[sdiy] Chromatic Tuner Design

Scott Young thebot at btinternet.com
Thu Feb 7 12:29:01 CET 2019


I have one of these (tc electronic polytune I think it’s called).  It probably doesn’t work that well at extremes, but you don’t end up using it for extremes, just for quickly and easily figuring out which string is out of tune.  You can use it in individual string mode as well just by playing that string in isolation for more extreme tuning issues.  It’s a great little tool in my opinion.

Cheers,
Scott.

> On 7 Feb 2019, at 09:55, rburnett at richieburnett.co.uk wrote:
> 
> Polyphonic pitch detection is a different kettle of fish altogether.  FFT surely works better than AC or AMDF/ASDF for polyphonic content, but I wonder how well it works when one of the strings is badly out of tune and close to the frequency of another string?! :-O  It's going to take a big FFT on a relatively long recording to be able to resolve frequency components that are close together into separate bins.
> 
> -Richie,
> 
> 
>> On 2019-02-07 09:30, Steve wrote:
>> Ah, thanks for the details. It does all make sense.
>> What I remember this being used in was a tuner that would show you the
>> tuned/sharp/flat status of 6 strings of your instruments in parallel
>> for one strum, and thus which ones needed tuning. There were
>> considerations of not minding a missing fundamental, depending on some
>> conditions. Perhaps more of a hack that seemed to work for the
>> intended scenario. I Don't remember all the details, and I obviously
>> did not implement that part :)
>> VON: "Richie Burnett" <rburnett at richieburnett.co.uk>
>> Interpolating the spectrum with a polynomial is one way to infer the
>> true frequency of spectral peaks that lie in between FFT bins. This
>> works with some success provided there isn't too much noise or other
>> tones contributing to the adjacent bins.
>> Another method of getting finer frequency resolution from the FFT is
>> to perform two consecutive FFTs on the signal and then look at how
>> much the phase has changed in the dominant bins from one FFT frame to
>> the next. Again this is susceptible to noise though and doesn't work
>> if there is more than one tone contributing to the magnitude and phase
>> in a given spectral bin!
>> In my opinion Autocorrelation or AMDF deal much better with complex or
>> difficult signals for pitch detection than the FFT does. For example
>> think of what the first dominant spectral bin would be for the FFT of
>> a harmonic waveform that just happens to have its fundamental
>> frequency absent. Whereas AC or AMDF don't care if the fundamental
>> frequency is missing, they just look for time-domain periodicity, so
>> aren't so easily tricked.
>> They also take the entire waveform into account so aren't so easily
>> tricked by complex waveforms that cross zero several times during each
>> period. Whereas squaring up the audio waveform with a comparator and
>> either counting the zero-crossings or measuring the time between
>> zero-crossings are both doomed to failure on all but the simplest of
>> waveforms like sine, tri, sqr, etc...
>> FFT also has a lot of latency for pitch detection because you have to
>> buffer up a full frame of audio data before you can even start to
>> transform it into the frequency domain and begin looking for spectral
>> peaks.
>> -Richie,
>> Sent from my Xperia SP on O2
>> ---- Steve wrote ----
>>> Don't both with FFT for pitch detection. It has loads of latency,
>> poor pitch resolution
>> Although that can be alleviated by using a center frequency estimator
>> on FFT peaks?
>> I have no mathematical clue how real the seeming gain in accuracy is,
>> I just know this has been used in a tuning product.
>> Some are described here:
>> http://www.ericjacobsen.org/fe2/fe2.htm
>> VON: "Richie Burnett" <rburnett at richieburnett.co.uk>
>> Look up autocorrelation and AMDF. These are techniques used for
>> detecting periodicity in complex waveforms.
>> Don't both with FFT for pitch detection. It has loads of latency, poor
>> pitch resolution, and is easily tricked by a missing fundamental.
>> -Richie,
>> Sent from my Xperia SP on O2
>> ---- oren levy wrote ----
>>> Hi All,
>>> I realize this isn't directly synth related but it's audio related
>> electronics.
>>> What are the preferred methods nowadays for designing accurate
>> chromatic tuners for a range of instruments? I'd like to put together
>> a chromatic tuner that I can hang on the wall that has a built in
>> microphone to help tune instruments ranging from a bass (let's say
>> 30Hz) up to maybe 2k with better resolution than a snark tuner. The
>> idea being that I can just play an open string on an acoustic guitar
>> and look at the wall to see how far off I am. Or play an oscillator on
>> my modular through my speakers and tune it.
>>> I am aware of two ways of going about it at the moment: measuring the
>> period of the wave and doing an FFT.
>>> I can take the mic signal and condition it and square it off through
>> a comparator to get a clean pulse to measure the period on a uC but I
>> worry that some instruments put out some funky waves.
>>> Are there any newer or more novel ways to measure the frequency of
>> funky waves as low as 30Hz?
>>> Rock & Roll,
>>> Oren Levy
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