SOUND recorded on antique wax cylinders and early 78 rpm records can now be recovered without damaging the precious, fragile media. This means recordings that archivists have never dared play could be heard for the first time.
The trick is to use a high-resolution imaging system to create a computer map of the groove pattern on the cylinder or record. Software then works back from the pattern to recreate the sounds that made them, Carl Haber of the Lawrence Berkeley National Laboratory in California told delegates to an acoustics meeting in New York last week.
Sound archives such as the Library of Congress, which is funding Haber鈥檚 work, and the British Library Sound Archive, have many rare recordings. Some, such as the British Library鈥檚 1924 recording of James Joyce reading from Ulysses, are irreplaceable. But every time these recordings are played they are damaged by the needle. And the British Library has some early tinfoil cylinders which were only designed to be played once, as the needle tears the foil, but it has never risked playing them. Now it can.
Advertisement
The imaging technique is a spin-off from the particle physics work that is Haber and his colleague Vitaliy Fadeyev鈥檚 bread and butter. They have been developing imaging systems that reveal the ephemeral tracks made by subatomic particles in atom-smashing accelerators. But when they heard about the threat to old recordings, they realised their technology might help.
Different techniques were used to record sound onto cylinders and records, so different methods are needed to retrieve it. In wax cylinders, the undulating pattern representing sound waves was cut into the bottom of the groove, creating peaks and troughs. In records, it was usually cut into the side of the groove, causing the stylus to sway from side to side.
To retrieve the sound, the researchers need an accurate image of the grooves and the undulating shapes within them. With a wax cylinder, they use an optical probe to sample the depth of the groove at 12 points across its width, and from this they create a profile of the groove鈥檚 cross section at that point.
They then repeat the process at intervals, and from this succession of 2D images they can build up a 3D image of the groove. By turning the cylinder through an angle of just 0.01 degrees between each image they sample the sound 96,000 times per second, which is more than twice the sample rate of a CD recording. They then take the complete 3D map, apply their lab鈥檚 noise-elimination routines to clean up noise from dust and scratches, and retrieve the recorded sound.
For 78 rpm discs, with the sound recorded laterally, the researchers found they could reproduce the sound from a series of simple 2D digital images. It takes between 5 and 15 minutes to copy a 3-minute record.
For cylinders, the process is much lengthier. It can take up to 24 hours to make the 3D map of the groove on a 4-minute wax cylinder. Haber hopes to speed up the process for both types of recording.
