Experiments in Audio

Category: Audio DSP Blog Published: Monday, 01 April 2013 Written by steve

This blog is a step by step account of audio DSP experiments in an effort to assess the quality of open source DSP code for the simulation of audio effects plugins.

Initially experiments have been carried out within the Guitarix project framework, a guitar amp simulation for Linux.

 I am particularly interested in the simulation of audio valve ( tube ) based amplifiers and effects as over the years I have built many amplifiers for both use as musical instrument amplifiers and hi-fi.

Whatever the reasons there is something magical about the technology that, in my opinion, cannot be matched with solid state technology.

Recent developments in digital technology, faster computers and research into simulation algorithms now make it possible to simulate such circuits.


The Quest begins.....

Category: Audio DSP Blog Published: Tuesday, 02 April 2013 Written by steve

Choosing the tools.

As my interest had been sparked by the open source project Guitarix I decided to use that as a framework for my experiments, this would save a lot of programming time, as the framework already existed, and ease the learning curve for DSP prigramming as it makes use of the excellent Faust DSP programming language and its FaustWorks user interface. This enables the user to very quickly develop DSP algorithms and test them as it autogenerates highly optimised DSP code and builds a variety of different plugin architectures on the fly. Check it out at http://faust.grame.fr/.

 After a very short time I was happily writing DSP code in Faust and so ready to start the experiment.



Where to start....

Category: Audio DSP Blog Published: Wednesday, 03 April 2013 Written by steve

As my aim was both to learn the intricacies of audio DSP programming and to assess the quality of valve simulations using DSP code I decided that the first project should just be a very simply one stage valve preamplifier, a simple task as the valve models are already coded and provided in the Guitarix framework.

Real valve amplifiers are incredibly simple in construction as valves are already basic voltage amplifiers ( unlike transistors which are basically high speed switches ). A single stage amplifier consists of one half of a small triode valve, 3 resistors and 1 or 2 capacitors plus the power source. The operating point of the valve is controlled by the values of these components and can be tuned for the intended purpose ( highest gain, cleanest signal etc etc ). In guitar amps it is not always required to find the operating point with least distortion so chosen values are mostly different from those found in hi-fi amps as some form of harmonic distortion is a necessary part of the final sound.

The valve models in Guitarix are based on ........ and are implemented using a lookup table generated with various parts of the circuit predefined ( power supply voltage, anode resistor, input resistor ). The resulting model can be tweaked at run time as the cathode resistor and resulting bias value are variables. The following values were used for initial experiments as they are the most common in guitar preamplifiers :

Read more: Where to start....