The Library of Congress, working with the Lawrence Berkeley National Laboratory, has unlocked historical recordings made by Alexander Graham Bell and others in the 1880s. The (quite technical) explanation of how the recordings were converted into digital form is below. The recordings are available here.
From a Library of Congress news release:
In the early 1880s, three inventors _ Alexander Graham Bell, Chichester Bell and Charles Sumner Tainter, collectively making up the Volta Laboratory Associates _ tried to record sound. In one experiment, on Nov. 17, 1884, they recorded the word “barometer” on a glass disc with a beam of light. This disc, along with nearly 200 other experimental recordings, was packed up for safekeeping and deposited at the Smithsonian Institution.
In the past year, Library of Congress Digital Conversion Specialist Peter Alyea; Lawrence Berkeley National Laboratory Scientists Carl Haber and Earl Cornell; and National Museum of American History Curators Carlene Stephens and Shari Stout worked jointly to recover sound from those recordings. Using high-resolution scans made from the original Volta discs, they were able to hear the word “barometer.”
The team has successfully submitted six discs—all experimental recordings made by Volta—to the sound recovery process known as IRENE/3D (Image, Reconstruct, Erase Noise, Etc.), a process developed by Berkeley Lab in 2003-04 and installed at the Library of Congress in 2006 and in 2008.
The process creates a high-resolution digital map of the disc or cylinder. This map is then processed to remove evidence of wear or damage, such as scratches and skips. Finally, software calculates the motion of a stylus moving through the disc or cylinder’s grooves, reproducing the audio content and producing a standard digital sound file.