This format dates from 1963-1964 and did not gain wide acceptance. It utilized two 70 mil wide tracks per channel (the same track width as 4-track 1/2″ and 8-track 1″ tapes).
In addition to the electrical performance improvements described below, the 3M team introduced the Iso-Loop tape transport which substantially reduced scrape flutter and other transport-related audible degradations.
Two tracks per channel were continuously recorded. They were called an L and an H track. The H track was the standard NAB equalization while the L track had a shelving pre-emphasis built with a simple double RC filter that started at 400 Hz, approached 12 db/octave and produced a signal 15 dB higher than the H channel at 15-20 kHz. A complementary de-emphasis was provided upon playback.
During playback, there was an automatic switch that seamlessly switched between the two tracks, controlled by the program signal level. This was a system to reduce tape noise that slightly predated the introduction of the Dolby System. The need for more tracks, and the subsequent availability of Dolby Noise Reduction caused the DynaTrack system to fade from the market.
The change in background noise was apparently inaudible on most program material, but was reported to be quite noticeable on acoustic guitar solos in early versions. By making the L/H switch in the playback electronics insensitive to any signal below 400 Hz in later versions of the product, there would be enough midrange signal to mask the increase in tape noise after switching to the H track.
The pilot run of six machines were available in the 2- and 3-channel formats, on 1/2″ and 3/4″ tape, respectively. New information provided 2014-02-11 by Tom Fine indicates that the 3-channel 3/4″ machines never produced any commercial work products and apparently were scrapped. Later M23-based machines were available in 2- and 4-channel formats, on 1/2″ and 1″ tape, respectively. Apparently the proposed mono version with 1/4″ tape was never constructed.
Several major labels, including RCA Red Seal, Columbia, and Capitol bought and used these machines to record the likes of Nat King Cole, Nancy Wilson, and George Shearing. For more of the story, as told by Dale Manquen, a member of the original design team, click here. For Dale’s technical discussions, click here and here.
Resources for transfer: California — Dale Manquen