All amplifiers are nonlinear. In most communication systems this is an unwanted feature however the nonlinearity can in the case of a guitar amplifier be desirable.
Guitar amplifiers are typically deliberately designed with enough gain for the guitar, when played hard, to overdrive one or more tube stages and thus generate gross distortion. The characteristics of the tube and the circuit directly influence the nature of the sound produced. Even the power supply can influence the tonal shape, with relatively undersized power supply capacitors producing a characteristic “sag” at instants of peak output and power draw, and subsequent recovery, that is often considered musically engaging. In addition, guitarists may employ acoustic feedback, further modifying the resulting sound (noting that the feedback signal has a slight time lag relative to the original signal).
Figure 1. Tube amplifier app from Slash (Guitarist with Guns and Roses)
The proposed FYP aims to model the nonlinear distortion and produce an equivalent guitar amplifier in software. A polynomial model will be used to model the guitar amplifier to reproduce the “warm” distortion characteristics of such amplifiers.
The student will be expected to:
- Review a set of functions for processing an audio signal (approx. 6).
Mixing, Filtering etc.
- Implement the effects caused by power amplifier nonlinearity and the above audio processing functions in Simulink.
- Design, implement, and test a polynomial model equivalent of an audio amplifier in Matlab and/or Simulink. At this stage the output signals from an amplifier and the Matlab equivalent can be compared.
- Implementation of functional blocks in digital hardware. This stage will involve the transfer of coefficients to their fixed point equivalents and subsequent use in hardware to distort an audio signal in real-time. The interfacing of an audio signal into a Xilinx Nexys4 board or similar hardware.
- Compare the resulting sound from the implemented hardware setup with the equivalent software setup.
Brief statement on relevance to engineering programs offered at NUI Maynooth
This is a good project for an engineer with an interest in intelligent systems, algorithm development and FPGAs.
Some Typical Progress Requirements for:
Pass: Completion of steps 1–2.