ULTRASONIC ANALYSIS
This post seems like a good introduction to the project. You clearly define your goals of wanting to work out the physics/ technical issues of shifting pitches, such as the nyquist frequency limitation and the means of computing the frequencies of a sound file using the Fourier Transform. Your scheduling is also reasonably divided into these two categories, with plenty of breathing room for research, and troubleshooting. You also list your references which is helpful for someone following your research.
AUDIO SIGNAL PROCESSING-PRELIMINARY DATA
I think this is a great post for detailing four possible technical approaches to shifting pitches. Each technique is explained well enough for the phenomena to be generally understood, and your audio aids help make the theory more palpable. In changing the sample rate, I was unclear how the reading of the sound file is connected to a frequency value such as 88.2 kHz. Is it as simple as the speakers sending each sound element out twice as fast? Also in the upsampling/downsampling what elements are you adding to the array? Are they empty elements, or is the sound file processed and the elements added are continuous or wave-dependant. For the phase shifting, do all the frequency x-values of the fourier transform simply have a constant added? Finally when finding peaks of the fourier transform, how are the sine waves shifted? By a constant multiplier?
MORE RESULTS -SHIFTING FREQUENCIES UP AND THE EFFECTS OF LINEAR SHIFTING
Although recording limitations stopped you from accomplishing the goals you initially described (shifting an ultrasound frequency into the audible range), you were able to maintain the fundamental goal of this project, pitch shifting, by deciding to manipulate an infrasound wave instead. I thought it was interesting how the nyquist frequency you mentioned before seems to have come back as an obstacle. Also using a frequency source that is very simple and well understood, such as the red and green laser interference, was probably a good idea in order to have a good control over what your shifting model without unreliable frequency generators. Also, it would be nice to have the ‘circshift function’ you applied to the fourier transform, better explained in the post. You detailed the pitch relationship issue in class very well, and it seems a very interesting problem, one with possible solutions in linear or nonlinear scalar multiplied shifting.
AUDIO SIGNAL PROCESSING
It seems the idea of shifting infrasound to the audible range has fallen out, but I think taking a previously recorded sound wave is an excellent idea, especially for the scope of a computational physics course. The results are fascinating as well. I wonder if the large amount of audible frequencies was not added for nefarious reasons, but rather to make sure the owner knows there is a signal output, although I reasonably agree with you. Your remarks of sine waves at the end are very interesting as well, I wonder if biologically, rats have a different reaction to different pitch intervals than humans. Your future work seems very cool with having a working pitch shifting mechanism, and would be interesting to see finalized and applied practically or creatively, such as for a musician, or in smartphone apps (though I’m sure both exist in some form).
