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Stereo has only been around for ˜60 years.Aside from aircraft detection, Bell Labs experiments, and Disney's
Fantasia....
I have been trawling old AUDIO mags.
Commercial production of stereo tapes and players in late 1956. However the early ones used a second mono head mounted 1.5" (or 1.8" or 1.6125") over from the existing mono head. Even if there had been a standard distance, phase coherence was improbable; but golly, *two* channels! Stacked (two in one) heads were already coming, but the market was chaotic, tape was 4X the price of disk, and only bleeding-edge enthusiasts bought into stereo tape.
November 1957: JBL's Ranger-Paragon stereo speaker is reported to be 18 feet long (correction, 8'10"), gave "a marvelous spread of sound".
The real butt-kick for home stereo was the stereo LP. Announced in late 1957, Westrex 3A Stereophonic Disc Recorder head featured on cover of STEREO Jan 1958. Competing systems quickly worked-out, and disks/systems followed the next year.
Feb 1958, Radio Shack was offering their $19.95 speaker "2 for stereo" at $37.50. (No other stereo gear listed.) (AR2 was $96, each, at Harvey's.)
April 1958: Audio Fidelity has "no less than four Stereodiscs on the market". Fairchild released a "248 stereo preamplifier" which was very clearly two 245 mono preamps plus a stereo volume control in one cabinet. Pilot's SP-215 was a more integrated stereo preamp but very costly. Also Ampex and Sony(!)/Superscope offered stereo tape machines with stereo speaker systems. May, Bogen offers a 2*12W integrated stereo amplifier DB212, Fisher 400 stereo control preamp, Scott "StereoDaptor" passive switch and volume knob.
Stereo FM was in FCC-hell for a long time (AM+FM simulcasts were tried for years); yet in 1958 there was a tuner with "multiplex" jack to go to a hoped-for stereo demultiplexer. (I remember Fisher external stereo decoders.)
2015-1956= 59 years, give/take.
2015-1958= 57 years, give/take.
http://www.americanradiohistory.com/Audio-Magazine.htm BTW: 17 June 1955 (60 years last month), the ISO agreed to shift Tuning A from 435cps to 440cps. Handel had used 422, Mozart 421; London Philharmonic was up to 455 by 1874, but in 1879 Adelina Patti flatly(sic) refused to sing at that pitch. 1834 a bunch of physicists proposed 440, but nobody cared. 1858 the French commissioned a study and got 435. English then used both 439 (Philharmonic) and 452.5 (Military).