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[AV Media Matters] Speed correction (was playingat1 7/8 and 3 3/4 ips)
Stuart Rohre writes:
<< Another speed correction technique for audio recordings from poorly
regulation portables is the play the originals on an instrumentation
playback deck that has external servo speed control. You can connect
audio oscillator of the appropriate servo control frequency, and tweak
exact speed control frequency to improve the voice quality.
Record copies on a high quality speed controlled deck. >>
Many years ago, when I was still in short trousers, the BBC used the
powered EMI L2 recorder for location radio reports. The reporters were
very fastidious about checking battery condition and the recorders would
often run more and more s-l-o--w---l----y. When played back in the
of course, the voice got higher and higher.
The solution, rather as Stuart suggests, was to modify a studio replay
machine such that the deck mains supply was powered from a separate
This was fed from an enormous ex-army audio amplifier - allegedly,
for instructing trainee parachutists - on the way down! The amplifier
fed with a signal generator, capable of giving 50 Hz of less and the
frequency was judiciously tweaked as the variation in the voice seemed
require. The synchronous motor followed the false mains frequency down
very low speeds.
It all sounds very crude now, but it worked extremely well. How much
though, in these days of crystal control.
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