Audacity as an oscilloscope

Special Interest Groups Audio Processing
1

Audacity forum,

I have been using Audacity for some time and recently I’ve
been trying to use it as an oscilloscope.

I am feeding it a square wave that swings to .5v for 500ms
and 0v for 500ms in the microphone port.

The problem is all I see on Audacity is a sharp negative spike at
the falling edges.

Any suggestions as to what I’m doing wrong.

Thanks for any reply.
Jerryd

2

Q.what I’m doing wrong.

A. Potentially feeding signals into your microphone socket which could damage the soundcard.

Depending on the source of your 0.5V square wave you could deliver excessive current to the sound card : potentially frying components.

BTW a DC blocking capacitor is probably responsible for the display only showing spikes, not a squarewave.
The signal you are feeding it is only 2Hz, sound cards are designed for audio signals which are in the range 20Hz - 20000Hz.


http://mackys.livejournal.com/926903.html
http://www.ehow.com/how_2278973_use-sound-card-oscilloscope.html

3

Trebor,

Thanks for the reply.

The signal is right off a 555 timer and there is a series
resistor and resistor to ground making a voltage divider.

I’m now getting a spike, positive and negative, whenever the
signal is changing state.

I think I don’t see a square wave because the sound card
wants to level the signal for any standing DC so any time
my signal is not rising or falling it goes flat at 0 on the
Audacity display.

Can I somehow get the sound card to put out actual levels?

jerryd

4

IIRC a 555 can be a source of ~100mA which is 100x greater current than you should apply to the mic input.
You need to prevent too much current or too high a voltage being applied to the mic input (see the links in my previous post).


spikes.gif
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The spikes are due to DC blocking capacitor. Soundcards are designed for audio frequencies 20-20000Hz AC, not DC signals (which are rightly blocked).

5

Trebor,

I changed the frequency and, just as you said, I now have
a square wave displayed.

I also understand the spikes are caused by the DC blocking
capacitor.

Thanks, this helped me a lot.

I’ll probably be back with more questions.

jerryd

6

Not if you damage your computer, (no computer, no internet).

If I was doing what you are doing I’d use one of these to protect the computer … http://en.wikipedia.org/wiki/Opto-isolator