Meeting Topic: Audio Restoration, A Case of "Not Good Enough"
Moderator Name: Bob Meganta
Speaker Name: Oliver Masciarotte
Meeting Location: Cogswell Polytechical College, Sunnyvale, CA, USA
Oliver Masciarotte spoke about audio restoration at AES San Francisco's January meeting. Fifty people attended.
Masciarotte owns and operates Seneschal, in the Presidio, San Francisco, California. Seneschal provides technical professional services, including consulting on the efficient production and preservation of digital assets.
Masciarotte started with a brief history of audio. The earliest known audio recordings were made in 1860, by Édouard-Léon Scott de Martinville, of a series of tones, and the song, "Au clair de la lune." The recording device was a phonautograph. A pattern was inscribed on a paper cylinder coated with soot, with no means of playback. Phonautograms look like crude waveforms on an oscilloscope.
In a century and a half, sound recording has evolved through several media, such as magnetic wire and tape, optical film, vinyl, digital recordings on video tape, film and compact discs.
Computer file-based storage has the advantage of no degradation over time, and is the present state of the art.
Sound recordings are subject to impairments, either in the original recording, or because of the effects of time. Noise, either acoustic or electronic, is often present. There may also be harmonic or intermodulation distortion, impulse noise or dropouts. Channel decoding errors can happen, in the digital domain. Impulse noise, such as clicks and pops, is common on vinyl records.
Signal analysis is an essential first step in restoration. The most common way of looking at signals is amplitude vs. frequency, as a waveform on an oscilloscope. Frequency vs. amplitude, as viewed on a spectrum analyzer, is more useful, as it readily shows artifacts, like spurious harmonics and noise.
Film soundtracks are among the greatest challenges of audio restoration. Film moves intermittently through a projector. It is subject to mechanical and thermal stresses, in addition to decomposing with time. Decomposed film smells like vinegar.
Equalization, de-clicking, removing distortion products, noise filtering and post processing are some of the main steps in the restoration work flow. Several restoration applications are available, including RX, Frequency, and NoNOISE II.
Masciarotte cautioned about the dangers of progressive hearing loss. Long-time exposure to lower-intensity sound, such as while flying on aircraft, can be as damaging as brief exposure to higher sound intensity. His presentation was frequently punctuated by questions and comments from attendees.