Raw Voltage from the sound card redux

[cmyr]

Hi all,

A while ago I picked up an antique analog chart recorder, and I've been playing around with trying to get it to chart arbitrary data. I've had it plugged into a sound card and have then run various signals at it, but I'm not that familiar with the workings of PCM or how signals are converted to analog, and I've run out of ideas. I thought I'd ask the forum quickly before I gave up on using audio and tried other techniques.

There was an earlier thread detailing something similar, here: http://forum.audacityteam.org/viewtopic ... 0&start=10


Essentially, I have a device that receives a DC input, and then charts the voltage of that input using a pen connected to a small motor, which moves across a spool of paper. The spool can turn at a variety of speeds, which provides an X-axis. The chart recorder can be tuned to register voltage changes from 1 mV up to 10v.

what I would like to do, ideally, is write a program that will take an arbitrary input and then create an analog signal through the sound card which has a voltage corresponding to the input. I have no idea whether or not this is particularly.

At this point, I think I'm mostly unsure of how sound is represented in voltage, and whether or not outputting a constant voltage is even possible. I've played around with generating various tones, as well as with using octave (matlab) to create RAW files of various constant values; in the first case I just get constant voltage fluctuations, and in the second I don't really get anything at all.

Anybody have any idea what I could try, at this point? I'm starting to run out of ideas.


thanks in advance.

[kozikowski]

Audio in general is varying voltages both plus and minus. That's why the blue waves go to plus and minus with zero in the middle in this illustration...

http://kozco.com/tech/audacity/Audacity1_playback.jpg

So if your instrument will respond to both polarities of voltages, you're golden.

Or you would be golden if the waves didn't change so fast. A 3000Hz tone will rip your ear off. It's right in the middle of the most sensitive part of your ear response. Think baby screaming. That means its voltage is changing direction 6000 times a second. Do you think you can make a pen draw and change direction 6000 times a second -- particularly an old one?

That's not to say you can't make it work. Classic American VU meters would work up to 20000Hz...

http://en.wikipedia.org/wiki/VU_meter

...but they weren't measuring each and every change in the wave. They "rectified" the waves to make them all go positive, and then they used the inertia and weight of the meter movement to ride over the average of the waves. This is an accurate electronic simulation of the original meter.

http://www.kozco.com/tech/betaSP_VU_H264.mov

Don't try to play it on-line. Download it.

You could do that, too if you're handy with tools and Radio Shack electronic parts.

Koz

[cmyr]

Thanks for the reply. I'm starting to think that my device is only registering positive voltages, which means I'm going to have to look at another solution, or rolling something myself. The only hardware output I can think of right now that might use a variable voltage output is an old two-pin fan connector, so I'm not holding my breath. I'll figure something out...

[kozikowski]

You are, or should be "Signal Conditioning." Preparing the signal so it's compatible with your instrument.

Koz