Yesterday I attended the College of Engineering and Physical Sciences Frontiers Lecture by UNH Math Professor Kevin Short, entitled “Disassembly, Repair and Rebuilding of Music using Mathematics.” As the title suggests, Kevin discussed his work on restoring old music recordings, including his Grammy Award-winning work on restoring an old Woody Guthrie recording. Kevin won his Grammy in collaboration with Nora Guthrie, Jorge Mateus, Steve Rosenthal, Jamie Howarth and Warren Russel-Smith. Jamie Howarth has a special place in this group as the founder of Plangent Processes, a pioneering company in the field of music repair.
The talk was excellent. Kevin introduced several complicated digital signal processing ideas and made them accessible to us all. In this post I will concentrate on only one topic from the talk – Kevin’s work on the Woody Guthrie recording.
When I first heard about Kevin’s work on restoring old music recordings I also heard the term wire recording, but never followed up on what it really meant. Well, it turns out that wire recording is a technique that magnetizes a steel wire to record sound. This technology was used to create the Woody Guthrie recording and, as you can imagine, it’s a challenge to get a 50 year old coil of steel wire to reproduce sound with any reasonable quality.
One problem with a wire recording has to do with the uneven speed of motion of the wire under the recording head during recording (all recording techniques that use moving parts have this problem to some extent). The uneven speed of the recording medium stretches out some sound segments in time, and it compresses others. Another problem is that handling of the medium may damage that medium, and this in turn results in similar stretching and compression of the sound in time. An extreme case of this latter problem is a cassette player chewing up your tape. The tape may still be playable but the sound it produces isn’t that great any more.
The Woody Guthrie recording suffered from this type of stretching and compression. However, Kevin and Jamie Howarth were able to restore the recording by taking advantage of a particular type of noise in the recording: powerline hum. You see, the wire recorder was plugged into a wall outlet, which provided AC current at a frequency of 60 Hz, and this frequency was pretty constant. The recording was contaminated with a 60 Hz sinusoidal noise that originated from the powerline. However, when you play back the wire recording, the frequency of this noise sinusoid fluctuates. Kevin and Jamie realized that they had a known source in the 60 Hz powerline and that, by observing the fluctuations of the frequency of the noise sinusoid on the wire, they could understand how all the other sounds were distorted as well. This knowledge can then be used to reverse the distortions (compress stretched out parts of the recording, and stretch out compressed ones). What an elegant idea!