Alumnus Marc Hildebrant (BSEE, ’71) of the Electrical and Computer Engineering Department was featured earlier this year in IEEE Spectrum’s "Geek Life," describing his work in restoring old music, particularly that recorded by Thomas Edison. The article, dealing with Hildebrant using digital techniques to restore mechanically recorded music to its full glory, also appeared in the February 2017 print version of The Music Man. According to the article written by Stephen Cass, most sound-restoration projects focus on the obvious issues, such as scratches or hissing, characteristic of decades-old analog storage media. “But Hildebrant is going deeper and fixing some of the distortions introduced by the original mechanical recording process,” Cass wrote.
As Hildebrant told Cass, “That area has always been kind of abandoned by the restoration people.... Pretty much what’s available today are recordings where they just clean up the noise. They don’t really change any of the sound of the music.”
According to the article, the earliest sound-recording and playback technology was pioneered by Edison in 1877.
“The system was purely mechanical,” as Cass wrote. “To record a performance, musicians arranged themselves around a recording horn, at the other end of which a diaphragm vibrated in accordance with the incoming sounds. A needle attached to the diaphragm etched a track in the recording medium—first cylinders, and later discs.
Edison’s early mass-market catalog includes many famous musicians of the day, but these recordings have significant sound-quality problems.
As Cass explained, “The distortion comes from the limited frequency response of the mechanical recording system—essentially many high and low frequencies simply couldn’t be captured, so even after being cleaned up, the recordings sound tinny. Hildebrant concentrated on restoring the missing frequencies below 300 hertz to fill out the bass notes. Trying to simply boost low frequencies below 300 Hz would serve only to amplify noise.”
Instead, wrote Cass, Hildebrant realized that voices or instruments present in the medium frequencies would likely have had harmonics in the missing frequencies originally and so could be used to reconstruct the missing sound signals. So, using commercially available software from Diamond Cut Productions, which can generate subharmonics as part of a digital signal-processing chain, Hildebrant was able to generate missing bass frequencies in old Edison recordings, as well as filter out high-frequency noise and make other adjustments.
“The process depends to a large extent on Hildebrant tweaking the processing chain for each track, guided in part by his extensive experience with audio recording,” explained the Cass article. “The result is a digital file that sounds much more natural than the original recording.”
Currently semiretired, Hildebrant told Cass he has been working on this audio-rejuvenation project for about five years, after working on restorations of early electrically recorded music. “Like everything in engineering, you’re always working on improving,” Hildebrant said. “An engineer’s life is never done.” (September 2017)