|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.
download the audio file 14.wav.gz
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
window to focus and maximize the color resolution to the -94 dB to -136 dB
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
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.
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.
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.
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.