ETDs, PCs, PDAs & cellphones vs tuning fork : how accurate are they ?

[Porritt, David]

Robert:

I have a VOIP phone so I tried your experiment.  I held the VOIP phone
up to my right ear and the cell to my left.  I couldn't detect a beat or
any pitch difference.  I could notice the short delay that the cell
phone has so that the second "ticks" don't sound together.  The VOIP
phone was ahead of the cell by about the amount I notice when making a
voice call (you know, you're talking to some one and they finally come
to the door and you hear them live and on the phone!)  

As a side note, I've certainly been happy with my VOIP phone and it is
the only phone I have since we moved to the country.  No phone line,
just a fixed wireless internet connection which I've recently upgraded
to 1.5 Mbs.  I love that!

dave

David M. Porritt
dporritt at smu.edu

-----Original Message-----
From: pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] On
Behalf Of Robert Scott
Sent: Saturday, May 27, 2006 5:25 PM
To: pianotech at ptg.org
Subject: Re: ETDs, PCs,PDAs & cellphones vs tuning fork : how accurate
are they ?

If I may dare to jump into the discussion with Philippe and Geoff, let
me say that for five years now I have been recommending that TuneLab
customers use the NIST tones through their telephone to calibrate their
Pocket PCs and laptops.  Throughout all that time, no one has ever
reported to me that when they check the calibration using their
cellphone that the pitch is anything other than rock-solid and
repeatable.  If there were any day-to-day pitch changes due to something
that happens through the cellphone network, someone would have noticed
it by now.  I am reasonably sure that cellphones produce faithful
representations of the pitch that was sent.  Precise time-base
synchronization is the bread-and-butter of the cellphone companies.
They use it extensively to synchronize all the data transmission their
network.  If there is a phase-locked-loop in the cellphone, you can bet
that it has no audible frequency modulation.

However, because the question came up a few days ago, I am still
investigating VoIP (Voice Over Internet Protocol) - the service that
uses the Internet to place telephone calls.  Because of the lack of
precise synchronization over the Internet, it is hard to see how pitch
precision can be transferred over the Internet.  VoIP was one of the
topics on last Friday's "Science Friday" on NPR.  After the program I
e-mailed the the guest (the author of a book on VoIP) asking about
possible pitch distortion through VoIP.  Unfortunately, this "expert"
was actually unaware that audio CDs can have pitch distortion, depending
on the precision of the playback speed.  So I was unable to get any
useful information from him.  However, if any of you have VoIP, you can
perform the following easy experiment and report back to the rest of us:

Call the NIST at (303)499-7111 in Colorado on your VoIP connection and
simultaneously on your regular phone.  Listen for any beat between the
two sounds.  There should be none.  But if there is, then we have
evidence of pitch distortion through VoIP.

Robert Scott
Ypsilanti, Michigan