I do believe in feedback

If I would have to state just one single universal principle which life as a whole rests upon then I would probably refer to the principle of feedback. But don’t worry, I don’t want to indulge in rather esoteric directions here but just move on a little bit towards basic system theory and how this relates to audio processing.

(source: wikipedia.org)

According to classical control theory, such systems are taking its output information and feeding it back in to the process input and that way closing the loop – hence the name closed loop system. In DSP audio land, the information is the audio signal itself and the audio (feedback) path constitutes the closed loop system. Audio signal processors such as feedback compressors or guitar amp effects are good examples for specific applications. While the control theory provides a lot of guidance for closed loop systems e.g. on handling differential equations and stability criteria, this can get pretty much nasty in practise because of the potential manifold of feedback loops in the circuit and not just only the main audio path.

myths and facts about aliasing

A recent trend in the audio producer scene seems to be to judge an audio effect plug-in just by analyzing the harmonic spectrum, which is usually done by throwing a static sine-wave right into the plug-in and then look at the output with a FFT spectrum analyzer afterwards. In this article I’m going to talk about what this method is capable of and where its limitations and problems lie and that aliasing gets confused with a lot of other phenomenons quite often. I’m also clearly showing that this method alone is not sufficient enough to judge an audio plug-in’s quality in a blackbox situation.

a harmonic spectrum plot showing quantization noise, harmonic distortion and aliasing effects