Re: "all pass" thought about (analogue) compression

From: Ken Smith (kensmith_at_green.rahul.net)
Date: 01/07/05 

Date: Fri, 7 Jan 2005 02:59:19 +0000 (UTC)

If you take a sine wave and run it through a circuit that does:

     Y = X ^(17/19)

the sine wave's RMS amplitude will be compressed towards about 0.98V RMS
and there will be some distortion. The 3rd harmonic will be about 2.7%.

Assume that the sine wave we start with is 300Hz.

A phase shifter (all pass filter) can be made with a Q such that the
900Hz, 3rd harmonic is shifted by 180 degree relative to the 300Hz
sinewave.

If we take this shifted signal and do another X^(17/19) operation on it,
the 3rd harmonic will only be about 0.2%

You don't need the phase shift to be exactly 180 degrees. Any non-zero
phase shift and two steps of (17/19) soft clipping will result in less
harmonic content than one step of (17/19)^2 clipping would produce.

If more distortion can be lived with, a lower power such as (11/13) could
be used.

Since the band of interest is 300Hz to 3KHz, we don't have to worry about
the harmonics of the frequencies above 1KHz. Those can be removed with a
simple low pass filter. I haven't verified it yet but it seems to me that
3 stages of phase shifter and 4 clippers should be able to make a
significant compression of amplitude but make less that 5% distortion on a
sine wave.

The intermodulation distortion will not be made zero by this method. If
the input has more than one frequency component, the distortion will be
much higher. 

-- 
--
kensmith@rahul.net   forging knowledge

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