The Fifth Element

Sidebar: The Dynatrack Conundrum

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.

This playback system was something Rube Goldberg would have loved. It was based on a time-weighted Schmitt trigger, an early digital-logic circuit made from analog components and originally designed in 1934. Otto H. Schmitt modeled his circuit on the propagation of neural impulses in the nerves of squid.

Time weighting was used so that the Schmitt trigger would not be fooled into switching tracks on the basis of merely momentary peaks. Additionally, the Dynatrack machines included a sophisticated real-time cross-fade circuit with an optical switch that was claimed to effect undetectable transitions between the two pairs of tracks. According to contemporary accounts, the Dynatrack system fooled EMI engineers in the UK into thinking that test records cut in the US by Capitol had been cut direct to disc, with no tape machine in the loop.

Alas, Dynatrack was overwhelmed in the marketplace by two developments. First, improved tape formulations made tape hiss less of a problem to start with. Second, Dolby A needed only one track per channel.

My working hypothesis has been that the in-machine tape in Cleveland was almost certainly, barring accident, an 8-track Dynatrack. Tom Fine and Richard Lane (the latter quoting analog tape pioneer Jack Mullin) assure me that in 1969 3M's Dynatrack was EMI engineer Carson Taylor's machine of choice. Assuming that a Dynatrack was indeed used, the multitrack tape would have been mixed down to an NAB stereo tape, which then would have been edited with a razor blade to make the cutting master for the US LP edition, which then was copied to make the tapes sent to Angel/Capitol's corporate siblings.

So, to get one generation closer to David Oistrakh's tone of liquid gold, we either must find the razor-edited stereo mixdown tape and hope it is undamaged, or find the Dynatrack 8-track and, if it is undamaged, give mastering maven Bob Ludwig a challenging new assignment!

Peesacake.—John Marks