First, thanks a lot to everyone supporting our Democracy DEV project.
We are receiving very good feedback from many developers and researchers working in the audio processing field. We look forward to get more and more backers, and reaching our funding goal!
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In this first update, we are going to provide some technical details on the wah guitar effect demo shown in the video above.
Also, you can download the MATLAB code from here!
The wah effect is based on a bandpas filter with variable center frequency and a small bandwidth.
Changing the bandpass center frequency leads to a spectrum shaping similar to speech and produces a kind of
The filtered wah signal is then mixed with the direct one to produce the final effect.
function y = wah(x,Fw,damp,mix,Fs,minf,maxf) % difference equation coefficients Q1 = single(2*damp); % this dictates size of the pass bands delta = single(Fw/Fs); yw = zeros(size(x),'single'); y = zeros(size(x),'single'); % apply difference equation to the sample for n=1:length(x) % auto wah function Fc = triWaveUpdate(minf,maxf,delta); F1 = single(2*sin((pi*Fc)/Fs)); yw(n) = wahFilter(x(n),F1,Q1); % wet/dry mix y(n) = (1 - mix)*x(n) + mix*yw(n); end end
Moving the center frequency may be achieved either with a control pedal or by defining a specific time varying function, for example a triangle wave.
function FcOut = triWaveUpdate(minf,maxf,delta) persistent iFc zFc FcUp; if isempty(iFc) iFc = 1; zFc = single(minf); FcUp = true; end if (zFc >= minf) && (zFc < maxf) && FcUp Fc = zFc + delta; elseif (zFc >= minf) && (zFc < maxf) && ~FcUp Fc = zFc - delta; elseif zFc >= maxf FcUp = false; Fc = zFc - delta; elseif zFc < minf FcUp = true; Fc = zFc + delta; else Fc = single(minf); end FcOut = zFc; zFc = Fc; end
The time varying filter is implemented as a State Variable Filter, which allows independent control over the center frequency and the damping factor in charge of tuning the filter bandwidth.
function yb = wahFilter(x,F1,Q1) persistent zyl zyb if isempty(zyl) zyl = single(0); zyb = single(0); end yh = x - zyl - Q1*zyb; yb = F1*yh + zyb; yl = F1*yb + zyl; zyl = yl; zyb = yb; end