Pitch Raises ... Overpull Percentage

[Robert Scott]

Conrad Hoffsommer asks:

 >For the benefit of newbies and non-ETD users, could you review just what
 >that is 20%/25%/30% OF?
 >I think I know, but I get confused ezee...

The question is a good one.  Different ETDs apply the 20%/25%/30% to 
different things.  I think the SAT applies the percentage to the flatness 
of a single note.  You can periodically measure a new note as you tune so 
as to update what the percentage applies to, but it is still one 
note.  TuneLab and the RCT both measure every note before it is 
tuned.  They use running averages to determine the flatness of the region 
of the piano where you are tuning, and apply the overpull percentage to 
that running average.  I think RCT uses what is sometimes called a "boxcar" 
average, which is an ordinary average of the last few notes.  TuneLab uses 
a declining weight average to estimate the flatness of the piano where you 
are tuning.  A weighted average gives more weight to the more recent notes 
and less and less weight to notes that are more distant from where you are 
tuning, in order to form the average.  In any case, however it is 
determined, the overpull percentage is applied to the flatness of the 
piano, and then the result is added on to the target pitch for the note you 
are about to tune.  For example, if the measurements say the piano (or the 
region where you are tuning) is 40 cents flat, and the overpull percentage 
is 25%, then the ETD will set a target pitch that is 10 cents higher than 
normal, in anticipation of the pitch falling that 10 cents by the time you 
finishing tuning the whole piano.

-Robert Scott
  Real-Time Specialties