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Challenging mic vs sensor with independent measurements

  • 1.  Challenging mic vs sensor with independent measurements

    Posted 02-13-2024 04:44
    No app can self evaluate. The rep for the app cannot use the app to self evaluate. The data collected by the app must be subject to a higher authority measurement like Matlab. It is the only way to get an independent evaluation at a much higher level of accuracy. The app cannot do that.
    An electronic string sensor makes most of the apps tested so far reduce this variance as the app reports, but then the app itself has its own variance due to limited FFT length and window size and limited inter-bin measurement. 
    The particular app is not named because most of the tuning apps have a similar mic placement variance. 
    An app representative told me he saw no significant difference (?) in measuring his Yamaha C7 with the mic and then the sensor. That was simply uncorrelated with vast data I have already collected but not using his app. It also says, effectively, that his app is completely insensitive to mic placement variation, a statement that does not hold up, as we shall see.  
    I don't want to pit app against app. But I had to clear the air with this issue. It left me with no choice, but to do independently have someone conduct an experiment on a Yamaha C7 with his app.
    I had another well respected professional use this same app and go to a Yamaha C7 and take comprehensive measurements. He used 9 different mic positions per note and measured  C1, C2, C3,  C4, C5, C6, C7 and did a screen video recording of what App X was reporting and captured the .wav file audio so that I could then take the audio into Matlab to see what it actually is doing, and correlate it to that app.
    The app in this case could not resolve the variations in the spectra. The mic variance of the 4th harmonic, for example, of C3 was off by almost 1 cent. I typically looked at the first 5 harmonics in Matlab but have all the higher ones of course available. The C6 2nd harmonic was 9 cents off, yes that much. The C7 2nd harmonic was off by 19 (nineteen) cents. The upper register of the piano is literally filled with all kinds of false beats that show up in the spectra in matlab with great precision. We hear this junk with our ears also. The sensor is able to pick out the dominant one as previously shown in my video presentations.
    The app reported the following mic spreads. The actual was far worse with the same data evaluated in Matlab. In Matlab, the variances and ranges were significantly worse and wider than what the app reported, but the app had ranges as bad as 2 cents.
    The ranges as reported in the app were (C1 through C7);
    1.1, 0.66, 2.34, 0.46, 0.21, 033, 0.61, in cents!
    In Matlab the ranges were wider, depending on the length of FFT in the measurement. Since the FFT length and window functions and inter-bin calculations of App X were not known, there was no way of correlating in a general sense. I typically used 500 msec or 1 sec for the FFT length in Matlab. Most apps cannot handle this long so the variances are greater since the timeframes of the windows are less.
    Remember the sensor does not vary and this 'invariance' has previously been presented with extensive tests.
    Here are the details from strike to strike as reported by his app. Again, these are not my readings, but the readings of the app in question, and these were not done by me, but by an independent party.
    These are the mic measurements in cents offset from zero reported by the app:
    C1 = 1.1500    0.0500    0.1100    0.7000    0.5400    0.7000    0.6400    0.1900    0.2800
    Range = 1.1 cents 
    C2 = -0.8200   -0.7300   -0.8500   -0.4300   -0.6200   -0.1900   -0.7100   -0.6600   -0.5700
    Range = 0.66 cents
    C3 = 2.2500    0.3500    0.7300    0.4000    0.2400    0.3300    0.4200   -0.0900    0.0800
    Range = 2.34 cents
    C4 = -0.6100   -0.6800   -0.5500   -0.8400   -0.6600   -0.7400   -0.8900   -1.0100   -0.9700
    Range =  0.46 cents
    C5 = 0.1400    0.2300    0.2200    0.2700    0.0600    0.1000    0.1700    0.1900    0.2600
    Range =  0.21 cents
     
    C6 = -0.2900   -0.2600   -0.1700   -0.2400   -0.3000   -0.5000   -0.2600   -0.3700   -0.2200
    Range = 0.33 cents
    C7 = 0.9800    0.9100    0.5000    0.7600    0.8500    0.7600    0.4700    0.6400    1.0800
    Range = 0.61 cents

    More later, with spectral pictures to come.

    Best to all,

    Steve



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    Steven Norsworthy
    Cardiff By The Sea CA
    (619) 964-0101
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  • 2.  RE: Challenging mic vs sensor with independent measurements

    Registered Piano Technician
    Posted 02-13-2024 08:58