Remove the original audio from the induced distortion, normalize to a percentage of the level of the original audio signal and add the distortion to the original signal again.
Well, this was mild nonsense. It wouldn't have fixed the THD% calculation for music, because there is already spectral content outside of the fundamental. Therefor, the calculation cannot work for music in the iteration I am using for this project. This was something Adam and I spoke about in the first place.
The new plan (which was really the old plan before the tangent), is to modify quantities in the nonlinear equations, to give a distortion % to the 1kHz sine tone that everyone uses (normalized to 1). Then apply that filter to the audio samples and the different percentages and have some samples for testing.
These will be ready by tomorrow (with luck). After speaking with Adam, I have decided to vito the rectifying NLDs because they aren't similar to a normally working device in saturation. Although, interestingly, squaring the wave pitch shifts it if it is a sine wave. The behavior to other wave types is even more interesting. I would love to start looking at distortion characteristics in deeper detail for post-grad.
Back to the scheduled program.
To decide what percentages of THD are worth exploring, I fist looked at the Geddes & Lee paper 'Auditory Perception of Nonlinear Distortion', which discussed a new method for measuring the perception of distortion (THD, IMD, ETC) in listening tests. This was all good, but I wanted to see what else had been done before I commit to that method. I looked through 'Nonlinear Distortion Measurement in Audio Amplifiers: The Perceptual Nonlinear Distortion Response' by Phil Minnick, which pointed me towards the Voishvillo paper 'Measurements and perception of nonlinear distortion - comparing numbers and sound quality'; which . There was also 'a new method for measuring distortion using a multitone stimulus and non-coherence' by Temme and Brunet.
Got that?
Finally, I came to the paper 'Measurement of Harmonic Distortion Audibility Using a Simplified Psycho-acoustic Model - Updated' by Temme, Brunet and Qarabaqi. This paper suggested that distortions that THD% samples between 0 and 9 had a lower standard deviation of subjective grading, where as at 10%, the deviation of this grading became much wider. These ratings were from 1 - 5, good to terrible. At 1-% some ratings jumped up from below 2.5 to above and up to 3.4. This is in line with a number of other papers, which suggest that THD becomes more perceptible above 10%.
As such I will use samples at THD%s in steps of 0.5% between 0% to 20%, and then steps of 10% up to 80%. This is because above the threshold conscious of perception of distortion, the listener will know they are hearing a strongly distorted signal (as proposed in the results of the paper mentioned above), and therefor the accuracy of the effect may be less.
The order of test signals in listening tests will be randomized, to fit in with the methods I will discuss in the next post. "I got it rapped like a mummy", Dr Dre.
From the hawksford paper, I am going to focus on the Cubic/SQS NLDs, as they may follow the behavior of saturated amplifiers/loudspeakers more readily.
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