MOTM

Tip for cheap and easy filter plots:

Since I wasn't able to find a software sweep generator and plotter that worked 
correctly, anytime I needed to look at the freq response of a phaser, filter or 
other device, I had to run white noise through it and then look at an fft 
analysis of it.  However, that produces an ugly and crude plot.  So I turned a 
synth into a sinewave sweep generator, run that through the device in question, 
and record into a wave editor application.  The way it squashes down a wave to 
fit on the screen ends up producing just the plots I wanted.  In something like 
CoolEdit, one can open up the fft analysis and move the line cursor over the 
plot in the background and it will tell you the freq at that point, so you can 
tell what frequencies the peaks and notches are at.  Here's a plot of the Nord 
phaseshifter:

http://home.att.net/~elhardt/Nord_Phaser_Plot.jpg

Good enough for publishing.  If you don't want a bipolor plot, just chop it off 
across the center.  Only drawback is you don't have the log graph overlaid 
ontop.  That could be applied in photoshop (or equivilant) though.

-Elhardt

Yup, an excellent technique. You can also synthesize frequency sweeps in the
computer and use those as the test signal. There is a freeware program
called MakeATestTone which generates AIFF files of frequency sweeps of
whatever duration, range, and amplitude you want. Play the test tone out
through one channel of your audio interface and record the output of the
device through an other channel.

I suppose the first thing one should do when employing this technique is to
loop the test tone output straight back into the recording input, so as to
measure the frequency response of your audio interface.

--Adam

Adam Schabtach writes:
>>Yup, an excellent technique. You can also synthesize frequency sweeps in the 
computer and use those as the test signal. There is a freeware program called 
MakeATestTone which generates AIFF files of frequency sweeps of whatever 
duration, range, and amplitude you want. Play the test tone out through one 
channel of your audio interface and record the output of the device through an 
other channel.<<

Yes, that sounds just like a freeware program I tried, but there were so many 
problems. Just listening by ear I could tell by the changing timbre and hollow 
sound of the swept sinewave that it was generating other harmonics. Can't get a 
pure sinewave out of this computer. The input back into the computer was so 
weak no matter what I tried, the plot was almost non-existant and kind of 
sloppy looking anyway. And it only plotted on a linear scale, not log scale.  
That's why I dropped that method and turned my Nord Mod into a sweep generator.

>>I suppose the first thing one should do when employing this technique is to
loop the test tone output straight back into the recording input, so as to
measure the frequency response of your audio interface.<<

And I forgot about this one. When doing that, one should get a straight line, 
but I was getting all kinds of bumps and ripples so any plot wasn't accurate 
anyway.

-Elhardt