Quote:
Originally Posted by Hartley118
I find this rationalisation of the 78 rpm standard speed pretty puzzling. Maybe in the US there were fixed-speed synchronous phonograph motors back in the day, where 78.26 rpm could be calculated from 60 Hz mains and an integer gear ratio, but was there ever such a fixed-speed 78 rpm synchronous gramophone motor in the UK?
AFAIK all UK pre-LP 78 rpm gram motors used a governor-controlled variable-speed motor. Even the exotic Garrard 201 direct drive turntable had a speed control. So has the neat and tidy 78.26 rpm calculation ever had any relevance at all here in the UK?
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The attached Wireless World article (from 1931) indicates that gear-drive turntables were found in British practice.
But I doubt that the precise standardization, to 78.26 rev/min, had much to do with domestic turntables, which often had a variable speed facility anyway. More likely it came about because of professional transcription needs, where gear-driven turntables with synchronous motors were found, and where precise timing was needed. The latter indicated a need for precision with both recording and playback speeds. 78.26 rev/min was a 23:1 reduction from 1800 rev/min, the speed of a 4-pole synchronous motor operating from a 60 Hz supply. Also, where variable speed turntables were used for transcription purposes, 78.26 rev/min was easily set with a (60 Hz) stroboscope.
The 50 Hz case would give 78.95 rev/min as a 19:1 reduction from 1500 rev/min. Whether that was the actual recording speed in 50 Hz areas I do not know. But with stroboscope-equipped turntables, it would often have been the playback speed. The RCA 70-series synchronous motor, gear-driven professional turntables were available in both 60 Hz and 50 Hz forms. My best guess from the nature of the mechanism is that the “78” speed was 78.26 rev/min at both supply frequencies. (See:
https://www.vintage-radio.net/forum/...0&postcount=59.)
(Typical stroboscopes also produced a slight error with the 50 Hz. 45 rev/min combination, “locking” at 45.11 rev/min.)
The old British Standard BS1928 for gramophone records – which I have not seen - might provide some clues, in that it probably specified actual recording speeds, and so might tell us whether late-era British 78s were recorded at 78.26 or 78.95 rev/min.
Cheers,