Math Restores Woody Guthrie Recording
An article entitled “Math Trek: The Grammy in Mathematics” from Science News Online explains how Jamie Howarth, with the help of mathematician Kevin Short, used an awesome technique to restore an old Woody Guthrie recording (emphasis mine):
Howarth had developed algorithms to correct these recordings. He looks for extraneous sounds, like an air conditioner or fan in the background that creates a rhythmic sound. Instead of simply removing these sounds, he uses them as a clock, a kind of built-in foot-beat in the recording that tells him what the true timing should be. When a recording is made, this background rhythm is even. But when it’s played back, it speeds up and slows down in perfect timing with the errors in the recording. That allows Howarth to adjust the timing of the recording to make it much more similar to the original sound.
When Howarth isn’t lucky enough to find a rhythmic background noise, he has another technique. He has found that all analog recordings contain a sort of rhythmic buzz at a specific frequency way above human hearing. This buzz can substitute for a background fan.
Howarth had successfully used these techniques to restore other old recordings, like the film soundtracks for Oklahoma! and Close Encounters of the Third Kind. But the Guthrie recording was such a mess that it forced him to develop new techniques. He turned to Kevin Short, a mathematician at the University of New Hampshire who had done work on signal processing for sound compression.
The team discovered the many ways that wire makes a lousy material for sound recording. One problem is that wire’s round. When the wire kinked, it would twist out of position and the head would no longer be reading the proper side of the wire. The machine still read the low and medium frequencies, but the very high frequency sounds dropped out—including the signal Howarth used as his foot-beat.
Short developed techniques to interpolate the missing information. “We could actually pick up a hum from the Con Edison power supplies,” Short says. “It’s a pretty nasty noise.” Because that hum was lower frequency, it remained even in the twisted sections. Short also brought in more sophisticated techniques to shift the pitches once the algorithm had identified what needed to be done.
That is really really cool. Be sure to check out the article to hear the before and after soundclips, which are impressive.
Comments
Dan McKeown
I cannot seem to get the link to work, any suggestions?
Alexander Micek
It’s currently being hammered by the interwebbingtubes because so many people linked to it today, but I refreshed a couple of times and it eventually came through. Keep trying!