Michael Berry suggests a way of measuring Total Harmonic Distortion (THD) using plug-ins in Pro Tools and as it is a detailed tip we said we would post it separately….

Just a few years ago every studio had  some basic test equipment including a distortion analyser.  That seems to be the exception rather than the rule these days.  A distortion analyser can be very handy to have around.  One common use is to adjust bias and balance after replacing tubes in a guitar amplifier. It is possible to create a distortion analyser with free/bundled plug-ins.  The residual distortion will be limited by the quality of the interface used but these days that is more than adequate for most studio uses.

You need the signal generator, the Bomb Factory noise meter, and the channel strip.  Additionally, the free Blue Cat spectrum analyzer, FreqAnalyst can show the character of the harmonic products very nicely.  If you don’t have the new channel strip, the EQ-3 1-band will do.  The channel strip is nicer as it can go up to a Q of 100 while the EQ-3 is limited to 10. You simply use the signal generator plug-in to send a signal out of your interface, through whatever you want to measure, and return it to another track.  

On the return track, use the new channel strip filter to notch out the tone that you sent out.  Set the unfiltered tone to zero on the Bomb Factory meter and when you insert the filter, the meter will read the distortion products left over.  To get a more conventional reading of distortion in percent, here is a link to an on line calculator so you don’t have to do the math.  It is really nice to use the Freqanalyst as you can see if the products are even (nice sounding) or odd (bad sounding). Optionally you can add a high pass filter just below the fundamental which will let you measure even in the presence of hum.

Thanks Michael for this idea.