WaveSpectra and WaveGene THD measurement
To align the FFT bins correctly, perform the following steps:
a) Right click on the frequency box and change "FFT Samples" to match the samples you selected in WaveSpectra (see below).
b) Again, right click on the frequency, and select "Optimize for FFT" and you will see the frequency change.
c) Set the output level to somewhere between -3dB and -6dB - but note with some sound cards, the THD+N measurement will get worse due to increased noise as the main signal is reduced in amplitude, but experimenting with this setting will find a suitable optimum.
WaveGene is now optimised for THD measurements.
a) Select an appropriate FFT window size (sample points) in the settings.
b) Select an appropriate window function - for distortion measurements, my understanding is that "Blackman- Harris 7 Term" is the optimum here.
c) Ensure you have "Hz/dB" selected, in order to display the distortion measurements on the left hand side.
d) Change the averaging "Avg:" to 20
e) Turn off the main spectrum display by clicking the "Main" button once, this will leave the averaged display.
Check the distortion at different sample rates as with some sound cards, the distortion actually increases as the sample rate rises.
WaveSpectra can be downloaded Here (Site is in Japanese)
WaveGene can be downloaded Here (Site is in Japanese)
NOTE: The above site appears to be offline now, but you can still download the files from a local mirror or the Internet Archive (links below).
Local mirror, site translated to English from the original Japanese
You can use this tool to translate the entire site if your browser doesn't have a translation facility. NOTE: this doesn't seem to work with the archive.org mirror