Re: Instantaneous (analogue) compression of speech signals

From: Roger Hamlett (rogerspamignored_at_ttelmah.demon.co.uk)
Date: 01/04/05 

Date: Tue, 04 Jan 2005 16:56:19 GMT

"John Woodgate" <jmw@jmwa.demon.contraspam.yuk> wrote in message
news:2pRIs3CKdb2BFwac@jmwa.demon.co.uk...
> Does anyone here have any experience of instantaneous (analogue)
> compression (aka soft clipping) of speech signals? I've been doing a
> little work on it but I'm unable to judge the resulting sound quality.
> Why do treble boost controls no longer have any audible effect for me?
> (;-)
It really does depend what you want. Generally, you can clip quite
agressively, _provided_ you have an automatic gain adjustment before the
clipping, and may actually improve intelligability if this is done right.
The IC that used to be commonly used, was the Plessey SL6270 VOGAD (voice
operated gain adjustment device), which massively reduced the dynamic
range needed for speech. A search on this may find an equivalent. Are you
trying to listen while speaking, or recording the sound. It is terribly
difficult to 'judge' your own voice through such a circuit. Try using a
text message recorded by somebody else, and playing it through the
circuit.
I might even be able to find a couple of the original Plessey IC's. We
used them many years ago, as part of a speech digitisation system, and
there may still be some in my cupboards somewhere!...
Presumably you have lost most of the higher frequency response in your
hearing. This has practically no effect on speech (you can filter
everything above about 2.5Khz, and still understand speech perfectly), but
would massively reduce the effect of treble 'boost' controls.

Best Wishes

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