I am grateful to Richard Lane, Tom Fine, Bob Ludwig, and Dale Manquen for educating me about a fascinating footnote in the history of sound recording: 3M’s Dynatrack tape machines of the 1960s.
Dynatrack was an ambitious, pre-Dolby noise-reduction scheme that recorded two tracks for each channel: an NAB-standard track, and a “boosted” track with fixed high-frequency pre-emphasis to overcome tape hiss. Therefore, a stereo recorder required four tracks, while a four-channel recorder required eight tracks.
The clever feature of Dynatrack was that it recorded both tracks for each channel continuously. It was only on playback that an “intelligent” cybernetic playback system would automatically switch between the boosted tracks, which were overloaded on dynamic peaks, and the standard NAB tracks, in which loud music would tend to mask the hiss.…
