Alan Blumlein

Alan Dower Blumlein (29 June 1903 – 7 June 1942) was an English electronics engineer, notable for his many inventions in telecommunications, sound recording, stereophonic sound, television and radar.[1] He received 128 patents and was considered as one of the most significant engineers and inventors of his time.[2][3]

He died during World War II on 7 June 1942, aged 38, during the secret trial of an H2S airborne radar system then under development, when all on board the Halifax bomber he was flying in were killed when it crashed at Welsh Bicknor in Herefordshire.[3]

Alan Dower Blumlein
Born29 June 1903
Hampstead, London, England
Died7 June 1942 (aged 38)
Welsh Bicknor, Herefordshire, England
EducationHighgate School
Imperial College London
OccupationElectronic Engineer
Spouse(s)Doreen Lane
ChildrenSimon Blumlein
David Blumlein
Parent(s)Semmy Blumlein
Jessie Dower
Engineering career
ProjectsH2S radar
Significant designUltra–Linear amplifier
Significant advanceStereophonic sound

Early life

Alan Dower Blumlein was born on 29 June 1903 in Hampstead, London. His father, Semmy Blumlein, was a German-born naturalised British subject. Semmy was the son of Joseph Blumlein, a German of Jewish descent, and Philippine Hellmann, a French woman of German descent.[Note 1] Alan's mother, Jessie Dower, was Scottish, daughter of William Dower (born 1837) who went to South Africa for the London Missionary Society. Alan was christened as a Presbyterian; he later married in a Church of England parish church.[4]

Alan Blumlein's future career seemed to have been determined by the age of seven, when he presented his father with an invoice for repairing the doorbell, signed "Alan Blumlein, Electrical Engineer" (with "paid" scrawled in pencil). His sister claimed that he could not read proficiently until he was 12. He replied "no, but I knew a lot of quadratic equations!"

After leaving Highgate School in 1921, he studied at City and Guilds College (part of Imperial College). He won a Governors' scholarship and joined the second year of the course. He graduated with a First-Class Honours BSc two years later.

In mid-1930, Blumlein met Doreen Lane, a preparatory school teacher five years his junior. After two-and-a-half years of courtship the two were married in 1933. Lane was warned by acquaintances before the wedding that, "There was a joke amongst some of his friends, they used to call it 'Blumlein-itis' or 'First Class Mind'. It seems that he didn't want to know anyone who didn't have a first class mind." Recording engineer Joseph B. Kaye, known as J. B. Kaye, who was Blumlein's closest friend and best man at the wedding, thought the couple were well matched.[5][6]

Career and inventions


In 1924 Blumlein started his first job at International Western Electric, a division of the Western Electric Company. The company subsequently became International Standard Electric Corporation and then, later on, Standard Telephones and Cables (STC).

During his time there, he measured the amplitude/frequency response of human ears, and used the results to design the first weighting networks.

In 1924 he published (with Professor Edward Mallett) the first of his only two IEE papers, on high-frequency resistance measurement. This won him the IEE's Premium award for innovation. The following year he wrote (with Norman Kipping) a series of seven articles for Wireless World.

In 1925 and 1926, Blumlein and John Percy Johns designed an improved form of loading coil which reduced loss and crosstalk in long-distance telephone lines. These were used until the end of the analogue telephony era. The same duo also invented an improved form of AC measurement bridge which became known as the Blumlein Bridge and subsequently the transformer ratio arm bridge. These two inventions were the basis for Blumlein's first two patents.

His inventions while working at STC resulted in another five patents, which were not awarded until after he left the company in 1929.

Sound recording

In 1929 Blumlein resigned from STC and joined the Columbia Graphophone Company, where he reported directly to general manager Isaac Shoenberg.

His first project was to find a method of disc cutting that circumvented a Bell patent in the Western Electric moving-iron cutting head then used, and on which substantial royalties had to be paid. He invented the moving-coil disc cutting head, which not only got around the patent but offered greatly improved sound quality. He led a small team which developed the concept into a practical cutter. The other principal team members were Herbert Holman and Henry "Ham" Clark. Their work resulted in several patents.

Early in 1931, the Columbia Graphophone Company and the Gramophone Company merged and became EMI. New joint research laboratories were set up at Hayes and Blumlein was officially transferred there on 1 November the same year.

During the early 1930s Blumlein and Herbert Holman developed a series of moving-coil microphones, which were used in EMI recording studios and by the BBC at Alexandra Palace.

Ultra-linear amplifier

In June 1937, Blumlein patented the Ultra-Linear amplifier (US Patent 2,218,902, dated 5 June 1937). A deceptively simple design, the circuit provided a tap on the primary winding of the output transformer to provide feedback to the second grid, which improved the amplifier's linearity. With the tap placed at the anode end of the primary winding, the tube (valve) is effectively connected as a triode, and if the tap was at the supply end, as a pure pentode. Blumlein discovered that if the tap was placed at a distance 15–20% down from the supply end of the output transformer, the tube or valve would combine the positive features of both the triode and the pentode design.

Long-tailed pair

Blumlein may or may not have invented the long-tailed pair, but his name is on the first patent (1936). The long-tailed pair is a form of differential amplifier that has been popular since the days of the vacuum tube (valve). It is now more pervasive than ever, as it is particularly suitable for implementation in integrated circuit form, and almost every operational amplifier integrated circuit contains at least one.

Stereophonic sound

In 1931, Blumlein invented what he called "binaural sound", now known as stereophonic sound.[1] In early 1931, he and his wife were at the cinema. The sound reproduction systems of the early talkies only had a single set of speakers – the actor might be on one side of the screen, but the voice could come from the other. Blumlein declared to his wife that he had found a way to make the sound follow the actor.

Blumlein explained his ideas to Isaac Shoenberg in the late summer of 1931. His earliest notes on the subject are dated 25 September 1931, and his patent had the title "Improvements in and relating to Sound-transmission, Sound-recording and Sound-reproducing Systems". The application was dated 14 December 1931, and was accepted on 14 June 1933 as UK patent number 394,325.[7]

The patent covered numerous ideas in stereo, some of which are used today. Some 70 claims include:

  • A "shuffling" circuit, which aimed to preserve the directional effect when sound from a spaced pair of microphones was reproduced via stereo loudspeakers instead of a pair of headphones
  • The use of a coincident pair of velocity microphones with their axes at right angles to each other, which is still known as a "Blumlein Pair"
  • Recording two channels in the single groove of a record using the two groove walls at right angles to each other and 45 degrees to the vertical
  • A stereo disc-cutting head
  • Using hybrid transformers to matrix between left and right signals and sum and difference signals

Blumlein's binaural experiments began in early 1933, and the first stereo discs were cut later the same year. Much of the development work on this system for cinematic use was completed by 1935. In Blumlein's short test films (most notably, "Trains at Hayes Station", which lasts 5 minutes 11 seconds, and, "The Walking & Talking Film"), his original intent of having the sound follow the actor was fully realised.[8]

In 1934, Blumlein recorded Mozart's Jupiter Symphony conducted by Sir Thomas Beecham at Abbey Road Studios in London using his vertical-lateral technique.[1]


Television was developed by many individuals and companies throughout the 1920s and 1930s. Blumlein's contributions, as a member of the EMI team, started in earnest in 1933 when his boss, Isaac Shoenberg, assigned him full-time to TV research.

His ideas included:

  • Resonant flyback scanning (the use of a tuned circuit in the creation of a sawtooth deflection waveform). (British Patent No. 400976, application filed April 1932.)
  • Use of constant-impedance network in power supplies to obtain voltage regulation independent of load frequency, extending down to DC (421546, filed 16 June 1933).
  • Black-level clamping (422914, filed 11 July 1933 by Blumlein, Browne and Hardwick). This is an improved form of DC restoration, compared to the simple DC restorer (consisting of a capacitor, diode and resistor) which had been patented by Peter Willans three months earlier.
  • The slot antenna. (515684, filed 7 March 1939.)

Blumlein was also largely responsible for the development of the waveform structure used in the 405-line Marconi-EMI system – developed for the UK's BBC Television Service at Alexandra Palace, the world's first scheduled "high definition" (240 lines or better) television service – which was later adopted as the CCIR System A.

H2S radar

Blumlein was so central to the development of the H2S airborne radar system (to aid bomb targeting), that after his death in June 1942, many believed that the project would fail. However it survived and was a factor in shortening the Second World War. Blumlein's role in the project was a closely guarded secret at the time and consequently only a brief announcement of his death was made some two years later, to avoid providing solace to Hitler.[3]

His invention of the line type pulse modulator (ref vol 5 of MIT Radiation Laboratory series) was a major contribution to high-powered pulse radars, not just the H2S's system, and continues to be used today.

Death and investigation

Halifax V9977
Halifax II V9977, which crashed on 7 June 1942, pictured at RAF Defford. Note the H2S radome under the belly.[3]

Blumlein was killed in the crash of an H2S-equipped Handley Page Halifax test aircraft while making a test flight for the Telecommunications Research Establishment (TRE) on 7 June 1942. During the flight from RAF Defford, whilst at an altitude of 500 ft the Halifax developed an engine fire which rapidly grew out of control.[9] The aircraft was seen to lose altitude, then rolled inverted and struck the ground.[9] The crash occurred near the village of Welsh Bicknor in Herefordshire.[10] Two of Blumlein's colleagues, Cecil Oswald Browne and Frank Blythen also died in the crash.

The Halifax was carrying a highly-secret cavity magnetron as part of the H2S test system, and the immediate recovery of the device was essential. A team led by Bernard Lovell arrived at the crash scene the same night, and took the magnetron.[11]

"Then reports of a crash in south Wales began to come in and the rest of that night was just a nightmare. I was driven by the C-in-C of the aerodrome [Defford], a man called King,[12] and winding through these lanes near Ross-on-Wye searching for this wreckage, and then the field with the burnt-out Halifax, and of course it was wartime, there was no time for emotions, our first duties were to search for the precious highly-secret equipment, and collect the bits-and-pieces of it." - Bernard Lovell. [13]

After the RAF investigative board completed its report on the Halifax crash on 1 July 1942, it was distributed to a restricted list of approved recipients, but not publicly divulged.[9] In the interests of wartime secrecy, the announcement of Blumlein's death was not made for another three years.[14] The investigative board, headed by AIB Chief Inspector Vernon Brown – who later also investigated the post-war Star Tiger and Star Ariel disappearances – and assisted by Rolls-Royce, who had made the Halifax's Merlin engines, found that the crash was caused by engine fire, attributed to the unscrewing of a tappet nut on the starboard outer engine, which had been improperly tightened by an RAF engine fitter while inspecting the engine some three hours prior to the crash.[9]

During the flight the loosened nut caused increasingly excessive valve clearance eventually allowing collision of the valve head with the rising piston fracturing the valve stem, which then allowed the inlet valve to drop open, resulting in the ignition by the spark plug of the pressurised fuel/air mixture within the inlet manifold and, eventually, the pumping of the ignited fuel outboard of the rocker cover and along the outside of the engine, leading to an extensive fire in the engine nacelle.[9] Due to the fire originating in the induction system, where the supercharged fuel/air mixture was at higher pressure than atmospheric, the heart of the fire was much hotter burning and intense than would be the case in a simple fuel fire.

Constantly fuelled by the broken intake, the fire burned rapidly along the wing and fuselage, eventually causing the outboard section of the starboard wing to separate from the centre section at approximately 350 feet of altitude.[9] With the loss of a substantial part of the starboard wing, all control over level flight was lost, and the plane rolled inverted and struck the ground at approximately 150 mph.[9]

The board found that the crew and passengers had not jumped immediately from the aircraft owing to several factors, including a loss of altitude while attempting to find an emergency field, the rapidly spreading fire, which blocked or impeded egress from the plane, and the fact that a sufficient number of parachutes were either not on board or were not being worn.[9] Almost immediately following the crash, Prime Minister Churchill issued a directive requiring any test flights with civilians or scientific personnel to carry a sufficient number of parachutes for all individuals involved.[9]

After the RAF investigative board completed its report on the Halifax crash, it was ordered to be kept secret by Prime Minister Churchill, and the cause of the crash was not revealed publicly, even to the relatives of the deceased.[9] As a result, numerous unfounded rumours of German sabotage as the cause of the crash would circulate for many years afterwards.[9]

Memorial window in Goodrich Castle
Memorial window at Goodrich Castle commemorating Blumlein and the other engineers, scientists and servicemen involved in radar development

Personal life

Alan Blumlein had two sons, Simon Blumlein and David Blumlein, Headmaster Emeritus of a prep school in Ealing, London.

Outside his work Blumlein was a lover of music and he attempted to learn to play the piano, but gave it up. He enjoyed horse riding and occasionally went cub hunting with his father-in-law.[15]

He was interested in many forms of engineering, including aviation, motor engineering and railway engineering. He obtained a pilot's licence and flew Tiger Moth aircraft of the London Aerodrome Club at Stag Lane Aerodrome.[16] On one occasion, he persuaded a bus driver to allow him to drive the vehicle from Penzance to Land's End. On another he spent several hours assisting the operator of a railway signal box in his duties at Paddington Station.[17]


ALAN DOWER BLUMLEIN 1903-1942 Electronics Engineer and Inventor lived here
Blue plaque at Blumlein's former home in Ealing

See also


  1. ^ Semmy Blumlein's father, Joseph B. Blumlein was Jewish, see Burns, p. 2


  1. ^ a b c "Early stereo recordings restored". BBC. 1 August 2008. Archived from the original on 7 August 2008. Retrieved 7 August 2008. Blumlein lodged the patent for 'binaural sound', in 1931, in a paper which patented stereo records, stereo films and also surround sound. He and his colleagues then made a series of experimental recordings and films to demonstrate the technology, and see if there was any commercial interest from the fledgling film and audio industry.
  2. ^ "Alan Blumlein – the man who invented stereo". AbbeyRoad. Retrieved 18 May 2009. In his short life, Blumlein devised over 120 patents and is considered as one of the most significant engineers of his time.
  3. ^ a b c d Fox, Barry (16 June 1990). "Mystery of the missing biography: A look at the life of Alan Blumlein" (1721). New Scientist. Retrieved 19 June 2009.
  4. ^ Burns (2000), pp. 2 and 4
  5. ^ Burns (2000), p.49
  6. ^ Alexander (1999), p. 10 and Chapter 3
  7. ^ ‹See Tfd›GB patent 394325, ‹See Tfd›Alan Dower Blumlein, "Improvements in and relating to Sound-transmission, Sound-recording and Sound-reproducing Systems.", issued 14 June 1933, assigned to Alan Dower Blumlein and Musical Industries, Limited
  8. ^ Robert Alexander (2013). "The Inventor of Stereo: The Life and Works of Alan Dower Blumlein". p. 83. CRC Press,
  9. ^ a b c d e f g h i j k Alexander (1999), pp.322–339
  10. ^ "The crash of Halifax V9977". The Official Alan Dower Blumlein website. Archived from the original on 8 April 2013. Retrieved 17 November 2012.
  11. ^ Burns (2000), p. 461
  12. ^ Note; Gp. Capt. P J R King.
  13. ^ The Secret War, Episode 2 - "To See A Hundred Miles", BBC Television, 1977.
  14. ^ Fox, Barry, The Briton Who Invented Electronics, New Scientist, Vol. 94 No. 1308 (3 June 1982), p. 641
  15. ^ Burns (2000), p.242
  16. ^ Burns (2000), p.234
  17. ^ Burns (2000), p.243
  18. ^ "Blumlein, Alan Dower". English Heritage. Retrieved 22 July 2015.
  19. ^ "New IET London: Savoy Place room names confirmed". IET. Retrieved 22 July 2015.

External links

1903 in the United Kingdom

Events from the year 1903 in the United Kingdom.

1931 in science

The year 1931 in science and technology involved some significant events, listed below.

1937 in science

The year 1937 in science and technology involved some significant events, listed below.

1942 in the United Kingdom

Events from the year 1942 in the United Kingdom. The year was dominated by the Second World War.

405-line television system

The 405-line monochrome analogue television broadcasting system was the first fully electronic television system to be used in regular broadcasting.

It was introduced with the BBC Television Service in 1936, suspended for the duration of World War II, and remained in operation in the UK until 1985. It was also used between 1961 and 1982 in Ireland, as well as from 1957 to 1973 for the Rediffusion Television cable service in Hong Kong.

Sometimes called the Marconi-EMI system, it was developed in 1934 by the EMI Research Team led by Sir Isaac Shoenberg. The figure of 405 lines had been chosen following discussions over Sunday lunch at the home of Alan Blumlein. The system used interlacing; EMI had been experimenting with a 243-line all-electronic interlaced system since 1933. In the 405 system the scanning lines were broadcast in two complementary fields, 50 times per second, creating 25 frames per second. The actual image was 377 lines high and interlaced, with additional unused lines making the frame up to 405 lines to give the slow circuitry time to prepare for the next frame; in modern terms it would be described as 377i.

At the time of its introduction the 405-line system was referred to as "high definition" - which it was, compared to earlier systems, although of lower definition than 625-line and later standards.

Arnold Wilkins

Arnold Frederic Wilkins OBE (20 February 1907 – 5 August 1985) was a pioneer in developing the use of radar. It was Arnold Wilkins who suggested to his boss, Robert Watson Watt, that reflected radio waves might be used to detect aircraft, and his idea led to the initial steps in developing ground-to-air radar in the UK. Wilkins also provided all the theoretical calculations to back-up his idea of aircraft detection, and it was his lashed-up system that he used in the Daventry Experiment to demonstrate that his idea would work. With the Daventry experiment, Wilkins successfully detected an aircraft (up to eight miles away) by reflection of radio waves for the first time in history.


Blumlein is a surname. Notable people with the surname include:

Alan Blumlein, electronic engineer

Blumlein Pair, a stereo recording technique invented by Alan Blumlein

Blumlein transmission line, used to create high-voltage pulses with short rise and fall times

Michael Blumlein, fiction writer and physician

Blumlein pair

Blumlein pair is the name for a stereo recording technique invented by Alan Blumlein for the creation of recordings that, upon replaying through headphones or loudspeakers, recreate the spatial characteristics of the recorded signal.

The pair consists of an array of two matched microphones that have a bi-directional (figure-8) pickup pattern. They are positioned 90° from each other. Ideally, the transducers should occupy the same physical space; since this cannot be achieved, the microphone capsules are placed as close to each other as physically possible, generally with one centered directly above the other. The array is oriented so that the line bisecting the angle between the two microphones points towards the sound source to be recorded (see diagram). The pickup patterns of the pair, combined with their positioning, delivers a high degree of stereo separation in the source signal, as well as the room ambiance.

The Blumlein pair produces an exceptionally realistic stereo image, but the quality of recordings is highly dependent on the acoustics of the room and the size of the sound source.

Both ribbon and condenser microphones can be used for Blumlein-pair recording. A few types of stereo ribbon microphones (B & O, Royer, AEA) have even been purpose-built for just this type of recording. Several types of stereo condenser microphone (Neumann, AKG, Schoeps) have also offered a Blumlein arrangement as one of their possible configurations. Individual microphones with variable patterns (such as those from Pearl/Milab) can be used. The Soundfield microphone used to make Ambisonic recordings can be adjusted to mimic two microphones of any pattern at any angle to each other, including a Blumlein pair.

In his early experiments at EMI with what he called "binaural" sound, Blumlein did not use this actual technique because he did not have access to figure-8 microphones. This meant that he had to develop ways of using omnidirectional microphones to record what we now know as stereo sound. In the claims he made in his U.K. patent application in 1931, as well as details of these techniques, he included the theoretical possibility of using directional microphones in what later became known as a Blumlein pair. During the period when Blumlein's patent (British Patent 394325) was being written, Harry F. Olson published a patent for the first practical ribbon microphone, and much of the later experimental work was carried out with this type of microphoneIt is unclear when this approach became known as the Blumlein pair, although it does not appear to have been known by that name during his lifetime.

David Hafler

David Hafler (February 7, 1919 – May 25, 2003) was an American audio engineer. He was best known for his work on an improved version of the Williamson amplifier using the ultra-linear circuit of Alan Blumlein.

History of multitrack recording

Multitrack recording of sound is the process in which sound and other electro-acoustic signals are captured on a recording medium such as magnetic tape, which is divided into two or more audio tracks that run parallel with each other. Because they are carried on the same medium, the tracks stay in perfect synchronisation, while allowing multiple sound sources to be recorded asynchronously. The first system for creating stereophonic sound (using telephone technology) was demonstrated by Clément Ader in Paris in 1881. The pallophotophone, invented by Charles A. Hoxie and first demonstrated in 1922, recorded optically on 35 mm film, and some versions used a format of as many as twelve tracks in parallel on each strip. The tracks were recorded one at a time in separate passes and were not intended for later mixdown or stereophony; as with later half-track and quarter-track monophonic tape recording, the multiple tracks simply multiplied the maximum recording time possible, greatly reducing cost and bulk. British EMI engineer Alan Blumlein patented systems for recording stereophonic sound and surround sound on disc and film in 1933. The history of modern multitrack audio recording using magnetic tape began in 1943 with the invention of stereo tape recording, which divided the recording head into two tracks.

The next major development in multitrack recording came in the mid-1950s, when the Ampex corporation devised the concept of 8-track recording, utilizing its "Sel-Sync" (Selective Synchronous) recording system, and sold the first such machine to musician Les Paul. However, for the next 35 years, multitrack audio recording technology was largely confined to specialist radio, TV and music recording studios, primarily because multitrack tape machines were both very large and very expensive - the first Ampex 8-track recorder, installed in Les Paul's home studio in 1957, cost a princely US$10,000 - roughly three times the US average yearly income in 1957, and equivalent to around $90,000 in 2016. However, this situation changed radically in 1979 with the introduction of the TASCAM Portastudio, which used the inexpensive compact audio cassette as the recording medium, making good-quality 4-track (and later 8-track) multitrack recording available to the average consumer for the first time. Ironically, by the time the Portastudio had become popular, electronics companies were already introducing digital audio recording systems, and by the 1990s, computer-based digital multitrack recording systems such as Pro Tools and Cubase were being adopted by the recording industry, and soon became standard. By the early 2000s, rapid advances in home computing and digital audio software were making digital multitrack audio recording systems available to the average consumer, and high-quality digital multitrack recording systems like GarageBand were being included as a standard feature on home computers.

Jecklin disk

A Jecklin disk is a sound-absorbing disk placed between two microphones to create an acoustic "shadow" from one microphone to the other. The resulting two signals can possibly produce a pleasing stereo effect. A matching pair of small-diaphragm omnidirectional microphones is always used with a Jecklin disk.

The technique was invented by Jürg Jecklin, the former chief sound engineer of Swiss Radio now teaching at the University for Music and Performing Arts in Vienna. He referred to the technique as an "Optimal Stereo Signal" (OSS). In the beginning Jecklin used omnidirectional microphones on either side of a 30 cm (1 ft.) disk about 2 cm (3/4") thick, which had a muffling layer of soft plastic foam or wool fleece on each side. The capsules of the microphones were above the surface of the disc, just in the center, 16.5 centimeters (6½") apart from each other and each pointing 20 degrees outside. Jecklin found the 16.5 cm (6½") ear spacing between the microphones too narrow. In his own paper, he notes that the disk has to be 35 cm (13¾") in diameter and the distance between the microphones should be 36 cm (14 3/16"). The concept is to make use of the baffle to recreate some of the frequency-response, time and amplitude variations human listeners experience, but in such a way that the recording also produces a useful stereo image through loudspeakers. Conventional binaural or dummy head recordings are not as convincing when played back over speakers; headphone playback is needed.

The Jecklin disk is a refinement of the baffled microphone technique for stereo initially described by Alan Blumlein in his 1931 patent on binaural sound.

There is a noteworthy change from the original small version: Instead of 30 cm, the disk now has a slightly larger diameter of 35 cm. But what stands out to an even greater degree, is the greatly enlarged microphone spacing – rather than formerly 16.5 cm as a human "head diameter" (ear distance) there is now a distance of 36 cm (double-headed?). Jecklin's German from his script: "Zwei Kugelmikrofone sind mit einem gegenseitigen Abstand von 36 cm angeordnet und durch eine mit Schaumstoff belegte Scheibe von 35 cm Durchmesser akustisch getrennt." Translated: Two omnidirectional microphones are placed with a distance between them of 36 cm (14 3/16"), and acoustically separated by a foam-covered disk having a diameter of 35 cm (13¾"). That shows a great difference to the initial smaller Jecklin Disk of 30 centimeters diameter and the distance between the microphones of 16.5 centimeters.

June 7

June 7 is the 158th day of the year (159th in leap years) in the Gregorian calendar. There are 207 days remaining until the end of the year.

List of electrical engineers

This is a list of electrical engineers (by no means exhaustive), people who have made notable contributions to electrical engineering or computer engineering.

Michael Gerzon

Michael Anthony Gerzon (4 December 1945 – 6 May 1996) is probably best known for his work on Ambisonics and for his work on digital audio. He also made a large number of recordings, many in the field of free improvisation in which he had a particular interest.

Slot antenna

A slot antenna consists of a metal surface, usually a flat plate, with one or more holes or slots cut out. When the plate is driven as an antenna by a driving frequency, the slot radiates electromagnetic waves in a way similar to a dipole antenna. The shape and size of the slot, as well as the driving frequency, determine the radiation pattern. Often the radio waves are provided by a waveguide, and the antenna consists of slots in the waveguide. Slot antennas are often used at UHF and microwave frequencies instead of line antennas when greater control of the radiation pattern is required. Slot antennas are widely used in radar antennas, particularly marine radar antennas on ships, for the sector antennas used for cell phone base stations, and are often found in standard desktop microwave sources used for research purposes. A slot antenna's main advantages are its size, design simplicity, and convenient adaptation to mass production using either waveguide or PC board technology.

Due to Babinet's principle a slot in a waveguide has the same radiation pattern as a driven rod antenna whose rod is the same shape as the slot. This can be used to determine the radiation pattern using calculations for a more conventional antenna.

The slot antenna was invented in 1938 by Alan Blumlein, while working for EMI. He invented it in order to produce a practical type of antenna for VHF television broadcasting that would have horizontal polarization, an omnidirectional horizontal radiation pattern and a narrow vertical radiation pattern.

Stereophonic sound

Stereophonic sound or, more commonly, stereo, is a method of sound reproduction that creates an illusion of multi-directional audible perspective. This is usually achieved by using two or more independent audio channels through a configuration of two or more loudspeakers (or stereo headphones) in such a way as to create the impression of sound heard from various directions, as in natural hearing. Thus the term "stereophonic" applies to so-called "quadraphonic" and "surround-sound" systems as well as the more common two-channel, two-speaker systems. It is often contrasted with monophonic, or "mono" sound, where audio is heard as coming from one position, often ahead in the sound field (analogous to a visual field). Stereo sound has been in common use since the 1970s in entertainment systems such as broadcast radio, TV, recorded music, internet, computer audio, and cinema.


A tappet is a projection that imparts a linear motion to some other component within a mechanism.


Ultra-linear electronic circuits are those used to couple a tetrode or pentode vacuum-tube (also called "electron-valve") to a load (e.g. to a loudspeaker).

'Ultra-linear' is a special case of 'distributed loading'; a circuit technique patented by Alan Blumlein in 1937 (Patent No. 496,883), although the name 'distributed loading' is probably due to Mullard. In 1938 he applied for the US patent 2218902. The particular advantages of ultra-linear operation, and the name itself, were published by David Hafler and Herbert Keroes in the early 1950s through articles in the magazine "Audio Engineering" from the USA. The special case of 'ultra linear' operation is sometimes confused with the more general principle of distributed loading.

Wheatstone bridge

A Wheatstone bridge is an electrical circuit used to measure an unknown electrical resistance by balancing two legs of a bridge circuit, one leg of which includes the unknown component. The primary benefit of the circuit is its ability to provide extremely accurate measurements (in contrast with something like a simple voltage divider). Its operation is similar to the original potentiometer.

The Wheatstone bridge was invented by Samuel Hunter Christie in 1833 and improved and popularized by Sir Charles Wheatstone in 1843. One of the Wheatstone bridge's initial uses was for the purpose of soils analysis and comparison.

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