What is baudline?
Mystery Signal
Mystery Signal of the Month   Mar 10 2006
We've got signal, but what the heck is it?
That's your mission.  Analyze this bébé.

mystery spectrogram                                                                                                                                                          

download the audio file 15.wav.bz2

hints :

  • No need to bunzip2 this file since baudline can automatically uncompress bzip2 files.
  • Setup baudline to be a Web Browser helper application as described in the FAQ.
  • Listen to the whole 11 second audio file because there are a number of different clinks, clunks, and dings.  Notice how the cadence changes.
  • Slow down the audio with the Play Deck's speed control and listen for sonic details that would otherwise be missed.
  • Use the Color Aperture window to focus and maximize the color resolution.  An upper -20 dB and lower -110 dB range is a good starting point.
  • Use the Kaiser or Gaussian windows with a high beta value to improve the time resolution in the spectrogram display.
  • Use the periodicity bars to make an accurate time measurement between the clicks.  How constant and repetitive is the mystery signal?

What is it?

The source of this tick-tick-ta-tick mystery signal is an inexpensive windup kitchen timer.  It is a popular cooking aid that is used for timing tasks like boiling eggs or pasta.  The dial is turned by hand and has a range from zero to 60 minutes. 

This particular kitchen timer makes a very complex mechanical sound that has a number of interesting features.  Let us first analyze the unique ticking sounds and then measure the time keeping accuracy.

The strongest spectral pulse is the main timing tick and it is fairly regular with a period of 0.218 seconds (~4.6 Hz).  Occasionally the cadence changes and an extra beat is inserted or a clunk is heard.  Suprisingly this slop does not adversely affect the periodicity, the clock rhythm keeps on ticking.  Zooming into the spectrogram timebase using the Gaussian window with a beta value of 6 will improve time domain resolution and allow extra spectral details to become visible. 

Two faint clicks precede the main tick by 0.012 and 0.024 seconds.  These pre-clicks are believed to be some sort of latch and release mechanism that is required for the firing of the main click.  A fourth faint click is also visible that trails the main click by 0.085 seconds.  The timing of these pre and post events are very consistent from click to click. 

If the SNR is good then the same delta 0.012 second measurement result can be accomplished using the waveform window. 

The clicks have a broadband spectral structure which is the common response for impulse (impact) signals.  The wideband click pulses have spectral energy that extend up to the 24 kHz Nyquist frequency limit which suggests that significant energy may exist past that point, possibly to 100+ kHz.

The average spectrum display above is a copy-n-paste accumulation of the bell-like tones that ring after the main impact click.  Many tones are visible; 7400 Hz, 10500 Hz, and 12500 Hz being the strongest.  The 7400 Hz tone has a decay time of about 0.085 seconds which signifies that the ringing is being damped by the action of the fourth click.  These non-harmonically related ringing tones are the source of the characteristic metallic sound.

Baudline's periodicity bars were used to make highly accurate timing measurements.  The periodicity of the click spacing changes from 0.218 seconds at the start of the .wav file to 0.216 seconds at the end which means that the speed of the ticking is increasing as the clock winds down.  This delta of 0.002 seconds is a change of -9000 PPM over a duration of 11 seconds for a -800 PPM/second drift.  This kitchen timer could be classified as an extremely poor time keeping device.

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