ACG for PC soundcard input?

- J
- Jim Thompson
  
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posted
18 years ago

Sun, Oct 16, 2005 10:56 PM

Use a fast-attack compressor, but follow with a clipper for back-up safety.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

- P
- Pooh Bear
  
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posted
18 years ago

Mon, Oct 17, 2005 6:48 AM

The input clipper is the simplest answer. You'll have trouble guaranteeing that AGC will act fast enough to avoid trouble.

Graham

- D
- David King
  
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posted
18 years ago

Mon, Oct 17, 2005 2:49 PM

I have a notebook computer with line level input. I would like to use shareware PC based oscilloscope software designed to input to the soundcard ADC via a switchable attenuating amp. Yes, I do know the limitations with this.

The problem with these setups is how to avoid inadvertently damaging the card with over voltage. IOW if the attenuator is manually set too low or the signal level suddenly changes.

Can someone possibly recommend a circuit for a suitable automatic gain control such that it would not significantly alter the acquired signal?

My frequency of concern is 20-500Hz at 1-5Vpp. Typically these devices run off 5V available at the games or USB ports.

There are several circuits for the basic principle on the net, some use a clipping indicator, or zeners for crude limiting, but I always wondered why no ACG. That would be a logical and welcome addition.

Can anyone help?

Dave King

- R
- Roger Lascelles
  
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posted
18 years ago

Mon, Oct 17, 2005 4:02 PM

AGC and clipping indicators aren't much use with a PC oscilloscope. AGC would make all signals look the same size, thus hiding all the amplitude information. Clipping is immediately visible on screen as the signal hits the limits.

You won't blow up your sound card if you have enough series resistance, because the input circuit will almost certainly safely take 10 mA to +5V or ground when the signal goes outside those limits. 1K ohm in series, for example, would let you go 10 volts off scale, and 10K would make it good for

100V.

If that is not enough peace of mind, I suggest an external opamp driving the series resistor. The opamp can only swing as far as its power supplies permit. For example, a low voltage +5V powered opamp could *not* go outside

0 to 5V, or a +15V powered opamp with 2K2 of series resistance could only deliver 6mA etc. Even with a 5V powered opamp, I'd still want some series resistance, to handle the instant of connection etc.

I'm sure you can find a circuit on the Internet with an opamp and series resistance. A switched attenuator on the input would be nice too.

Roger Lascelles