What is baudline?
Mystery Signal
Mystery Signal of the Month   Dec 20 2003
We've got signal, but what the heck is it?
That's your mission.  Analyze this bébé.

mystery spectrogram                                                                                                                                                          

download the audio file 11.wav.gz

hints :

  • No need to gunzip this file since baudline can automatically uncompress files.
  • Setup baudline to be a Web Browser helper application as described in the FAQ.
  • Use the delta measurement bars to measure pitch change.
  • Use a Windowing function such as Kaiser or Gaussian and increase the beta variable to improve the time resolution.
  • Due to deficiencies in the .wav file format, the sample rate is not 1.0000 but it is 0.9766 samples per second.  Also note that the base frequency (0 Hz) is 59.8144 Hz.

What is it?

It is the power grid.  60 Hz AC (alternating current) from the power lines that have bled into the microphone.  The interesting thing is that the fundamental randomly wanders about 60 Hz with a delta of 0.05 Hz (50 mHz).  The frequency is not constant.

Keep in mind that you are looking at almost 3 hours worth of samples with a bin resolution of 0.00095 Hz/bin.  This signal was captured with an ADC sample rate of 4000 samples/second and a decimation rate of 4096 for an effective rate of 0.9766 samples per second.  The input device's down mixer was set to a frequency range of 59.81445 ... 60.30273 Hz.  This is an extreme amount of tuned frequency zoom.  Just like an electron microscope, with enough gain even the smoothest surface will look rough.

The two important questions at this point are "can this be error?" and "why is 60 Hz wandering?"

Is it error or an artifact?  Since this signal is microphone-bleed-in then it is possible that the computer power supply, the ADC clock, or the microphone itself are creating this wander / drift.  So an important test is to use completely different computer hardware in a different room and see if the shape of the wandering is identical.  This is easy to do with a local network, X-Windows, and the X11 remote display feature.  When you do this side-by-side the curves are the same.  The signal really exists.

Why is the 60 Hz power line frequency changing?  Power is created by huge spinning turbines that turn massive generators.  Also the demand for power is dynamic.  I would guess that these two items coupled together make it rather difficult to maintain a rock steady RPM and AC frequency.  The system control tolerances are probably greater than 0.05 Hz.

We have seen a lot of other strange behavior while monitoring the power grid.  Large phase shifts that show up as a wideband pulse.  Frequency discontinuities, the frequency instantly jumps 0.05 Hz or more.  Both of these are probably caused by quick grid switching of power plants that have not been synchronized.  What we haven't witnessed yet and would likely be fascinating to see are a brownout and a cascading power outage.

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