The phonautograph is the earliest known device for recording sound. Previously, tracings had been obtained of the sound-producing vibratory motions of tuning forks and other objects by physical contact with them, but not of actual sound waves as they propagated through air or other media. Invented by Frenchman Édouard-Léon Scott de Martinville, it was patented on March 25, 1857. It transcribed sound waves as undulations or other deviations in a line traced on smoke-blackened paper or glass. Intended solely as a laboratory instrument for the study of acoustics, it could be used to visually study and measure the amplitude envelopes and waveforms of speech and other sounds, or to determine the frequency of a given musical pitch by comparison with a simultaneously recorded reference frequency.
Apparently, it did not occur to anyone before the 1870s that the recordings, called phonautograms, contained enough information about the sound that they could, in theory, be used to recreate it. Because the phonautogram tracing was an insubstantial two-dimensional line, direct physical playback was impossible in any case.
Several phonautograms recorded before 1861 were successfully played as sound in 2008 by optically scanning them and using a computer to process the scans into digital audio files.
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Édouard-Léon Scott de Martinville, a printer and bookseller by trade, was inspired when he happened to read about the anatomy of the human ear in the course of his business. His phonautograph was constructed as an analog of the ear canal, eardrum and ossicles. Scott created several variations of the device. The functions of the ear canal and eardrum were simulated by a funnel-like horn or a small open-ended barrel with a flexible membrane of parchment or other suitable material stretched over the small end. A pig bristle or other very lightweight stylus was connected to the membrane, sometimes by an indirect linkage which roughly simulated the ossicles and served as an amplifying lever. The bristle traced a line through a thin coating of lampblack--finely divided carbon deposited by the flame of an oil or gas lamp--on a moving surface of paper or glass. The sound collected by the simulated ear and transmitted to the bristle caused the line to be modulated in accordance with the passing variations in air pressure, creating a graphic record of the sound waves.
Martinville's first patent described a flat recording surface and a weight-driven clockwork motor, but the later and more familiar form of his invention, marketed by Rudolph Koenig in 1859, recorded on a sheet of lampblack-coated paper wrapped around a cylinder which was hand-cranked. The cylinder was carried on a coarsely threaded rod so that it progressed along its axis as it rotated, producing a helical tracing. The length of the recording that could be accommodated depended on the speed of rotation, which had to be rapid in order to resolve the individual waveforms of various sounds with good detail. If only longer-term dynamics such as the cadences of speech were being studied, the cylinder could be rotated much more slowly and a longer recording could be made. Some phonautographs included a tuning fork or other means of simultaneously recording a known reference frequency.
Several other inventors subsequently produced modified versions of the phonautograph and recorded the sound-modulated line by the use of various implements and in various formats, either in attempts to improve on Scott's apparatus or to adapt it to specific applications. In at least one instance, a complete return to the device's conceptual origins was made by employing the preserved parts of an actual human ear.
By mid-April 1877, Charles Cros had realized that a phonautograph recording could be converted back into sound by photoengraving the tracing into a metal surface to create a playable groove, then using a stylus and diaphragm similar to those of the phonautograph to reverse the recording process and recreate the sound. Before he was able to put his ideas into practice, the announcement of Thomas Edison's phonograph, which recorded sound waves by indenting them into a sheet of tinfoil from which they could be played back immediately, temporarily relegated Cros's less direct method to obscurity.
Ten years later, the early experiments of Emile Berliner, the creator of the disc Gramophone, employed a recording machine which was in essence a disc form of the phonautograph. It traced a clear sound-modulated spiral line through a thin black coating on a glass disc. The photoengraving method first proposed by Cros was then used to produce a metal disc with a playable groove. Arguably, these circa 1887 experiments by Berliner were the first known reproductions of sound from phonautograph recordings.
However, as far as is known, no attempt was ever made to use this method to play any of the surviving early phonautograms made by Scott de Martinville. Possibly this was because the few images of them generally available in books and periodicals were of unpromising short bursts of sound, of fragmentary areas of longer recordings, or simply too crude and indistinct to encourage such an experiment.
Nearly 150 years after they had been recorded, promising specimens of Scott de Martinville's phonautograms, stored among his papers in France's patent office and at the Académie des Sciences, were located by American audio historians. High-quality images of them were obtained. In 2008, the team played back the recordings as sound for the first time. Modern computer-based image processing methods were used to accomplish the playback. The first results were obtained by using a specialized system developed for optically playing recordings on more conventional media which were too fragile or damaged to be played by traditional means. Later, generally available image-editing and image-to-sound conversion software, requiring only a high-quality scan of the phonautogram and an ordinary personal computer, were found to be sufficient for this application.
No matter what hardware and software are used, the basic principle involved is relatively simple. If a greatly enlarged image of a segment of a phonautograph tracing were projected as a horizontally-oriented undulating line on a sheet of graph paper, a numerical description of the line could be created by proceeding from one grid column to the next, counting the number of squares between the line and a straight horizontal reference line, and making a list of the numbers. Such a list is, in fact, a digital audio file of the simplest kind. If entered into a computer in the required format and with the required file header information, it can be played as sound. Naturally, a computer needs no projector or graph paper to convert a scanned phonautogram into a playable digital audio file by comparable procedures.
One complication is that Scott de Martinville's phonautograms were recorded on machines which were hand-cranked rather than motor-driven, resulting in unsteady rotation of the cylinder. The irregular wavering of pitch caused by playing back such recordings at a constant speed can make speech much more difficult to understand and has obvious dire effects on the reproduction of music. Fortunately, several phonautograms had a separate parallel track, inscribed simultaneously with the voice track, in which a constant reference tone had been recorded. By working with short segments of the paired tracks and adjusting both so that the reference tone was held to a steady pitch, it was possible to correct the irregularity and greatly improve the results.
One phonautogram, created on April 9, 1860, was revealed to be a 20-second recording of the French folk song "Au clair de la lune". Due to some confusing 1860 technical terminology, it was initially played at double the original recording speed and believed to be the voice of a woman or child. At the correct speed the voice of a man, almost certainly de Martinville himself, is heard singing the song very slowly. Also recovered were two 1860 recordings of "Vole, petite abeille" ("Fly, Little Bee"), a lively song from a comic opera. Previously, the earliest known recording of vocal music was an 1888 Edison wax cylinder phonograph recording of a Handel choral concert.
A phonautogram containing the opening lines of Torquato Tasso's pastoral drama Aminta has also been found. Probably recorded in April or May 1860, this phonautogram is the earliest known recording of intelligible spoken words to be played back, predating Frank Lambert's 1878 talking clock recording. Earlier recordings, made in 1857, 1854, and 1853, also contain de Martinville's voice but are unintelligible because of their low quality, brevity and irregularity of speed.