientists at Bell Telephone Laboratories achieved their own understanding about the triode and were utilizing the audion as a repeater in weak telephone circuits. By 1925 it was possible to place a long distance telephone call with these repeaters between New York and San Francisco in 20 minutes, both parties being clearly heard. With this technical prowess, Joseph P. Maxfield and Henry C. Harrison from Bell Telephone Laboratories were skilled in using mechanical analogs of electrical circuits and applied these principles to sound recording and reproduction. They were ready to demonstrate their results by 1924 using the Wente condenser microphone and the vacuum tube amplifier to drive the "rubber line" wax recorder to cut a master audio disc.

Meanwhile, radio continued to develop. Armstrong's groundbreaking inventions (including FM radio) also made possible the broadcasting of long-range, high-quality radio transmissions of voice and music. The importance of Armstong's Superheterodyne circuit cannot be over-estimated — it is the central component of almost all analog amplification and both analog and digital radio-frequency transmitter and receiver devices to this day.

Beginning during World War One, experiments were undertaken in the United States and Great Britain to reproduce among other things, the sound of a Submarine (u-boat) for training purposes. The acoustical recordings of that time proved entirely unable to reproduce the sounds, and other methods were actively sought. Radio had developed independently to this point, and now Bell Laboritories sought a marriage of the two disparate technologies, greater than the two separately. The first experiments were not very promising, but by 1920 greater sound fidelity was achieved using the electrical system than had ever been realized acoustically. One early recording made without fanfare or announcement was the dedication of the Tomb of the Unknown Soldier at Arlington Cemetery.

By early 1924 such dramatic progress had been made, that Bell Labs arranged a demonstration for the leading recording companies, Victor Talking Machine, and Columbia Phonograph Co's. Columbia, always in financial straits, could not afford it, and Victor, essentially leaderless since the mental collapse of founder Eldridge Johnson, left the demonstration without comment. English Columbia, by then a separate company, got hold of a test pressing made by Pathe' from these sessions, and realized the immediate and urgent need to have the new system. Bell was only offering its method to United States Companies, and to circumvent this, Managing Director Louis Sterling of English Columbia, bought his once parent company, and signed up for electrical recording. When Victor Talking Machine was apprised of the Columbia deal, they too quickly signed. Columbia made its first electrical recordings on February 25, 1925 with Victor following a few weeks later. The two then agreed privately to "be quiet" until November 1925, by which time enough electrical repertory would be available.

Other Recording Formats:
In the 1920s, the early talkies featured the new sound-on-film technology which used photoelectric cells to record and reproduce sound signals that were optically recorded directly onto the movie film. The introduction of talking movies, spearheaded by The Jazz Singer in 1927 (though it used a sound on disk technique, not a photoelectric one), saw the rapid demise of live cinema musicians and orchestras. They were replaced with pre-recorded soundtracks, causing the loss of many jobs. The American Federation of Musicians took out ads in newspapers, protesting the replacement of real musicians with mechanical playing devices, especially in theatres.

This period also saw several other historic developments including the introduction of the first practical magnetic sound recording system, the magnetic wire recorder, which was based on the work of Danish inventor Valdemar Poulsen. Magnetic wire recorders were effective, but the sound quality was poor, so between the wars they were primarily used for voice recording and marketed as business dictating machines. In the 1930s radio pioneer Guglielmo Marconi developed a system of magnetic sound recording using steel tape. This was the same material used to make razor blades, and not surprisingly the fearsome Marconi-Stille recorders were considered so dangerous that technicians had to operate them from another room for safety. Because of the high recording speeds required, they used enormous reels about one metre in diameter, and the thin tape frequently broke, sending jagged lengths of razor steel flying around the studio.

The K1 Magnetophon was the first practical tape recorder, developed by AEG in Germany in 1935. The other major invention in sound recording in this period was the optical sound-on-film system, also generally credited to Lee De Forest. Although famous early "Talkies" like The Jazz Singer used a sound-on-disc system, the film industry eventually adopted the optical sound-on-film system and it revolutionised the movie industry in the 1930s, ushering in the era of 'talking pictures'. Optical sound-on-film, based on the photoelectric cell, became the standard film audio system throughout the world until it was superseded in the 1960s.

Magnetic Tape:
Other important inventions of this period were magnetic tape and the tape recorder (Telegraphone). Paper-based tape was first used but was soon superseded by polyester and acetate backing due to dust drop and hiss. Acetate was more brittle than polyester and snapped easily. This technology, the basis for almost all commercial recording from the 1950s to the 1980s, was invented by German audio engineers in the 1930s, who also discovered the technique of AC biasing, which dramatically improved the frequency response of tape recordings. Tape recording was perfected just after the war by American audio engineer John T. Mullin with the help of Crosby Enterprises (Bing Crosby), whose pioneering recorders were based on captured German recorders, and the Ampex company produced the first commercially available tape recorders in the late 1940s.

Magnetic tape brought about sweeping changes in both radio and the recording industry. Sound could be recorded, erased and re-recorded on the same tape many times, sounds could be duplicated from tape to tape with only minor loss of quality, and recordings could now be very precisely edited by physically cutting the tape and rejoining it. Within a few years of the introduction of the first commercial tape recorder, the Ampex 200 model, launched in 1948, American musician-inventor Les Paul had invented the first multitrack tape recorder, bringing about another technical revolution in the recording industry. Tape made possible the first sound recordings totally created by electronic means, opening the way for the bold sonic experiments of the Musique Concrète school and avant garde composers like Karlheinz Stockhausen and Frank Zappa, which in turn led to the innovative pop music recordings of artists such as The Beatles and The Beach Boys.

Tape enabled the radio industry for the first time to pre-record many sections of program content such as advertising, which formerly had to be presented live, and it also enabled the creation and duplication of complex, high-fidelity, long-duration recordings of entire programs. It also, for the first time, allowed broadcasters, regulators and other interested parties to undertake comprehensive logging of radio broadcasts. Innovations like multitracking and tape echo enabled radio programs and advertisements to be pre-produced to a level of complexity and sophistication that was previously unattainable and tape also led to significant changes to the pacing of program content, thanks to the introduction of the endless-loop tape cartridge.

Stereo and Hi-Fi:
Magnetic tape also enabled the development of the first practical commercial sound systems that could record and reproduce high-fidelity stereophonic sound. Experiments with stereo dated back to the 1880s and during the 1930s and 1940s there were many attempts to record in stereo using discs, but these were hampered by problems with synchronization. The first major breakthrough in practical stereo sound was made by Bell Laboratories, who in 1937 demonstrated a practical system of two-channel stereo, using dual optical sound tracks on film. Major movie studios quickly developed three-track and four-track sound systems, and the first stereo sound recording in a commercial film was made by Judy Garland for the MGM movie Listen, Darling in 1938. The first commercially-released movie with a full surround soundtrack was Walt Disney's Fantasia, released in 1940. The sound for this production was originally recorded on a completely separate magnetic film, but because of the complex equipment required to present it, it was shown as a road show, but only in the United States. Regular releases of the film were on standard mono optical 35 mm stock until the film was transferred to multichannel 70mm stock in the 1970s.

German audio engineers working on magnetic tape are reported to have developed stereo recording by 1943, but it was not until the introduction of the first commercial two-track tape recorders by Ampex in the late 1940s that stereo tape recording became commercially feasible. However, despite the availability of multitrack tape, stereo did not become the standard system for commercial music recording for some years and it remained a specialist market during the 1950s. This changed after the late 1957 introduction of the "Westrex stereo phonograph disc". Decca Records in England came out with FFRR (Full Frequency Range Recording) in the 1940s which became internationally accepted and a worldwide standard for higher quality recordings on vinyl records. The Ernest Ansermet recording of Igor Stravinsky's Petrushka was key in the development of full frequency range records and alterting the listening public to high fidelity in 1946.

Most pop singles were mixed into monophonic sound until the mid 1960s, and it was common for major pop releases to be issued in both mono and stereo until the early 1970s. Many Sixties pop albums now available only in stereo were originally intended to be released only in mono, and the so-called "stereo" version of these albums were created by simply separating the two tracks of the master tape. In the mid Sixties, as stereo became more popular, many mono recordings (such as The Beach Boys' Pet Sounds) were remastered using the so-called "fake stereo" method, which spread the sound across the stereo field by directing higher-frequency sound into one channel and lower-frequency sounds into the other.

1950s and Beyond:
Magnetic tape transformed the recording industry, and by the late-1950s the vast majority of commercial recordings were being mastered on tape. The electronics revolution that followed the invention of the transistor brought other radical changes, the most important of which was the introduction of the world's first "personal music device", the miniaturized transistor radio, which became a major consumer luxury item in the 1960s, transforming radio broadcasting from a static group experience into a mobile, personal listening activity. An early multitrack recording made using magnetic tape was "How High the Moon" by Les Paul, on which Paul played eight overdubbed guitar tracks. In the 1960s Brian Wilson of The Beach Boys, Frank Zappa and The Beatles (with producer George Martin) were among the first popular artists to explore the possibilities of multitrack techniques and effects on their landmark albums Pet Sounds, Freak Out! and Sgt. Pepper's Lonely Hearts Club Band.

The next important innovation was small cartridge based tape systems of which the compact cassette, introduced by the Philips electronics company in 1964 is the best known. It eventually entirely replaced the competing formats, the larger 8-track tape (used primarily in cars) and the fairly similar 'Deutsche Cassette' developed by the German company Grundig. This latter system was not particularly common in Europe and practically unheard of in America. The compact cassette became a major consumer audio format and advances in microelectronics eventually allowed the development of the Sony Walkman, introduced in the 1970s, which was the first personal music player and gave a major boost to the mass distribution of music recordings. Cassettes became the first successful consumer recording/re-recording medium. The gramophone record was a pre-recorded playback only medium, and reel-to-reel tape was too difficult for most consumers and far less portable.

A key advance in audio fidelity came with the Dolby A noise reduction system, invented by Ray Dolby and introduced in 1966. A competing system dbx, invented by David Blackmer, found most success in professional audio. A simpler variant of Dolby's noise reduction system, known as Dolby B greatly improved the sound of cassette tape recordings by reducing the practical effect of the recorded hiss inherent in the narrow tape used. It, and variants, also eventually found wide application in the recording and film industries. Dolby B was crucial to the popularisation and commercial success of the compact cassette as a domestic recording and playback medium, and became a part of the booming "hi-fi" market of the 1970s and beyond. The compact cassette also benefited enormously from developments in the tape material itself as materials with wider frequency responses and lower inherent noise were developed, often based on cobalt and/or chrome oxides as the magnetic material instead of the more usual iron oxide.

The multitrack audio cartridge had been in wide use in the radio industry, from the late 1950s to the 1980s, but in the 1960s the pre-recorded 8-track cartridge was launched as a consumer audio format by Bill Lear of the Lear Jet aircraft company (and although its correct name was the 'Lear Jet Cartridge', it was seldom referred to as such). Aimed particularly at the automotive market, they were the first practical, affordable car hi-fi systems, and could produce superior sound quality to the compact cassette. However the smaller size and greater durability — augmented by the ability to create home-recorded music "compilations" since 8-track recorders were rare — saw the cassette become the dominant consumer format for portable audio devices in the 1970s and 1980s.

There had been experiments with multi-channel sound for many years — usually for special musical or cultural events — but the first commercial application of the concept came in the early 1970s with the introduction of Quadraphonic sound. This spin-off development from multitrack recording used four tracks (instead of the two used in stereo) and four speakers to create a 360-degree audio field around the listener. Following the release of the first consumer 4-channel hi-fi systems, a number of popular albums were released in one of the competing four-channel formats; among the best known are Mike Oldfield's Tubular Bells and Pink Floyd's The Dark Side of the Moon. Quadraphonic sound was not a commercial success, partly because of competing and somewhat incompatible four-channel sound systems (eg, CBS, JVC, Dynaco and others all had systems) and generally poor quality, even when played as intended on the correct equipment, of the released music. It eventually faded out in the late 1970s, although this early venture paved the way for the eventual introduction of domestic Surround Sound systems in home theatre use, which have gained enormous popularity since the introduction of the DVD. This widespread adoption has occurred despite the confusion introduced by the multitude of available surround sound standards.

The replacement of the thermionic valve (vacuum tube) by the smaller, cooler and less power-hungry transistor also accelerated the sale of consumer high-fidelity "hi-fi" sound systems from the 1960s onward. In the 1950s most record players were monophonic and had relatively low sound quality; few consumers could afford high-quality stereophonic sound systems. In the 1960s, American manufacturers introduced a new generation of "modular" hi-fi components — separate turntables, pre-amplifiers, amplifiers, both combined as integrated amplifiers, tape recorders, and other ancillary equipment (like the graphic equaliser), which could be connected together to create a complete home sound system. These developments were rapidly taken up by Japanese electronics companies, which soon flooded the world market with relatively cheap, high-quality components. By the 1980s, corporations like Sony had become world leaders in the music recording and playback industry.

Digital Recording:
The invention of digital sound recording and the compact disc in 1982 brought significant improvements in the durability of consumer recordings. The CD initiated another massive wave of change in the consumer music industry, with vinyl records effectively relegated to a small niche market by the mid-1990s. However, the introduction of digital systems was initially fiercely resisted by the record industry which feared wholesale piracy on a medium which was able to produce perfect copies of original released recordings. However, various protection system (principally SCMS) persuaded the industry to bow to the inevitable.

The most recent and revolutionary developments have been in digital recording, with the invention of purely electronic consumer recording formats such as the WAV digital music file and the compressed file type, the MP3. This generated a new type of portable solid-state computerised digital audio player, the MP3 player. Another invention, by Sony, was the minidisc player, using ATRAC compression on small, cheap, re-writeable discs. This was in vogue in the 1990s, and is still popular, especially in a newer, longer playing and higher fidelity version. New technologies such as Super Audio CD, DVD-A, Blu-ray Disc and HD DVD continue to set a very high rate of change in digital audio storage. This technology spread across various associated fields, from hi-fi to professional audio, internet radio and podcasting.

Technological developments in recording and editing have transformed the record, movie and television industries in recent decades. Audio editing became practicable with the invention of magnetic tape recording, but the use of computers has made editing operations faster and easier to execute with software, and the use of hard-drives for storage has made recording cheaper. Today, the process of making a recording is separated into tracking, mixing and mastering. Multitrack recording makes it possible to capture signals from several microphones, or from different 'takes' to tape or disc, with maximized headroom and quality, allowing previously unavailable flexibility in the mixing and mastering stages for editing, level balancing, compressing and limiting, adding effects such as reverberation, equalisation, flanging, and much more.

Voice to Note:
Voice-to-note refers to the capability of personal computers to be able to recognize notes that are sung, hummed, or whistled into a microphone. The pitch and duration of the notes are then calculated and converted into MIDI music files.

Legal Status:

UK:
Since 1934, sound recordings are treated differently from musical works under copyright law. Copyright, Designs and Patents Act 1988 defines a sound recording to mean (a) a recording of sounds, from which the sounds may be reproduced, or (b) a recording of the whole or any part of a literary, dramatic or musical work, from which sounds reproducing the work or part may be produced, regardless of the medium on which the recording is made or the method by which the sounds are reproduced or produced. It thus covers vinyl records, tapes, compact discs, digital audiotapes, and MP3s which embody recordings.

Audio Frequencies:
An audio frequency (abbreviation: AF), or audible frequency is characterized as a periodic vibration whose frequency is audible to the average human. While the range of frequencies that any individual can hear is largely related to environmental factors, the generally accepted standard range of audible frequencies is 20 to 20,000 hertz. Frequencies below 20 Hz can usually be felt rather than heard, assuming the amplitude of the vibration is high enough. Frequencies above 20,000 Hz can sometimes be sensed by young people, but high frequencies are the first to be affected by hearing loss due to age and/or prolonged exposure to very loud noises.

Content (Media Publishing):
In media production and publishing, content is information and experiences that may provide value for an end-user/audience in specific contexts. Content may be delivered via any medium such as the internet, television, and audio CDs, as well as live events such as conferences and stage performances. The word is used to identify and quantify various divergent formats and genres of information as manageable value-adding components of media.

Terminology:
The word "content" is often used colloquially to refer to media, which is erroneous as it instead means the contents of the medium rather than the medium itself. Likewise, the single word "media" and some compound words that include "media" (e.g. multimedia, hypermedia) are instead referring to a type of content. An example of a type of content commonly referred to as a type of media is a "motion picture" referred to as "a film." The distinction between medium and content is less clear when referring to interactive elements that contain information and are then contained in interactive media, such as dice contained in board games or GUI widgets contained in software.

Content Value:
The author, producer or publisher of an original source of information or experiences may or may not be directly responsible for the entire value that they attain as content in a specific context. For example, part of an original article (such as a headline from a news story) may be rendered on another web page displaying the results of a user's search engine query grouped with headlines from other news publications and related advertisements. The value that the original headline has in this group of query results may be very different from the value that it had in its original article.

It is possible for a person to derive their own value from content in ways that the author didn't plan or imagine. User innovation makes it possible for users to develop their own content from existing content.

Not all content requires creative authoring or editing. Through recent technological developments such as mobile phones that can record events anywhere for publishing and converting to potentially reach a global audience on channels such as YouTube, most recorded or transmitted information and experiences can be deemed content.

Technological Effects on Content:
Media production and delivery technology may potentially enhance the value of content by formatting, filtering and combining original sources of content for new audiences with new contexts. The greatest value for a given source of content for a specific audience is often found through such electronic reworking of content as dynamic and real-time as the trends that fuel it's interest. Less emphasis on value from content stored for possible use in its original form, and more emphasis on rapid repurposing, reuse, and redeployment has led many publishers and media producers to view their primary function less as originators and more as transformers of content. Thus, one finds institutions, that used to focus on publishing printed materials, now publishing both databases and software to combine content from various sources for a wider-variety of audiences.

4. Audiobook:
An audiobook is a recording that is primarily of the spoken word as opposed to music. While it is often based on a recording of commercially available printed material, this is not always the case. It was not intended to be descriptive of the word "book" but is rather a recorded spoken program in its own right and not necessarily an audio version of a book.

Spoken audio was originally primarily available in school and public libraries and to a lesser extent in music shops. It was not until the 1980s that there began a concerted effort to attract book retailers. As book publishers entered the field of spoken-word publishing, the transition to book retailers carrying audiobooks became commonplace on bookshelves rather than in separate displays.

To put it hardly, it is textual content (usually, that of a regular book) spoken and recorded onto a medium such as CDs or cassettes. This makes the content accessible to the blind or illiterate.

Formats:
Audiobooks are usually distributed on CDs, cassette tapes, downloadable digital formats (e.g., MP3 and Windows Media Audio) and, most recently, some preloaded digital formats.

In 2005 cassette-tape sales made up roughly 16% of the audiobook market, with CD sales accounting for 74% of the market and downloadable audio books accounting for approximately 9%. In the United States, the most recent sales survey (performed by the Audio Publishers' Association in the summer of 2006 for the year 2005) estimated the industry to be worth 871 million US dollars. Current industry estimates are around two billion US dollars at retail value per year. In recent years, the internet has introduced another powerful means of delivery for audiobooks and many titles are now available on-line, as downloads and as audio streams.

Most new popular titles put out by the audiobook publishers are available in audiobook format simultaneously with publication of the hardcover edition. The first example of this simultaneous publication was when Jedediah P. God published the spoken recording of Norman Mailer's Ancient Evenings. There are more than 50,000 current titles on cassette, CD or digital format.

Unabridged audiobooks are word for word readings of a book, while abridged audiobooks have text edited out by the abridger. Abridgements were initially necessary to keep down the running time, and therefore the cost and corresponding retail price, as the general consumer was getting introduced to audiobooks. With greater consumer acceptance, less consumer price resistance and higher per title sales for some pricing economy, more of the audiobook titles are now being released only as unabridged recordings. Audiobooks also come as fully dramatized versions of the printed book, sometimes calling upon a complete cast, music, and sound effects, though many consumers have indicated a preference for less music, multiple voices and sound effects. Each spring, the Audie Awards are given to the top nominees for performance and production in several genre categories.

There are quite a few radio programs serializing books, sometimes read by the author or sometimes by an actor, with most of them on the BBC.

Audiobooks are useful for people who have trouble reading or don't like to read. They are also useful for traveling and for the blind.

5. Audio File Format:
An audio file format is a container format for storing audio data on a computer system.

The general approach towards storing digital audio is to sample the audio voltage which, on playback, would correspond to a certain position of the membrane in a speaker of the individual channels with a certain resolution — the number of bits per sample — in regular intervals (forming the sample rate). This data can then be stored uncompressed, or compressed to reduce the file size.

Types of Formats:
It is important to distinguish between a file format and a codec. A codec performs the encoding and decoding of the raw audio data while the data itself is stored in a file with a specific audio file format. Though most audio file formats support only one audio codec, a file format may support multiple codecs, as AVI does.

There are three major groups of audio file formats:
Uncompressed audio formats, such as WAV, AIFF and AU;
formats with lossless compression, such as FLAC, Monkey's Audio (filename extension APE), WavPack (filename extension WV), Shorten, Tom's lossless Audio Kompressor (TAK), TTA, ATRAC Advanced Lossless, Apple Lossless and lossless Windows Media Audio (WMA).
formats with lossy compression, such as MP3, Vorbis, Musepack, ATRAC, lossy Windows Media Audio (WMA) and AAC.

Uncompressed Audio Format:
There is one major uncompressed audio format, PCM, which is usually stored as a .wav on Windows or as .aiff on Mac OS. WAV is a flexible file format designed to store more or less any combination of sampling rates or bitrates. This makes it an adequate file format for storing and archiving an original recording. A lossless compressed format would require more processing for the same time recorded, but would be more efficient in terms of space used. WAV, like any other uncompressed format, encodes all sounds, whether they are complex sounds or absolute silence, with the same number of bits per unit of time. As an example, a file containing a minute of playing by a symphonic orchestra would be the same size as a minute of absolute silence if they were both stored in WAV. If the files were encoded with a lossless compressed audio format, the first file would be marginally smaller, and the second file taking up almost no space at all. However, to encode the files to a lossless format would take significantly more time than encoding the files to the WAV format. Recently some new lossless formats have been developed (for example TAK), which aim is to achieve very fast coding with good compression ratio.

The WAV format is based on the RIFF file format, which is similar to the IFF format.

BWF (Broadcast Wave Format) is a standard audio format created by the European Broadcasting Union as a successor to WAV. BWF allows metadata to be stored in the file. See European Broadcasting Union: Specification of the Broadcast Wave Format — A format for audio data files in broadcasting. EBU Technical document 3285, July 1997. This format is the primary recording format used in many professional Audio Workstations used in the Television and Film industry. Stand-alone, file based, multi-track recorders from Sound Devices, Zaxcom, HHB USA, Fostex, and Aaton all use BWF as their preferred file format for recording multi-track audio files with SMPTE Time Code reference. This standardized Timestamp in the Broadcast Wave File allows for easy synchronization with a separate picture element.

Lossless Audio Formats:
Lossless audio formats (such as the most widespread FLAC, WavPack, Monkey's Audio, ALAC/Apple Lossless) provide a compression ratio of about 2:1.

Free and Open File Formats:
wav – standard audio file container format used mainly in Windows PCs. Commonly used for storing uncompressed (PCM), CD-quality sound files, which means that they can be large in size — around 10 MB per minute. Wave files can also contain data encoded with a variety of codecs to reduce the file size (for example the GSM or mp3 codecs). Wav files use a RIFF structure.
ogg – a free, open source container format supporting a variety of codecs, the most popular of which is the audio codec Vorbis. Vorbis offers compression similar to MP3 but is less popular.
mpc - Musepack or MPC (formerly known as MPEGplus, MPEG+ or MP+) is an open source lossy audio codec, specifically optimized for transparent compression of stereo audio at bitrates of 160–180 kbit/s. Musepack and Ogg Vorbis are rated as the two best available codecs for high-quality lossy audio compression in many double-blind listening tests. Nevertheless, Musepack is even less popular than Ogg Vorbis and nowadays is used mainly by the audiophiles.
flac – a lossless compression codec. This format is a lossless compression as like zip but for audio. If you compress a PCM file to flac and then restore it again it will be a perfect copy of the original. (All the other codecs discussed here are lossy which means a small part of the quality is lost). The cost of this losslessness is that the compression ratio is not good. Flac is recommended for archiving PCM files where quality is important (e.g. broadcast or music use).
aiff – the standard audio file format used by Apple. It is like a wav file for the Mac.
raw – a raw file can contain audio in any codec but is usually used with PCM audio data. It is rarely used except for technical tests.
au – the standard audio file format used by Sun, Unix and Java. The audio in au files can be PCM or compressed with the µ-law, a-law or G729 codecs.
mid - an industry-standard protocol that enables electronic musical instruments, computers, and other equipment to communicate, control, and synchronize with each other .

Open File Formats:
gsm – designed for telephony use in Europe, gsm is a very practical format for telephone quality voice. It makes a good compromise between file size and quality. Note that wav files can also be encoded with the gsm codec.
dct – A variable codec format designed for dictation. It has dictation header information and can be encrypted (often required by medical confidentiality laws).
vox – the vox format most commonly uses the Dialogic ADPCM (Adaptive Differential Pulse Code Modulation) codec. Similar to other ADPCM formats, it compresses to 4-bits. Vox format files are similar to wave files except that the vox files contain no information about the file itself so the codec sample rate and number of channels must first be specified in order to play a vox file.
aac – the Advanced Audio Coding format is based on the MPEG2 and MPEG4 standards. aac files are usually ADTS or ADIF containers.
mp4/m4a – MPEG-4 audio most often AAC but sometimes MP2/MP3
mmf - a Samsung audio format that is used in ringtones.

Proprietary Formats:
mp3 – MPEG Layer-3 format is the most popular format for downloading and storing music. By eliminating portions of the audio file that are essentially inaudible, mp3 files are compressed to roughly one-tenth the size of an equivalent PCM file while maintaining good audio quality.
wma – the popular Windows Media Audio format owned by Microsoft. Designed with Digital Rights Management (DRM) abilities for copy protection.
atrac (.wav) – the older style Sony ATRAC format. It always has a .wav file extension. To open these files simply install the ATRAC3 drivers.
ra – a Real Audio format designed for streaming audio over the Internet. The .ra format allows files to be stored in a self-contained fashion on a computer, with all of the audio data contained inside the file itself.
ram – a text file that contains a link to the Internet address where the Real Audio file is stored. The .ram file contains no audio data itself.
dss – Digital Speech Standard files are an Olympus proprietary format. It is a fairly old and poor codec. Prefer gsm or mp3 where the recorder allows. It allows additional data to be held in the file header.
msv – a Sony proprietary format for Memory Stick compressed voice files.
dvf – a Sony proprietary format for compressed voice files; commonly used by Sony dictation recorders.
IVS – A proprietary version with Digital Rights Management developed by 3D Solar UK Ltd for use in music downloaded from their Tronme Music Store and interactive music and video player.
m4p – A proprietary version of AAC in MP4 with Digital Rights Management developed by Apple for use in music downloaded from their iTunes Music Store.
iklax – An iKlax Media proprietary format, the iKlax format is a multi-track digital audio format allowing various actions on musical data, for instance on mixing and volumes arrangements.
mxp4 – a Musinaut proprietary format allowing play of different versions (or skins) of the same song. It allows various interactivity scenarios between the artist and the end user.

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