Sujet Ardour (DAW pr Linux) Installation et Utilisation

Citation : Date Fri, 28 Mar 2003 14:11:54 +0100
De Jiøí Schimmel <schimmel@feec.vutbr.cz>
À CHRISTIAN.FRISSON.ETU@univ-lemans.fr
Objet Re: Amp simulation / EDU project

Hi Christian,
You have contacted the DSound company several days ago and asked us for some advice on your project.
I'm sorry for such delay but in last weeks we were very busy because of 114th AES Convention.
I work for DSound but I am phd student in Technical University Brno.
I designed and implemented DSound distortion and overdrive plug-ins and VL2 Powercore plug-in as well.

So here is my basic advice and comments:
For simple simulations of guitar amp you don't need schematics - just realize principles of particular blocks of guitar amp.
For example, the distortion, overdrive, and similar guitar amp effects are based on non-linear characteristic features, like limitation or rectification of signal.
For good simulation you also have to realize the topology of guitar amplifier, like feedbacks and frequency-dependent elements.

However you need schematics of real analog guitar amp for hi-quality simulations.
It's not good idea to use experimental topologies and values.
Definitely, the best way of simulation is computing of circuit equations.
However it's very power-consuming with more complex circuits and it makes this method unusable for real-time processing.

So I recommend you this procedure:
At the first you have to find out some schematics of guitar amp.
It's not problem to find something in the Internet.
Then you can design model of this guitar amp in the PSpice or similar program.
You should divide this model into several parts like input amplifer, non-linear amplifier, feedback, etc. and simulate their main features like the static transfer characteristic or frequency response.
These blocks, their main features, and their connections you can simulate afterwards in some suitable environment.
I use Matlab Simulink for this.
You can approximate non-linear transfer characteristic via pointed-line approximation or power-polynomial approximation.
You can use LMS or similar method for estimating power-polynomial coefficients according to measured or simulated transfer characteristic.
You can use splines as well.
There are also several methods for estimating FIR and IIR filter coefficients according to measured frequency response of the real model.
For the guitar amp simulation, blocks with non-linear amplifier are most important.
Other parts of guitar combo, e.g. equaliser, modulation effects, etc., can be simulated using general digital audio effect models.
Remember that frequency characteristic of guitar cabinet has significant effect to sound color of guitar combo!
In the end, I would like to call your attention to aliasing effect.
You have to realise that your algorithm is working in digital domain and non-linear transfer characteristic causes generation of higher harmonics.
It means the aliasing effect appears which causes that digital distortions sounds "sharply" and unpleasantly, unlike the analog distortions.
You have to use oversampling to avoid it.
For testing, you have to use soundcard with instrument (Hi-Z) inputs for connecting electric guitar.

Greetings,
Jiøí Schimmel
Dep. of Telecommunications FEEC
Technical University Brno
Czech Republic