"Self tuning piano" questions

[Don A. Gilmore]

----- Original Message ----- 
From: "Isaac sur Noos" <oleg-i@noos.fr>
To: "Pianotech" <pianotech@ptg.org>
Sent: Thursday, December 25, 2003 6:17 AM
Subject: RE: "Self tuning piano" questions

Merry Christmas, Mr. Oleg:

> Don,
>
> While i did not get really the inside of the ^process, I understood
> that you are able to measure pitch on a very short moment .
>
> Is the measure occurring as long as the system is on (and corrections
> applied with heat all along ?)

No.  The tuning process takes place only when the system is swiched on.  It
takes about 30-60 seconds and then the tuning is maintained as long as the
system is on.  In other words, the "in-tune" values of electrical current
for each string are stored and then held constant to keep the piano in tune
for that day's conditions.  The next time you turn on the system, you get a
new, custom tuning.

> Is the measure/pitch regulation occurring always at the same time, or
> is it the fact that the initial tuning is recorded by the same process
> that gives enough accuracy - the question is about the lack of
> stability of pitch, if the measure occur only once, it should be
> precisely at the same moment to be accurate.

The strings are sustained magnetically at a specific, repeatable volume
during the tuning process.  If you take the signal from the pickup coils,
amplify it and then apply this signal to another coil, the latter coil will
actually sustain the string as if drawing a violin bow across it.  This
allows the system to tune without the user having to play any notes.  It
also ensures that the string is always tuned at the same, constant volume.
After the piano is tuned at the factory, this is the volume that the tuning
is "recorded" at, so the tuning you get in your home will be very precisely
duplicating that at the factory.

> But I suppose it is a real time system is not it ?
>
> Why are the pickup not catching overtones (are they partials ?)

Yes, by "overtones" I mean partials.  There are two reasons why I don't get
overtones.

1.  Because the pickup is very small and cheap.  The larger, more complex
and expensive a coil is, the more partials it can reproduce.  This is the
goal in a guitar pickup.  But in our case, it behooves us to use a crappy
pickup.

2.  Because the feedback circuit that sustains the string tends to want to
only drive it at its fundamental.

The combined effect of these factors means that the signal shows up on an
oscilloscope as an almost perfect fundamental sine wave.

Don A. Gilmore
Mechanical Engineer
Kansas City