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

[Geoff Sykes]

If the transmitting or storage medium of your source was analog then clock
speeds, transfer, repeater and amplifier delays, and any number of other
mishaps could indeed alter the frequency. Think speeding up or slowing down
a record player. (You do remember records, don't you?)

Digital, on the other hand, is simply a collection of one's and zero's in a
very specific order. Regardless of clock speed, storage, transmission, noise
or any number of inconsistencies in the signal path, even if you
deliberately change the speed the transfer process or processor clocks,
those one's and zero's will ALWAYS arrive in exactly the same sequence in
which they were sent. When they don't, the result is noise and dropouts, not
speed or frequency shift. In other words, a calibrated 440Hz tone sent over
a digital signal path will still be 440Hz at the receiving end. Changing the
pitch of a digital signal is possible, but it involves some serious and
deliberate number crunching. 

Broadcast tones, ala WWV, I would not trust as absolutely accurate. Tones
over the phone, unless you are absolutely sure there is no analog stage in
the chain, I would also hold suspect. Also, once a digital signal has been
converted back to analog it passes through amplifiers and a speaker so we
can hear it. Unless those components are extremely high quality the end
result might not be exactly spot on. But unless you have a precision
frequency counter or a calibrated spectrum analyzer handy you probably would
not be able to tell. Just how accurate do you want to be? We're talkin'
pianos here, not rocket science. 

-- Geoff Sykes 
-- Assoc. Los Angeles







-----Original Message-----
From: pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] On Behalf
Of Robert Scott
Sent: Thursday, May 25, 2006 6:59 AM
To: pianotech at ptg.org
Subject: Re: ETDs, PCs,PDAs & cellphones vs tuning fork : how accurate are
they ?


Philippe Errembault writes:

> Do you usually have an idea of the precision of the A440 reference  
>you use ? I mean... I didn't get any the precision information with  my 
>tuning fork, I found it on the net. I wonder what are the precisions of 
>professional ETDs, and what  precision we can expect from a pocket PC, 
>about which I wonder  if it even can be as precise as a normal PC...

If you have back-issues of the PTG Journal you might want to take a look at
"Calibration of Pitch References" in the August 2001 issue.  It shows that
all major ETDs are calibrated to better than .01 cents. The article also
discusses tuning forks, their temperature dependency and accuracy, and how
to calibrate them.


> ...it might be that PDAs only contain ONE reference clock, for time, 
> for CPU and for sound processing.

I have found that not to be true.  In fact, even on the Pocket PCs, they
sometimes have separate audio sample rate oscillators for the listening mode
and the sound-generating mode (recording and playback).  As for the CPU
clock, that is definitely not tied to the audio clock because the Pocket PCs
take advantage of switching to slower CPU clock speeds during idle times to
save on power.  You can't have your audio processing clock tied to such a
variable clock source.

It really does not matter that the audio sample rate is not precise, as long
as it is stable.  Once you do a software calibration on your ETD, the
results are the same as if the oscillator were perfect to begin with.


> There was also someone who claimed that listening a tone reference 
> through a cellphone was as good as with a normal phone. I know the 
> buffering of cell phone is small, but this doesn't change the fact 
> that any shift in it's clock should be retrieved in the output!..

You can depend on the frequency of sounds being delivered accurately over
the cell phone network.  There is no pitch distortion.  What is unclear is
whether the same can be said for the Voice-over-Internet Protocol Internet
services that deliver telephone service through your computer.  This is
potentially subject to indeterminate TCP/IP buffering and perhaps pitch
distortion.

Robert Scott
Ypsilanti, Michigan