Electronic Sustain Timer

[Robert Scott]

Sarah Fox writes:

 >This isn't nearly as complicated as it sounds.

No, it's much more complicated.  I know because I have done it in TuneLab.

If you look closely at the amplitude variations that occur as a note 
decays, you will see that even if only one string is sounding, there is 
still a beat-like variation due to the precession of the string from up and 
down motion to its ultimate elliptical orbit.  Any falseness in the string 
makes matters even worse.

When I implemented this function in TuneLab, I used an approach that does 
not depend at all on detecting when an amplitude crosses a 
threshold.  Instead, I defined two sampling periods, based roughly on the 
expected sustain time (which I know based on the selected note).  The first 
sampling period covers a fairly loud portion of the decay.  A second 
sampling period immediately follows, and it is twice as long as the 
first.  In each of these two sampling periods, I form an integral of the 
instantaneous amplitude (based on peak detection) over time.  Then, 
modeling the decay as exponential, there is a straightforward computation 
to determine the time-constant of the decay as a function of those two 
integrals.  Because this method is based on integrals and not on 
threshold-detection, it is much more immune to the effects of false beats 
and related imperfections.  You can download the free trial version of 
TuneLab to check it out.  The ultimate sustain time reported is an 
arbitrary multiple of the computed exponential-decay time constant, 
normalized to give numbers that agree roughly with subjective reporting of 
sustain time.

I understand that the RCT also has a sustain time measurement 
feature.  Perhaps some RCT users might comment on how that works.

And Sarah, as far as developing ideas goes, showers are for showering, but 
mowing the lawn - that's where the best thinking happens :-)


-Robert Scott
  Real-Time Specialties