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A Simplified Frequency Shifter for Improving Acoustic Feedback Stability

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A non-critical system for improving acoustic feedback stability in sound reinforcing systems has been designed. Simple adjustments of the rate of signal phase change provides a feature not available previously. The technique may be viewed either as a continuous change of signal phase or as a shift of signal frequency.

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Scott Dorsey
Scott Dorsey

Comment posted June 25, 2020 @ 17:12:58 UTC (Comment permalink)

A frequency shift stops PA systems from howling due to feedback reinforcement.  Although intolerable for musicians, this method gives an enormous amount of gain before feedback for voice-grade systems.  In the modern DSP world it's very easy to do a frequency shift with a Hilbert transform... but how do you manage it in 1962 with only analogue electronics?   Why, the same way!  You generate two versions of the signal, one shifted by 90 degrees, and you mechanically shift between the sine of one and the cosine of the other by spinning the rotor of the signal transformer.  The faster you spin the rotor, the greater the frequency shift.  This is just so ingenious it makes my teeth hurt.  Frequency response?  Probably not very good; those resolvers were designed for 400 Hz narrowband use, not for audio, and those all-pass networks aren't perfect either.  But I bet it makes the stadium announcer audible!

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