Using Oscilloscope

dannyboy

Can the oscilloscope be used with probes or am I limited to only using it with a mic through a pre amp? 

benfaber

SignalScope and SignalScope Pro require audio hardware to be used for input signals, but you don't necessarily have to use mic preamps.  If you want to probe an electric circuit, you will want audio inputs with a high impedance to minimize the effect that your input hardware has on the signals you are trying to measure.

You might want to take a look at the SignalScope tips page (http://faberacoustical.com/SignalScope/Tips.php) for some hookup suggestions.

Ben

dannyboy

It really didnt take this long to reply I just mainly use Signal Scope for RTA, but have been using more for the o`scope and have been running into a problem hopefully you can help me out Ben.

I`m using it to measure an audio signal to check for clipping. I`ve fed the source a 0dbfs 1kb tone measured the rms voltage w/ a dmm .73 volts and set the full scale voltage according to that number...good so far?

Now,  I`m using an Mbox2 as the audio inputs. Is there any way I could be overdriving the inputs of the Mbox2? In other words could what the o`scope be showing as a  clipped signal not be the actual signal but something in the Mbox2?  I ask this because no matter what signal I measure I always get a reading of .73 volts even after setting the full scale voltage.

Here are some specs on the Mbox2:

Input impedance >1MOhm
Max input level 8volts Rms

Sorry for the long post.....any help is appreciated!

benfaber

I`ve fed the source a 0dbfs 1kb tone

The 0 dBFS tone is being generated by what device?

set the full scale voltage according to that number

So, in SignalScope, you tweaked the user-defined full-scale voltage until the oscilloscope indicated a level of 0.73 Vrms on the appropriate input channel.  What full-scale voltage did you settle on?

Getting the same reading suggests that you could be clipping on the input channel and that you are reading a peak voltage (Vpk) rather than an rms voltage (and that your full-scale voltage is incorrect).

What does the waveform look like in the oscilloscope?

dannyboy

FaberAST wrote
The 0 dBFS tone is being generated by what device?

Its a cd run through the head unit that I`m measuring. The cd was recorded @ odbfs according to the liner notes.

So, in SignalScope, you tweaked the user-defined full-scale voltage until the oscilloscope indicated a level of 0.73 Vrms on the appropriate input channel.  What full-scale voltage did you settle on?

I settled on 1.650 volts.

Getting the same reading suggests that you could be clipping on the input channel and that you are reading a peak voltage (Vpk) rather than an rms voltage (and that your full-scale voltage is incorrect).

Yeah the more I think about it the more I`m convinced my full scale voltage is wrong ( I think I read peak to peak as you suggest)which is something that I can fix. What worries me is clipping the inputs of my Mbox2 and if I am I dont know how to prevent that from happening. 

What does the waveform look like in the oscilloscope?

I guess its a bit squared not fully squared though.

dannyboy

sorry for that terrible use of the quote button!!!

benfaber

What it sounds like is that your head unit produces a higher output voltage than your Mbox can handle.  That's a bit surprising, though, since Digidesign claims the Mbox 2 will handle up to 8.7 Vrms on the input!  Perhaps you need to turn down the input gain or use the pad on the Mbox.

Do you have any specs (or measurements) that indicate what the full scale output of the CD player in your head unit is?