Guitar and Audio technology blog

Saturday, 13 November 2010

Project constraint considerations

Whilst writing the constraints section for the project proposal a number of considerations have been bought to attention.

Frequency range of modelling accuracy:

Time constraints mean that not all frequencies of a guitar will be recorded, analysed and modelled so how much will be?  The first 12 frets of a guitar contain 31 unique frequencies (which some cross over beyond the 12th fret).  

Volume, Gain and Tone controls:

The amplifier has three settings controlling the pre amplifier level, the power amplifier level and the tone stack of the amplifier.  Due to non linearity of analogue circuitry difference between 2 - 4 and 4 - 6 in terms of gain will not be the same however not all 1 - 10 values can be calculated.  Tone stack should prove easier to model as it acts as a single band EQ.  

Speaker emulation:

A key aspect of guitar amplifier sounds are due to the speaker and speaker configuration of an amplifier.  This project however does not include speaker emulation as one of its main aims and instead focuses on the pre and power amplifier valve sections.  There are two key issues with this, first of all if the output of the amplifier is to be recorded then using a microphone to measure the speaker will colour the output with the frequency response of both the speaker and microphone.  There is however a speaker output on the amplifier for connection to external cabinets with an 8/16 ohm resistance output.  If this could be used as a line in then an uncoloured signal could be used to truly represent the output of the amplifier sections.  The second issue is how to test the final model as a model of the amplifier, a similar speaker emulation (using convolution plug-ins) will need to be used to test the effectiveness of the model as a guitar amplifier.

Thursday, 11 November 2010

Final Rationale

There is a current trend amongst not only guitarists but general audiophiles to use older vacuum tube technology for amplifiers rather than more modern 'solid state' transistor amplifiers.  Often their argument is that although more expensive and far less reliable than transistor amplifiers they supply a 'warmth' that only analogue circuitry can provide.

Digital modelling of guitar amplifiers, an alternative to more traditional guitar amplification, has in the past decade become more realisable with advancements in DSP technology both on custom chip sets and on personal computers.  The major advantage of using a modelling program for amplification is the range of different amplifiers, cabinets, effects and studio conditions that can be recreated from a click of a button.  The other major advantage of using such software is also cost, with amplification modellers costing much less than a single tube amplifier it is no surprise that modelling programs are becoming so popular, especially with the newer generation of guitarists.  Currently there are several major guitar amplifier modelling programs, major names worth mentioning include Native instruments Guitar rig, IK Multimedia Amplitube and Line 6's POD/Gear box range.  With average prices of the major brands exceeding the £250 mark it is clear that the current range of amplifier modelling solutions is aimed at the professional end user.  Although some ‘lighter’ options are available offering a small range of the full packages these are still a considerable cost especially at an entry level price point.  It is proposed that there is a gap in the current market for a basic entry level vacuum tube amplifier emulator.   

The argument against the current range of modelling options is that they do not reproduce the same 'warmth' that a physical tube circuit creates.  This perception of 'warmth' cannot be scientifically calculated.  In theory reproducing the exact output waveform of a tube amplifier should create an exact model of the physical amplification system.  It is proposed that a VST plug-in is to be created, digitally modelling a physical tube guitar amplifier.  More specifically a Laney Cub 10, a small 10 watt class A/B tube amplifier.  The project aims to gain a deeper insight into the characteristics of tube amplifiers and the different ways in which digital modelling can be achieved.

Anthony Evans  

Wednesday, 3 November 2010

Max/MSP chebyshev polynomials

As an initial experiment using Chebyshev polynomials a basic max/msp patch has been created using the first 4 polynomials to create the first 4 harmonics of a sine wave.  An output wave has been recorded and analysed showing the effects of the polynomials on the signal.  The patch needs to be adapted to allow for individual amplitude control of each harmonic.
Max/MSP patch

Using Audacity to analyse the audio the frequency peaks can clearly be seen at integer multiples of the fundamental as expected.  Also noted the 'hollow' sound when only odd harmonics added.  Further polynomials are to be added to create a larger effect.

Frequency plot of max output

This was designed as an initial experiment and shall be further adapted to allow for better amplitude control.  The waveform was also analysed to show the effects of the harmonics on the input signal, as expected the wave changed and grew more square like with each additional odd harmonic added.  The next stage is to analyse the harmonic output of the amplifier at various different settings and frequencies.  A suspect limitation may be the frequency range that can be modelled accurately due to time constraints.  Meaning a certain range of input frequencies will be defined in the modeller, at least for the current time anyway.

Anthony Evans

Initial aims and objectives

The following are the key aims and objectives for the project.
  • To design, implement and fully test a wave-shaping model of a Laney Cub 10 vacuum tube guitar amplifier in VST plug-in format.
Secondary aims:
  • To critically evaluate wave-shaping techniques as a method of amplification modelling.
  • To show an insight into the alternative methods of amplification modelling.
  • To document the development process of the VST plug-in.
  • Record and analyse output from the amplifier within a certain frequency band.
  • Produce initial designs using a high-level graphical audio programming language of wave-shaping functions to be used to emulate amplifier output.
  • Design and implement a functioning filter modelled on the behaviour of the tone stack of the amplifier.
  • Review literature in the area of guitar amplification modelling, wave-shaping techniques and vacuum tube amplification.
  • Perform experiments using wave-shaping polynomials to evaluate different techniques.
  • Produce C++ algorithms for wave-shaping/harmonic polynomial designs to be implemented using the VST SDK.