The THD Meter is documented in the next revision of the ‘Foo manual. It
was originally intended for internal use only, but after many requests
it has been added to the release version. This is why the UI is not
polished.
To select your input, hold the control key and click on the settings
button. This will allow you to select your input signal.
The Frequency Locked indicator will come on when the THD Meter senses a
single frequency signal, and will display the frequency.
– The S/(N+D): display shows the signal to noise plus distortion
measurement.
– The THD+N display shows the total harmonic distortion plus noise
measurement.
Both measurements are shown as a percentage, as well as their levels in
relative (dBr) and full scale (dBFS) units.
The THD Meter also shows the input level referenced to full scale. By
clicking the “A-Weight” checkbox, the results may be weighted.
To properly use the THD meter, you must analyze a single frequency
signal. An easy way to do this is by using SpectraFoo’s Signal
Generator, with synchronization enabled.