Um, if i am understanding THD correctly, then the DD-1 must have an internal filter,(like a 39.99hz to 40.01hz, and one similar for 1k.
It samples both what goes through the filter and also samples what does not. So once whatever does not go through the filter is above .99% of the total signal it lights up the distortion light.
Or am i way off here.
That could possibly create a situation where if for some reason you were processing the signal and it was effecting 40hz, the dd-1 would show distortion sooner. Because on an o-scope you would just see a waveform, and not notice if the frequency shifted.
edit:
i mean the user may not notice an effected(sp) signal