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Instantaneous (analogue) compression of speech signals

J

John Woodgate

I read in sci.electronics.design that Ken Smith
about 'Instantaneous (analogue) compression of speech signals', on Wed,
5 Jan 2005:
How about this:


CA3080
---------!+\
! >---------+---------
--!+/ !
! !
---------------+
!
---
---
!
GND

The distortion should only be odd order harmonics since the CA3080 puts
out nearly the same current in each direction. Since it is slew rate
limiting, it is equivelent to a trebble boost, clip and then trebble
cut. This should reduce the amplitude of the harmonics. You can vary
the slew rate slope with the pin 5 current on the CA3080.

It looks very interesting in terms of functions. But the device is
obsolete and there is no recommended replacement.
 
J

John Woodgate

I read in sci.electronics.design that Reg Edwards
..com>) about 'Instantaneous (analogue) compression of speech signals',
There is a microphone gain control pot. Following the microphone
amplifier is a simple 400 Hz to 3 KHz filter. The filter should be
correctly terminated with Ro to minimise over-swing on sharp speech
transients. Maximum available filter output volts being about 6 volts
peak-to-peak.

The application is not for radio, and I need 100 Hz to 5 kHz bandwidth.
I have an active filter at the input, to condition mic and CD signals.
There is then a 10K resistor followed by a pair of back-to-back small
signal, high-gain transistors. The transistors, such as BC109's, are
diode connected with base connected to collector. The transistors behave
as clipping diodes with a much sharper than normal knee transition. The
sharper the better!

That's interesting. I have 10 kohm, and shunt 1N4148s with series
resistors to *soften* the knees. I'll try hardening them, your way.
The maximum output from the clipping circuit is plus or minus 0.6 volts.
The amplifier following the clipping circuit should have a high input
impedance, 100K or greater, so as not to interfere with clipping action.

Yes, I have a TL072 buffer. 1 Tohm should be high enough!
 
J

Jim Thompson

I read in sci.electronics.design that Jim Thompson
4ax.com>) about 'Instantaneous (analogue) compression of speech
signals', on Wed, 5 Jan 2005:


Sadist! Do you KNOW the series expansion of tanh? I realise that it is
easily implemented with a long-tailed pair, but I can't calculate the
harmonic spectrum; I'll have to write a Mathcad script.

But thanks for the suggestion - I think.

I've already run a simulation. Want to see it?

...Jim Thompson
 
J

Jim Thompson

I read in sci.electronics.design that Ken Smith
about 'Instantaneous (analogue) compression of speech signals', on Wed,
5 Jan 2005:

It looks very interesting in terms of functions. But the device is
obsolete and there is no recommended replacement.

I have a trivial solution for that, also. Is the concept valid, or
wild-ass?

...Jim Thompson
 
K

Ken Smith

I read in sci.electronics.design that Ken Smith
about 'Instantaneous (analogue) compression of speech signals', on Wed,
5 Jan 2005:
How about this:


CA3080
---------!+\
! >---------+---------
--!+/ !
! !
---------------+
!
[....]
It looks very interesting in terms of functions. But the device is
obsolete and there is no recommended replacement.

Ok, how about a LT1228?

You even get an added buffer stage.
 
J

John Woodgate

I read in sci.electronics.design that Jim Thompson
4ax.com>) about 'Instantaneous (analogue) compression of speech
signals', on Wed, 5 Jan 2005:
I've already run a simulation. Want to see it?

Yes, please. JMW[at]JMWA[dot]demon.co.uk
 
J

Jim Thompson

I read in sci.electronics.design that Jim Thompson
4ax.com>) about 'Instantaneous (analogue) compression of speech
signals', on Wed, 5 Jan 2005:
I've already run a simulation. Want to see it?

Yes, please. JMW[at]JMWA[dot]demon.co.uk

See...

Newsgroups: alt.binaries.schematics.electronic
Subject: Audio Clipping Question, ala S.E.D (Woodgate) -
TanhClipper.pdf
Message-ID: <[email protected]>

...Jim Thompson
 
R

Rich Grise

Keep us posted... this is becoming a fascinating subject.

It's sounding more and more like John Woodgate is looking for a log amp. I
had a class on them once. ;-)

And to Mr. Dyson - thanks for your exposition; I "get" the concept of FFT
- turn the signal sideways, nothing to it! I just don't want to plow
through all that arithmetic. I once did a bank of 8 bandpass filters, much
like a "graphic equalizer", in an effort to extract formant information
based on the _relationship_ of the components. I saw an experiment on
edjamacayshunal TeeVee, where they did a Pavlov's Dog-style experiment,
with human infants. They used, instead of a bell, a human voice saying one
phoneme, for example, "ah", or "ee". They trained the human infant to
respond to a particular phoneme, and the infant responded to the same
phoneme, _no matter who said it_. Old man, young girl, child, adult man,
adult woman - it didn't matter, as long as they were saying the same
phoneme. So clearly, the phoneme itself is determined by the _pattern_ of
the various formants, without regard to the fundamental, or even the
specific frequencies.

I got bogged down while trying to find some kind of pattern-matching
algorithm, and had to abandon the project because of other considerations.
It was intended to be a speakwrite. Or a first pass at one. To control a
handicap-assist robot.

Thanks,
Rich
 
R

Rich Grise

I read in sci.electronics.design that Pig Bladder
ruid.net>) about 'Instantaneous (analogue) compression of speech
signals', on Wed, 5 Jan 2005:

No, I have hearing aids and I make no secret of them. What I'm doing is
about:

- the effects of (non-linear) signal degradations on speech
intelligibility;

- the anomaly that some 'degradations' actually increase
intelligibility.

Ah! Finally! ;-P

First, get rid of the fundamental. That's almost a given - 300-3000 Hz was
what I read in TRAH, Reg Edwards says 400 at the bottom.

I wonder, since there's log amps, if there's log log amps. And when I had
the class on the LVAs (Log Video Amps - they call them video because
that's the part of the spectrum between audio and IF), I don't remember if
they accommodated bipolar signals, but I can't imagine them not. The
logarithmic part was only "gain".

But that "Apparent Anomaly" - now _there's_ some meat!

Have you any examples at hand?

Thanks,
Rich
 
R

Rich Grise

On Wed, 05 Jan 2005 20:55:43 +0800, budgie wrote:

.... I could send you a scan of the relevant
part from one type that we found surpisingly good when overdriven by a large
margin.

I've noticed that Mr. Woodgate said, "yes, please" - for the benefit of us
roobs, would you be so kind as to post it also to
?

Thanks,
Rich
 
R

Reg Edwards

John,

The filter preceding the clipping circuit can be a high-pass (first)
followed by a low-pass.

In a speech waveform, specially the male voice, most of the signal amplitude
is contained in the lower frequencies, say 400 Hz and below. So the
high-pass filter section tends to level the amplitude even before clipping.

Nearly all of the information is contained in the band 400 Hz to 3.3 KHz.

The clipper turns the large low frequency components which get through the
filter into square waves. There's a peculiar effect. The human ear and
brain partially succeeds in re-constituting the missing low-frequencies.
Imagination? Dunno about dogs' ears.

The higher frequencies are distorted into much higher frequency odd
harmonics. The function of the low-pass filter section is to minimise the
effect.

A second low-pass filter can be inserted after (not immediately after)
clipping to limit the RF bandwidth actually transmitted.

You can adjust filter frequencies to suit your own application. There is no
interaction between the various cascaded circuit sections.

Oscilloscope patterns obtained with speech, music and sinewave inputs are
very interesting while varying the two gain controls and listening on a loud
speaker. Despite the sharp clipping action, the onset of clipping and
distortion is quite soft.

An alternative clipping circuit is a cathode or emitter-coupled pair.

In my younger days I did a lot of testing and fault-locating on GPO music
circuit transmission lines into BBC broadcasting and TV stations. Used
headphones as the detector to balance impedance bridges up to 20 KHz. Found
it possible to train one's ears to hear up to 20 KHz. Saved time changing
from headphones to amplifier-plus-meter at 10 KHz where most people got
stuck.
 
P

Pig Bladder

I like the homomorphic compressor, not because it is better in any way but
because it is unusual.

I just LOVE that new word. "To change into oneself." ;-P
 
J

John Larkin

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?
(;-)

If you could tolerate a time delay, you could do a very nice smooth
AGC thing without the clipping problem that results from a fast-attack
signal. A negative delay line (future predictor) would be handy here,
too.

John
 
J

John Woodgate

I read in sci.electronics.design that John Larkin <[email protected]>
wrote (in <[email protected]>) about
'Instantaneous (analogue) compression of speech signals', on Wed, 5 Jan
2005:
If you could tolerate a time delay, you could do a very nice smooth
AGC thing without the clipping problem that results from a fast-attack
signal.

True, this technique is well-known, but it's costly. I'm looking for an
ingenious low-cost solution.
A negative delay line (future predictor) would be handy here,
too.
I can do that with a -2 ohm resistor, which has a conductance of -half a
mho.
 
B

budgie

Someone once told me that mixing a speech signal with itself but
shifted up one octave also makes it easier to understand. Never tried
it though. I think you are referring to a similar effect.

nope.
 
B

budgie

On Wed, 05 Jan 2005 20:55:43 +0800, budgie wrote:

... I could send you a scan of the relevant

I've noticed that Mr. Woodgate said, "yes, please" - for the benefit of us
roobs, would you be so kind as to post it also to
news:alt.binaries.schematics.electronic ?

probably - what's a roob? Is that a reclusive noob?
 
B

budgie

On Wed, 05 Jan 2005 20:55:43 +0800, budgie wrote:

... I could send you a scan of the relevant

I've noticed that Mr. Woodgate said, "yes, please" - for the benefit of us
roobs, would you be so kind as to post it also to
news:alt.binaries.schematics.electronic ?

Arrrrgggghhhh! My news service doesn't carry binary groups.
 
J

John Woodgate

probably - what's a roob? Is that a reclusive noob?

It's obviously a regressive boor, neatly tying in with another thread
(and being true!).
 
J

John Woodgate

I read in sci.electronics.design that Jim Thompson
4ax.com>) about 'Instantaneous (analogue) compression of speech
signals', on Wed, 5 Jan 2005:
See...

Newsgroups: alt.binaries.schematics.electronic
Subject: Audio Clipping Question, ala S.E.D (Woodgate) -
TanhClipper.pdf
Message-ID: <[email protected]>

It hasn't come through. Could you please email to me at JMW[at]JMWA[dot]
demon.co.uk?
 
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