analysis and implementation of the guitar amplifier tone stack david yeh, julius smith...
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Analysis and Implementation of Analysis and Implementation of the Guitar Amplifier Tone Stackthe Guitar Amplifier Tone Stack
David Yeh, Julius SmithDavid Yeh, Julius Smith
dtyeh,[email protected],[email protected]
CCRMACCRMA
Stanford UniversityStanford University
Stanford, CAStanford, CA
© 2006 David Yeh
Digital audio effects that emulate analog equipment are popular
““Modeling” amplifiersModeling” amplifiers
Products by Line 6, Yamaha, Products by Line 6, Yamaha, Roland, Korg, Universal Audio, etc.Roland, Korg, Universal Audio, etc.
CAPS open source LADSPA suiteCAPS open source LADSPA suite http://quitte.de/dsp/caps.htmlhttp://quitte.de/dsp/caps.html
Emulate behavior of classic analog Emulate behavior of classic analog gear in softwaregear in software As close to real thing as possibleAs close to real thing as possible
For portability and flexibilityFor portability and flexibility
© 2006 David Yeh
Guitar amp tone stack is a unique component in the sound of an amplifier
Almost every guitar amplifier, solid state or tube, has a tone control circuit – referred to as a tone stackPassive RC filter to audio signalLocated either directly after preamp stage or after stages of gain and buffer
© 2006 David Yeh
Prior work
Modeled by Line 6 (and others)Analyzed by Kuehnel (2005, book)Substituted in CAPS (LADSPA plugins for guitar effects) by shelving filter
© 2006 David Yeh
Parameter mapping from tone controls to frequency response is very complicated
Passive RC circuit Three real poles One zero at DC, one pair of zeros with anti-
resonance Shelving filter is close: 3 poles, 3 zeros
Frequency response depends on pole locations But no notch filtering
Circuit components are not isolated Component values are comparable Bridge topology
Tone controls affect location of multiple poles and zeros
© 2006 David Yeh
Tone Stack Transfer Function
Third order continuous time systemComplex mapping from component values/parameters to coefficients
© 2006 David Yeh
Poles depend only on Bass and Mid controls
© 2006 David Yeh
Zeros depend on all parameters
© 2006 David Yeh
Pole 1
Pole 2
Pole 3
Poles sweeping Bass and Mid Low freq
High freq
© 2006 David Yeh
Zeros plots for parameter sweeps
© 2006 David Yeh
Digitization as third-order filter
Straightforward approachFind continuous time transfer functionDiscretize by bilinear transformImplement as transposed Direct Form II (DFII)Pros: Perfect mapping of tone controls to frequency response within limitations of bilinear transformCons: Complicated formulas to compute coefficients
© 2006 David Yeh
Bilinear transformation of 3rd order system
© 2006 David Yeh
DFII block diagram
Mid
Treble
Bass
Compute DF coefs
Transposed DFII core
Audio out
Audio in
B[]
A[]
Component values
R, C
© 2006 David Yeh
DFII frequency response shows good match with continuous time version
© 2006 David Yeh
Error relative to continuous time
Worst case errors shown B=1, M=0, T=0
Discrete time reaches low pass asymptote but continuous time does not
© 2006 David Yeh
Reduced sampling rate
Commercial effects pedals commonly run at 31 kHzGuitar amplifier system is bandlimited by speaker response: 100–6000 Hz.For f_s = 20 kHz, error increases but only at high frequency because of asymptotic limits
© 2006 David Yeh
Table lookup implementation simplifies computation of coefficients
Lattice filter implementation for robustness to roundoff error in coefficients and to smoothly fade between coefficients as tone controls are variedTabulate 25 steps of each tone control parameterConvert from z-domain transfer function to lattice coefficients by step-down algorithmImplemented DFII and lattice filter in CAPS audio suite. Both run in real time.
© 2006 David Yeh
Sound samples
White noise at different settings Original white noise (2 sec) B=0 M=0 T=0 B=0 M=1 T=0 B=1 M=0 T=1 B=1 M=1 T=0 B=1 M=1 T=1 B=0.5 M=1 T=0.5
© 2006 David Yeh
Comparison of implementationsComparison of implementations
DFIIDFII Table lookupTable lookup
Exact parameterization Exact parameterization of tone stack behaviorof tone stack behavior
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Runs in real timeRuns in real time More efficient More efficient computation of filter computation of filter coefficientscoefficients
Arbitrary precision of Arbitrary precision of tone settingstone settings
Settings are quantized Settings are quantized – can interpolate– can interpolate
Easy to change circuit Easy to change circuit component valuescomponent values
Must tabulate each Must tabulate each circuit configurationcircuit configuration
Real time changes in Real time changes in tone settings not tone settings not audibleaudible
Robust to roundoff Robust to roundoff errors in coefficients – errors in coefficients – can fade between can fade between settingssettings