Phonautograph - Playback

Playback

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 inventor of the Gramophone, employed a recording machine which was in essence a refined version of the early 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. 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's phonautograms, stored among Scott's 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'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 rate of 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.