[ntp:questions] Using NTP to calibrate sound app

Jeroen Mostert jmostert at xs4all.nl
Mon Jan 28 22:51:59 UTC 2013

On 2013-01-28 23:04, unruh wrote:
> 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.

So for two pianos sounding together, the beating shouldn't be audible. OK.

> 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.

So the beating *should* be audible. OK. :-)

> 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.
Well, then, why measure at all? As long as it sounds good.

Surely to make it sound *as good as possible*, you need to be able to accurately 
measure how close you got to your goal. If you are able to tune a piano to 
within 1 cent (or thereabouts, granting that it may be difficult), you need to 
measure better than 1 cent to know how good your tuning actually is.

>> 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?
Don't ask me. Ask the piano tuners. Unless you *are* one, of course. In which 
case it sounds like you have to educate your fellow practitioners.


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