[ntp:questions] Using NTP to calibrate sound app

unruh unruh at invalid.ca
Mon Jan 28 22:04:48 UTC 2013

On 2013-01-28, Jeroen Mostert <jmostert at xs4all.nl> wrote:
> On 2013-01-27 23:43, unruh wrote:
>> On 2013-01-27, David Taylor<david-taylor at blueyonder.co.uk.invalid>  wrote:
>>> On 27/01/2013 19:33, unruh wrote:
>>>> On 2013-01-27, no-one at no-place.org<no-one at no-place.org>  wrote:
>>> []
>>>>> In case you are wondering, my app is a professional piano tuning app.
>>>>> The standard in this industry is that tuning devices should be
>>>>> accurate to 12 parts per million.  I know that is probably overkill
>>>>> for tuning pianos, but that is what the professionals expect from
>>>>> their equipment.
>>>> Ah. I would expect 1 cent, which is more like 500PPM.
>>> 1% (10,000 ppm) is a 4.4 cycles per second beat at 440 Hz!  Completely
>>> unacceptable.  You want an imperceptible beat, ideally, well under 1 Hz.
>>>    Agreed that 12 ppm is overkill.
>> I agree that 1% is pretty bad-- that is 1/6 of a semitone, which is
>> clealy preceptible. However 1 cent, 1/100 of a semitone, is the limit of
>> audibility
> Not really. A cent is simply 1/100th of a semitone, no more, no less. It's true 
> that few if any should be able to distinguish a note from a note that's one cent 
> off when heard in isolation, but the cent is not some sort of biological limit, 
> as far as I know. When played together, a difference of one cent between notes 
> is certainly audible in the beating (at least on artificial waves, I have no 
> idea if the same is true for physical pianos). Whether you can even physically 
> tune a piano that accurately is another matter altogether. Even if you can't, 
> you should still like to be able to tell that you didn't.

at 440 Hz, 1 cent is a frequency difference of .25Hz -- ie 4 sec, and
the piano note dies out faster than that. Also the various strings in a
note ( remember that pianos have 2 or three strings per note) cause
frequency difference of greater than that. The higher harmonics for
which the beating would be faster (eg 4th harmonic would have a 1 Hz
beating,) are also "out of tume"-- ie the 4th mode is not 4 times the
fundamental, on a piano but slightly greater. Also, the piano pegs have
a pretty coarse adjusting mechanism-- static friction significantly
higher than dynamic, so you tend to get stick-slip, making extremely
fine admustments in the tension hard. 

And who "should you still like to able to tell that you didn't"? A piano
tuner is there to make the piano sound good, not to engage in
unwarrented mathematical games. 

> Even if you consider accurate detection of 1 cent to be good enough for tuning 
> purposes, your measuring equipment still needs to be an order of magnitude 
> better. A 0.1 cent difference is 57.8 ppm. 12 ppm is 0.02 cent, which isn't 
> excessive if you're going for 0.2 cent accuracy. 

?? And if you are going for 10^-12 cent accuracy it is horrible. But why
would you be going for .2 cent accuracy?


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