[ntp:questions] Using NTP to calibrate sound app

unruh unruh at invalid.ca
Tue Jan 29 20:04:55 UTC 2013


On 2013-01-28, Jeroen Mostert <jmostert at xs4all.nl> wrote:
> 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.

Actually for a piano, there is ALWAYS beating. The three strings have
three modes, and it is impossible to tune all those three modes to the
same frequency, because the soundboard couples them. Thus the three
modes all sound to gether with slightly different frequencies. This is
part of what gives the piano its distinctive sound over say a
harpsichord. 

>
>> 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. :-)

No it means that when you play two notes an octave apart there will be
beating. 
>
>> 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.

And many great tuner do exactly that and have a very large disdain for
tuners that rely on instruments. 

>
> 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 

But the goal is not "measureable" by a simple thing like a frequency
tuner. 

> 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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