The Ozaphane Duo-Trac was released in February 1937. It used a unique sound-reproducing system that seems to have vanished almost without trace. I had never heard of it until I happened to pick up an advertising leaflet at an ephemera fair. The system relied on Ozaphane, described in the leaflet as: "...an entirely new material..." which appears to have been an alternative to cellophane, which reached the market in 1912.
This link gives a report dated 1938 from the US Department of Commerce, saying that Viscose Ozaphane is "a new type of fim with a base of regenerated cellulose sheeting". It describes a light-sensitive dye being incorporated in the base's material rather than being added as a coating as was usual with photographic film. The conclusion was that Ozaphane had advantages over acetate film in that it was grainless, had no emulsion coating to get scratched, and was a third of the thickness; all advantages for Duo-Trac use. The light-sensitive dye was too insensitive for photography, but this was no problem for the Duo-Trac as high-intensity light could be used for copying. The stability of the Ozaphone tape on aging was considered inferior and unsuitable for archive use.
This makes it clear that the Ozaphane Duo-Trac system was actually quite a radical departure from conventional cinema sound technology.
Left: Front cover of Duo-Trac leaflet: 1937
The Duo-Trac replayed music recorded optically on a 4mm strip of cellophane. This seems rather narrow, but 1/4-inch magnetic tape was only 6.35 mm wide. It was replayed in the same way as a cinema sound-track, by shining a light through the variable-area recording and onto a photocell valve. As the picture shows, two tracks were recorded on the strip. A 7-inch reel would play for 15 minutes, and was then reversed to play the other 15 minutes. A 9-inch reel would play for 30 minutes. The leaflet claims that "...10 hours of Duo-Trac music are held in 20 seven-inch reels weighing 4 pounds. Compare this with the same time on disc records which would require 100 double-sided discs weighting 25 pounds." That seems pretty convincing.
The great advantages of this optical system was that there was no groove noise or needle scratch, (serious problems on old 78 shellac-based discs) and there was no wear and degradation no matter how often the strip was played. The playing duration was much longer than for 78 discs.
Left: Page 2 of Duo-Trac leaflet: 1937
The console model was an imposing piece of furniture.
The Duo-trac technology was introduced well before plastic magnetic tape was introduced. The state of art in magnetic recording and playback at the time was the Blattnerphone, designed by early British film maker Ludwig Blattner, which used steel tape and was first installed at the BBC in 1930. The first practical recorder using oxide-coated paper tape was the Magnetophon K1 from AEG, demonstrated in Germany in 1935. By 1945 the Germans were using high-frequency bias (discovered accidentally by the Germans in 1940) and PVC-backed tape, and the essentials for analogue tape-recording were all in place.
Left: Page 3 of Duo-Trac leaflet: 1937
Note that the spec boasts of a 'large energised speaker', which I assume means a loudspeaker with the magnetic field generated by a DC-fed winding, rather than a permanent magnet. This was common at the time, for the comtemporary permanent magnets were rather feeble affairs. If you got clever the field winding could double as a smoothing choke in the power supply.
The instructions at the bottom of the page show that to play the second track it was not necessary to turn the reels over; the machine had auto-reverse and could play in both directions. I'm not sure when this feature appeared in the world of magnetic tape; certainly my cassette Walkman could do it in the early 1980's.
Given that there were two quite independent audio tracks, it would have been simple to adapt the system to stereo, though the playing time would have been halved.
Left: Rear cover of Duo-Trac leaflet: 1937
This is the Table Model; a snip at 33 guineas. It also incorporated a five-valve three-band superhet radio receiver.
Left: Console model of the Duo-Trac
This is in the possession of the Science Museum in London.
Shown are what appear to be 7-inch reels holding the strips, while the take-up reel on the right looks to be 9-inch.
The system is mentioned in Encyclopedia of Recorded Sound by Frank Hoffman, where he calls it a 'Cellophone'.
According to the Science Museum, the Duo-Trac system was a commercial failure. It must have been a rather complete one, because I have quite a collection of old radio magazines, but I have never seen an advert for it. The last advert known advert appeared on 29th July 1939, in Wireless Trader. By then people probably had other matters on their minds.
According to Hoffman, the main commercial problem was that all the established and popular bands and singers were already signed up to record companies.