How very convoluted

So the next step in the process I have deemed, is to find a way to quantify the THD of an audio signal. As it turns out, this is a hard thing to do. I have been trying to do this quite a lot.

In fact, a lot of the papers I have referenced so far (mostly in the proposal) are based on this particular point. The reality is, the original question is in itself questionable.

THD% is a way of measuring distortion. By definition, distortion and complex signals cause more distortion i.e. inter-modulation, and various other properties of non linearity.

The question shouldn't really be 'does total harmonic distortion have an effect on perceived loudness', but more likely 'does amplifier/loudspeaker distortion have an effect of perceived loudness'. These are not the easiest things to determine in some cases, but it would be a much tidier question all in all.

I have most recently been trying to use a home-brewed method to calculate the THD of a music signal. Convolving a 1kHz sine wave with the distortion characteristic of the music which has been processed with a nonlinear device. The problem I am having is that a 1kHz sine is spectraly sparse, and most of the distortion seems to occur in a wide band at high frequency.

Key papers at this time are:
Measurement and Perception on Nonlinear Distortion - Comparing Numbers and Sound Quality
- Alex Voishvillo
Testing Challenges in Personal Computer Audio Systems
- Wayne Jones et al.
Nonlinear Distortion Measurement in Audio Amplifiers: The Perceptual Nonlinear Distortion Response
-Phillip Minnick
Auralization of Signal Distortion in Audio Systems Part 1: Generic Modeling
-Wolfgang Klippel
Non-linear Convolution: A New Approach for the Auralization of Distorting Systems
- Angelo Farina

One of the biggest problems I am currently having, is that if I re-analyse a pre-distorted and analysed sinewave, I get a different value to what I can clearly see on the graphing of the function.


So it is time to step back, look at the question and make a decision.
Do I forge ahead and stick with THD as the metric for distortion measurement, or do I go freeform? I think the NLDs cause IMD and all other kinds of goodies for analysis, and convolution is going to become a big part of getting real answers from the complex program material. In the mean time I am going to see if I get start to get my understanding correct in relation to 2 sinewaves, some impulses and some noise.

I must also look towards perceptual models, in case analytical ones do not work out.