What is baudline?
Mystery Signal
Mystery Signal of the Month   Jul 29 2005
We've got signal, but what the heck is it?
That's your mission.  Analyze this bébé.

mystery spectrogram                                                                                                                                                          

download the audio file 14.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 Color Aperture window to focus and maximize the color resolution to the -94 dB to -136 dB range. 
  • Make an accurate Hz measurement of the main fundamental frequency.

What is it?

The mystery signal is a 1000.01 Hz sine wave that was created by baudline's Tone Generator with the dither feature disabled.  The strange crisscrossing trajectories are caused by quantization distortion due to the lack of dither.  The line reflections off the left and right edges are a form of aliasing.

Hz measurement frequency
The primary Hz window shown on the right is a highly accurate frequency measurement (uHz).  At an 8000 sample per second rate the 1000 Hz sine wave is highly repetitive with an exact 8 sample periodicity.  The addition of the .01 Hz offset causes the sample bin usage to slowly drift.  This combined with the lack of dither are the source of the interesting looking aliasing and distortion.

dither on
Below is a baudline spectrogram and spectrum image of a 1000.01 Hz sine wave with dither enabled.  The Color Aperture window was used to maximize the color resolution and highlight the noise floor.

dither spectrogram

Notice how the dither removes the aliasing and distortion.  Dither is important for clean function generation and this is why the baudline default is for dither to be turned on.

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