Guitar and Audio technology blog

Sunday, 16 January 2011

Official website

To view my official website please go to, website for myself Anthony Evans Audio technologist and developer.

Analysis of results

Low E 82.4Hz
The following image shows the peak frequency content when the Low E string is played with the gain and volume settings at 10 and the tone at 5.  The fundamental frequency is clearly visible at 82.4 Hz and the following peaks can be shown to be certain odd and even harmonics of the fundamental.  The second harmonic at 164.8Hz can be seen clearly, however there is also a peak at around 230Hz and 300Hz which does not match to a specific harmonic of the low E. 

 The image below shows the same audio halfway through the audible period and it can clearly be seen that most of the high end content is dropped, and it is observed throughout the clip that the high end content lowers considerably quickly with the first five or so harmonics remaining consistent throughout the audio.  It is also observed that during the audio the harmonics die off in an alternative cyclic manner, with peaks close to each other fluctuating alternatively producing the sustained cyclic period the amplifier produces.

 The image below represents the DI guitar played at the low e (82.4 Hz) both peak and half way through the audio.  Some of the odd harmonic content found in the previous amplifier output can also be found here, showing that the guitar itself produces harmonic content not related to the fundamental. The main difference in terms of frequency content is that the frequencies produced by the amplifier are much larger in amplitude and more exaggerated than the DI guitar.     

The image below shows the two waveforms next to each other (the amplifier output on top the DI below).  Just from looking it can clearly be seen that the amplifier adds a great deal of sustain and gain to the input.  From the odd shape of the waveform asymmetrical clipping can be observed as would be expected of tube applied overdrive.  Reproducing this asymmetrical distortion is key in producing the cyclic sustain and ‘warmth’ that tube overdrive offers.

Amplitude dependent wave shaping

One of the key characteristics of the amplifier even at peak distortion is the effect of input amplitude on the amount of distortion generated.  When playing softly even at full volume, even though there is an increase in overall volume/loudness there is less distortion than when playing harder.  It is important to incorporate this into my chebyshev waveshaping functions as the plug-in should only apply more distortion to higher input values.  A test has been produced separating the input into above and below 0.5 && -0.5 only applying the distortion polynomials above these values.  This works quite well however more subtlety is needed and is currently being developed. 


After some experimentation I have now got a working amplitude dependant distortion waveshaper using chebyshev polynomials.  The code looks at the amplitude level and depending on the number adds specific polynomials too the output variable.  The only problem is that it is extremely computationally excessive and needs to be tested on another pc to establish what the problem is.  I have a feeling as the amplitude changes so often there are too many process leading to too much distortion and sometimes crashing.  I am currently in the process of updating this as well as including a control on the amount of distortion in general.