audio mixer - graphical display of sound output? compare 2 signals?

[bonanz]

Hello,

I have a problem with a dj mixer.  There is a decay on the crossfader. for a scratch dj the cut in time on the crossfader is very important.  My particular mixer seems to have a problem with a rc time constant most likely that causes a slight muddy sound when doing fast and complicated scratch moves.  It is hard to notice alone, but when compared side by side with another (top of the line) dj mixer, the difference in the 'cleanliness' of the cuts is evident.

I'd like to graphically display the difference between the two.  I'd like to play a tone and use the crossfader to cut the sound in and out and compare the voltage, or volume level to inspect the slope of the decay.  This would probably be straight forward with a scope and measuring the voltage somewhere in the crossfader circuit, but I want to compare the two, but I am not willing to disassemble the 'top of the line' mixer I want to compare the faulty mixer to.  The faulty mixer I am willing to take apart, but I want to be clear that the problem is occurring.

Can anyone offer any suggestions on how to creat a graph from the audio how quickly it goes from full volume to 0.

here is an mp3 file of me playing a tone and cutting the sound with the crossfader

http://rapidshare.de/files/8264098/tone_cut.MP3.html

click on the link, then click free at the bottom, then it will make you wait for approx 15 secs or so and then offer the link for download.

[bonanz]

here is essentially what I want to be able to graph but I'm not so concerned about fader travel, maybe plotting volume against time would suffice

[audioguru]

Hi Bonanz,
The cut-in sounds good, but the fade has a "clunk" sound with it, like an abrupt  DC level-shift. Can you post the schematic of the fader circuit for us to see what might be causing it?

It would be easy to see at the output of the mixer on an oscilloscope. You could compare the input to the output or even compare the two mixers. ;D

[bonanz]

I haven't taken the one mixer apart yet, I was kinda hoping I could compare the cut in times without taking them apart first.

[audioguru]

I think the "clunk" sound when fading quickly is a fault of the VCA called "control signal feed-through". It would also occur as a "click" if the fading is extremely fast like a sudden mute, but it happens when the signal is at one of its peaks so the muting causes the output to suddenly drop its instantaneous DC level very far.
Digital pots have solved that problem by switching only at a zero-crossing of the signal.
Maybe a modern digital pot with that feature would be much better than the VCA for the cross-faders.

On your waveform pic the volume's change with time is difficult to see. If you use a much higher frequency then it will have much better resolution.