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Bell Laboratories and the Development of Electrical Recording
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Bell Laboratories and the Development of Electrical Recording
Fletcher-Munson Curve from Bell Labs audio research The Development of Electrical Recording at Bell Laboratories Radio broadcasting in the United States began in November, 1920 when KDKA, Pittsburgh was granted its call letters and began broadcasting 1. This development represented a challenge to the phonograph, since free music and entertainment was available "off the air". Phonograph recording companies sought to respond by improving the quality of their recordings by adopting new technology. A key innovator of this technology was Bell Laboratories. Bell Laboratories was the research division of the US telephone monopoly, American Telephone & Telegraph. Research had previously been done by Western Electric, the manufacturing arm of AT&T, and also by Bell System division until "...On January 1, 1925, AT&T established Bell Telephone Laboratories, Inc., in New York City as a joint venture with Western Electric. Gifford [President of AT&T] appointed Frank Jewett president of the new company in charge of 4,000 workers..." 2. Thereafter, a steady flow of ground-breaking technology was developed by Bell Labs during the next 60 years. Bell and Western Electric research had been working since at least 1914 on technologies for long-distance telephone transmission to span the US. This research also provided the components of what later became the Westrex electrical recording system. The first complonent was the condenser microphone, developed in 1916 by the brilliant electrical engineer Edward Christopher "E.C." Wente (Ph.D. from Yale in 1918). The condenser microphone became practical with the development of the vacuum tube (valve), used to amplify the low-level signal output of the condenser microphone 4. Prior to the condenser microphone, since the earliest days of Bell's telephone, the carbon microphone had been used as a transmitter (and continues to be used today). The carbon microphone is basically a generator of voltage, and requires no amplification, at least for telephone transmission over distances up to several hundred miles (or kilometers). However, it does not produce an electrical signal sufficient for transcontinental telephone transmission. The condenser microphone connected to an amplifier was found to produce strong, transcontinental signals. The condenser microphone design is innovative. It includes two parallel plates, each having an electrical charge, The front plate moves with the audio signal, while the rear plate is fixed. Sound waves moving the front plate cause a slight change in the capacitance between the plates, which allows a change in current flow in the electrical circuit connected to the microphone transmitter. The amplifier circuit then amplifies this low-level signal to a useable level 12. This condenser microphone, although first developed for long-line telephone transmission, when the US was beginning transcontinental telephone service, had remarkable acoustic performance. In its initial versions with thicker steel plates stretched to give stiffness (so as to raise its resonant frequency above the audio range), it had nearly flat frequency reproduction up to about 6,000 Hz. With improved, thinner plates of aluminium, this reproduction was expanded up to 15,000 Hz, at a time when the acoustic recording process did not record much above 2,400 Hz 9.
Edward C. Wente of Bell Laboratories holding his Condenser Microphone Earlier Development of Electrical Recording Technology Prior to the Bell Laboratories research, a number of attempts had been made to develop an electrical recording process that involved microphones and electrical disk cutting. This was to replace the acoustic or mechanical recording technique used since Edison, which involved no electricity or amplification. An early example of electrical recording was the work of Lionel Guest and Horace O. Merriman in Britain. They had developed and demonstrated an electrical recording system using carbon microphones connected to a moving coil recording head 15. Their system produced the earliest issued electrical recording which has come down to us. This was the 1920 recording of the burial of the Unknown Soldier in Westminster Abbey, London on November 11, 1920 14. Guest and Merriman placed 4 carbon microphones inside Westminster Abbey 15, with their recording apparatus outside. The resulting recording was issued by Columbia Graphophone Co. However, the sound quality of the Guest and Merriman system was not satisfactory. British recording companies, including Columbia Graphophone, after evaluation decided not to adopt the Guest and Merriman system. They subsequently went to the U.S., but with similar results, and their system was not further developed. The sound quality of the 1920 Westminster Abbey recording is sufficiently poor that it is difficult to discern if only instruments are playing, or if there is also singing. The result is significantly poorer than an acoustic recording of the period, except perhaps that it would be difficult for the acoustic horn to capture such a large group. You may hear and judge the sound quality of this 1920 recording for yourself by clicking on the link below. This reproduces the music Abide with Me from this 1920 Westminster Abbey service. Click here to listen to 'Abide with Me' recorded electrically by Guest and Meriman in 1920 Joseph P. Maxfield and Henry C. Harrison of Bell Labs Develop Electrical Recording In about 1920, two teams at Bell Laboratories were created to pursue electrical recording. Joseph P. Maxfield and Henry C. Harrison, two of the leading developers of modern electrical components at Bell Labs, were charged with developing a phonograph recording system. E. C. Wente, Donald MacKenzie and Irving Crandall (who had originally hired Wente in 1916) 19 were assigned to develop a sound system for films to be shown in cinema theaters5. Both groups were overseen by Dr. Harvey Fletcher , who previously joined Bell research in 1916.
Joseph P. Maxfield and Henry C. Harrison of the Bell Laboratories
Dr. Harvey Fletcher of Bell Laboratories Dr. Harvey C. Fletcher was a brilliant physicist born into a Mormon family in Utah on September 11, 1884. Fletcher studied and worked at the University of Chicago with Nobel Prize winner Robert A. Millikan. In 1911, Fletcher received his Ph.D. in Physics from the University of Chicago summa cum laude. After creating and heading the Physics Department at Brigham Young University in Utah, Fletcher in 1916 became Director of Research at Bell Laboratories, where he oversaw three decades of research and improvement in sound, hearing, transmission, and reproduction. At Bell Laboratories, Fletcher and his predecessor Irving B. Crandall oversaw research in this area by Joseph P. Maxfield (1887-1977), Henry C. Harrison (1887-1971), Edward L. Norton (1898-1983), K. P. Secord, Donald MacKenzie, Rogers H. Galt (1889- ) 16, and Harold Black (1898-1983). Harold Black had invented the negative feedback amplifier in 1927. Fletcher also supervised Arthur C. Keller (1901-1983), who worked extensively on recording improvements, including binaural and stereo recording (as described below).
Harvey Fletcher, K. P. Secord, and Rogers H. Galt at the Bell Laboratories in New York City Western Electric Carbon Microphones Microphone design was a key part of the Bell Laboratories development of electrical recording technology. Carbon microphones, using loose carbon granules were employed from the earliest days of telephone instruments until today. The carbon microphone generates its own current from acoustic vibration, so requires no electrical amplification (although it will require a circuit with voltage). In the 1920s, Bell Labs developed improved carbon microphones, including the so-called "double button" carbon microphone which kept the carbon powder from touching the microphone diaphragm.
The famous Western Electric double-button carbon microphone in the Western Electric 1B microphone housing circa 1925. The double-button carbon microphone reduced noise and also greatly reduced harmonic distortion. These improved carbon microphones and later the condenser microphones described above were manufactured by Western Electric (the manufacturing arm of the Bell system). The Western Electric double-button carbon microphone in the 1B and 1C housing was widely used in radio broadcasting. In broadcasting, the carbon microphone was connected directly to the engineering control panel and then to the radio transmitter. A radio studio from late 1924 with this microphone arrangement can be seen in in the photograph below.
Art Gillham (with glasses) standing to left of Will Rogers in November 4, 1924 during the Eveready Hour, broadcast by WEAF New York. AT&T sold WEAF in 1926 as part of a settlement with GE/RCA to exit broadcasting, and GE and RCA committed to transmit all their network broadcasts via AT&T long distance lines 18. (note the Western Electric carbon microphone in the 1B housing on a stand) The Matched-Impedance Electrical Recording System Harrison and Maxfield made a number of developments which, combined together, allowed the creation of an electrical disk sound recording system. Already described is the condenser microphone. Another of these key developments was the matched-impedance amplification system, having such a condenser microphone, linked to a tube (valve) amplifier, with the amplified output voltage driving a moving magnet (called also a "moving armature'") cutting head to scribe the sound into a wax master disk. Bell Laboratories had early licensed the patents covering Lee de Forest's Audion tube. Using this device, Bell Laboratories engineer Harold Arnold had by late 1914 improved the Audion by replacing gas by a vacuum and redesigning the electrodes and filaments, producing an amplifier having low distortion and linear amplification 20. Such an amplifier was the basis for the matched-impedance electrical recording system which was developed by Bell Labs. The early performance was encouraging. Initially, it had a a recording bandwidth from 50 Hertz to about 6,000 Hertz, beyond which its high frequency sensitivity declined11.
Frequency response of the electrical amplification and condenser microphone of the Westrex system 11 This frequency reproduction range was noticeably superior to the highly variable acoustic process, which (with the best equipment and with the best recording engineers) could reproduce from about 250 Hz to about 2,400 Hz, with rapid reproduction fall-off thereafter. This wider bandwidth added another octave of sound reproduction, along with reduced harmonic distortion and a generally more realistic sound image, including recording of higher harmonics of the musicians. The dynamic range of the sound recording and the background noise were both also somewhat improved. Light Valve Vacuum Tube for Movie Film Recording At the same time, Edward C. Wente and his team had developed the "light valve", a light-emitting vacuum tube that converted audio voltage into variable levels of light to expose movie film. This light output produced a variable density sound pattern on the film. Wente was granted a patent on this technology in 1923, and it was later the basis for the Western Electric "sound-on-film" process. Western Electric Model 361 and Model 394 condenser Microphone TransmittersWente's early condenser microphone designs continued to be developed into continuously improved microphone transmitter designs. The Western Electric Model 361 seems to have been available by late 1924 and the Model 394 transmitter by late 1926 (although these dates are my estimates based on contemporary literature). According to tests, the Model 361 had had several mid-range resonant frequencies, particularly "...a +7dB resonance at 2.9kHz..." 22. The later Model 394 microphone transmitter sought to improve this and other characteristics. The Model 394 transmitter was widely used in a number of Western Electric microphones for broadcasting, movie and disk recording in different versions from about 1927 6 and into the 1930s. The improvement of the condenser microphone and its use for 1925-1930 electrical recordings, was key in the quality of the Bell Labs system. It should be added that there is disagreement among experts as to whether the initial recordings made with the Western Electric electrical recording system were made with carbon microphones or with condenser microphones. However, it seems clear that by later 19269 or early 1927, condenser microphones were definitely used in the Western Electric system. This was important not only because of the condenser microphone's expanded frequency range and lower harmonic distortion, but also because even well-engineered carbon microphones produced a low level hissing or sizzling background sound from the vibration of the carbon granules. However, there were some inconveniences with early condenser microphones. The condenser microphone transmitter required vacuum tube (valve) circuitry to be located near the microphone, since the microphone transmitter required a polarizing voltage to be applied to its condenser plates, and it also required amplification. The output of the microphone transmitter was weak. It also had a high impedance. If this output were sent down a long electrical wire to the recording electronics, it would be degraded by noise. Therefore, in condenser microphones used for recording from in this period, the amplifier was placed in a box below the transmitter, or in another nearby housing. In later Western Electric models, the transmitter was sometimes attached to a tubular housing containing the amplifier circuits and the circuits to provide voltage to the plates. Such a base was initially large, as can be seen in the photograph below: the model 47A, containing the 394 transmitter at the top, and the electronics in the tubular base.
Western Electric 47A Condenser Microphone with 394 transmitter at top and a single vacuum tube (valve) amplifier in base The initial condenser microphones using the Model 361 transmitter were replaced by improved condenser microphones such as the Western Electric 47A having the Model 394 transmitter beginning in late 1926 and into 1927-1928 in both movie sound recording and phonograph disk recording 6. A Recording System based on the Theory of Electrical Filters Electrical recoding systems previous to and contemporaneous to the Bell Laboratories development may be fairly described as being mostly empirical or trial-and-error development. In contrast, Bell Laboratories based their design on scientific concepts from electrical theory, reflecting the advanced electrical engineering eduction of these scientists. In their development of an electrical phonograph disk recording system, the Maxfield and Harrison team based their system on the theory of a many-sectioned electrical filter 5. Bell Labs had applied filters to the improvement of long-distance telephone transmission. The "theory of electrical filters" was well-developed and mathematically well-characterized. This allowed them to apply these known mathematical solutions to the recording and reproduction problems they were attacking. Using this approach, a mechanical equivalent of each electrical component of such a filter was identified. This approach was used both for the electrical recording system and for sound reproduction systems such as loud speakers. For example, the air in an enclosed cabinet could be considered to function like a capacitor. Since the mathematical solutions in the theory of electric filters had already been solved, this accelerated a scientifically-based development of sound reproduction. In the new Bell Labs recording system, the microphone was connected to the matched-impedance amplifier (described above), whose electrical output was connected to and controlled the electrical disk cutting mechanism.
Henry C. Harrison and Norman H. Holland examine a phonograph tone arm
For the disk cutting mechanism, the Bell engineers developed a
"moving magnet", also called a "moving armature"
electromagnetic device. In this system, the electrical output of the
matched-impedance amplifier was fed to an electro-magnet, causing the cutting
stylus to move according to the changes in the magnetic field of the
electro-magnet. In this way, the music, in the form of the varying
electrical output of the amplifier, going to the cutting stylus via the moving
magnet would inscribe the music wave form into the wax master. So, in
this electrical system, the microphone and amplifier, moving the stylus,
replaced the old acoustic horn mechanically vibrating a sound box which would
move the stylus. Since the microphone and amplifier had a broader
frequency range and less harmonic distortion and less acoustic resonances,
the recording cut into the wax was dramatically better, as could immediately
be heard in the first electrical disks released by the Victor Talking
Machine Company. (See
1925 - The First Electrical Recording
for this first orchestral electrical recording.)
There were a number of limitations of this new electrical system which the
early recording engineer needed to overcome. If the electrical signal
for the music exceeded certain limits, it could cause the
electromagnet-stylus assembly to move too far and become stuck.
To solve this, a small spring was added to the cutting head to control stylus
excursion.
Another difficulty was in the conversion by the Bell engineers of an electrical
component into an equivalent mechanical component. As described above,
the Bell Labs design was in part based on the "theory of electrical
filters". Electrical filter theory required that the final section
of such a filter terminate in a resistor. The mechanical equivalent of
the electrical resistor that the Bell engineers developed was a 9 inch
long rubber line or rod. This rubber line was added at the end of
the system to provide mechanical resistance or damping
5,10. The rubber rod was placed up the inside
of the length of the cutting arm. This is the source of the
description of the system by the engineers as being the
"rubber line recorder".
Cutting Arm of the 'Rubber Line' Recorder with the rubber damping rod
inside 11
Without the damping resistance of the rubber line, the movement of the
recording stylus at high volume levels, particularly of low frequencies,
would have been excessive. The damping of the rubber line was one of
the essential developments that would make this Bell system linear in its
frequency response 8.
However, this rubber tube also introduced difficulties.
First, it required careful design and repeated testing of the stiffness and
compliance of the rubber column. The coupling of components,
the mass of the components, and characteristics of the rubber each had
to be varied and tested empirically to discover a combination that
gave the disk cutter an essentially flat frequency response. In
fact, in the lab, the Bell system was able to achieve a remarkably
flat response from 250 Hz to about 15,000 Hz 9, although
the total system in use commercially had an upper frequency response
to about 6,000 Hz in the early years (progressively expanded
each year thereafter). It was also found later, from experience,
that the recording engineer would need to carefully pack and adjust
the rubber line in the recording arm, prior to each recording session
to assure good linearity and response in the recorded sound. In
this way, the early Westrex electrical system, like the earlier acoustic
recording system. depended on the craft and resourcefulness of the
recording engineer to gain the best recording results from the system.
The Western Electric Electrical Recording System
The microphone, plus the amplifier, plus the electromechanical disk
cutting mechanism combined together was the new Western Electric
electrical recording system, commercialized as the
"Westrex" system.
Western Electric engineer George Groves (later a famous Hollywood sound
engineer) at a 1925 Western Electric Cutting Machine.
By early 1924, the Western Electric system was ready for demonstration.
Western Electric's next task was to interest the phonograph recording
companies to license and adopt the Westrex system, to replace the
acoustic recording processes used for the last 50 years. The
fascinating and often surprising story of this is quest to implement
electrical recording of the phonograph disk can be seen by clicking
on the link:
Licensing the Westrex Electrical Recording System to
Victor and Columbia.
Note: Of course, the Western Electric electrical recording system, while being the
most successful early electrical phonograph recording system was not the
only, or even the first electrical system. Allan
Sutton in his fascinating and meticulously researched book:
Recording the 'Twenties. The Evolution of the American Recording
Industry, 1920-29
13,
published in 2009 by Allan Sutton's excellent
Mainspring Press
describes the history of the development of the various technologies
leading to electrical recording. Well worth a purchase and
multiple readings.
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