Transcript Presentation 4

DSP Implementation of a
1961 Fender Champ Amplifier
James Siegle
Advisor: Dr. Thomas L. Stewart
April 22, 2003
Outline
• Background
• Progress/Project Changes
• Objectives Restatement
• Functional Description
• Block Diagram
• Previous EE452 Schedule
• Lab Work
• Current Objectives
Background
Solid-State Amplifiers
• As solid-state technology has become
more advanced in recent years,
devices, such as transistors and ICs,
are increasingly available to be used to
design inexpensive guitar amplifiers.
• However, these analog solid-state
designs require much feedback to
improve their linear transfer
characteristic.
Background
Solid-State Amplifiers
• This heavy feedback results in a sharp
clipping characteristic that produces
successive harmonics with high
amplitudes when the configuration is
driven at a high volume.
Reference: Barbour, Eric.
"The Cool Sound of Tubes.”
Ed., Michael J. Riezenman.
IEEE Spectrum August 1998.
Background
Tube Amplifiers
• There are several theories to explain the tube
guitar amplifier’s superior sound as compared to
the solid-state amplifier’s sound.
• Overall, the tube amplifier configurations result
in a frequency response with a dominant 1st
harmonic component, followed by a 2nd harmonic
component that is around half the magnitude of
the 1st harmonic, and higher harmonics with
decreasing amplitudes.
Background
Tube Amplifiers
• Lower harmonics have the most presence
and thus produce a louder sound than solidstate amplifiers at high volumes.
Reference: Barbour, Eric.
"The Cool Sound of Tubes.”
Ed., Michael J. Riezenman.
IEEE Spectrum August 1998.
Background
Tube Amplifiers
• Tube disadvantages:
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•
•
short life time
fragility
storage inconvenience (bulky size)
high power and heat dissipation
high voltage operation
high impedances requiring matching
transformers
• high cost (Fender Champ cost = $1,000)
Progress/Project Changes
Objectives
• The goal of the project is to reproduce
the output characteristics of a 1961
Fender Champ from a guitar input with a
DSP nonlinear modeling algorithm
• The Champ has been chosen due to its
popularity among vintage vacuum tube
amplifiers and its simple design
Progress/Project Changes
Objectives
Progress/Project Changes
Objectives
• The DSP available for this project is the
Texas Instruments TMS320C6711
• For MATLAB 6.5, there is an Embedded
Target for the TMS320C6711 where a
Simulink design can be translated to ANSI
C standard code
• This addition will allow more time to be
spent improving the DSP algorithm for the
amplifier model rather than spending hours
learning the subtlties of the DSP board
Progress/Project Changes
Objectives
• Several sets of data from sinusoidal and
guitar inputs to the amplifier will be used
to model the 1961 Fender Champ’s
distortion characteristics
• This approach was used in the patents for
similar projects
(PAT. NO. 5,789,689 - Tube modeling programmable digital
guitar amplification system)
(PAT. NO. 6,350,943 - Electric instrument amplifier)
Reference: http://www.uspto.gov/
Progress/Project Changes
Objectives
• Since there are several differing views on
the source of tube amplifiers’ unique
distortion, this data collection approach is
the most optimal and unified approach to
the problem
Progress/Project Changes
Functional Description
Analog Audio Signal
from Guitar
Interfacing Circuitry
to Guitar Cable
DSP with C/C++
or Assembly
Digital Filters
Audio Output with
Tube Amplifier Sound
Inputs/Outputs
• Inputs - analog audio signal from either a guitar A/D
interface or a saved audio file and software or
hardware based volume selection will regulate the
filters’ behavior
• Output - audio signal with tube amplifier effect
Progress/Project Changes
Functional Description
Analog Audio Signal
from Guitar
Interfacing Circuitry
to Guitar Cable
DSP with C/C++
or Assembly
Digital Filters
Audio Output with
Tube Amplifier Sound
Modes of Operation
• 12 volume settings similar to those provided with the
12-volume switch on the 1961 Fender Champ - (Only
three will be implemented where ‘3’ is the first audible
volume, ‘6’ is the middle selection, and ‘12’ is overdriven
level for amplifier)
• linear effects will be omitted due to lack of time
Block Diagram
Analog Audio Signal Input
from Guitar or File
Mode of Operation (Software)
BP
BP
BP
BP
BP
...
BP
...
BP
Nonlinear Transfer Characteristics
BP
BP
BP
BP
Summer
Parallel Bandpass FIR Filter Approach
Final BP
Equivalent Tube Amplifier
Signal Output
BP
Progress/Project Changes
Block Diagram
Analog Audio Signal Input
from Guitar or File
External Volume Selection
Mode of Operation (Software)
...
...
...
HP
2
...
LP
2
...
2
...
2
LP
...
2
LP
HP
...
2
HP
Nonlinear Transfer Characteristics
2
LP
2
2
HP
2
LP
2
LP
Equivalent Tube Amplifier
Signal Output
2
HP
HP
Multirate Signal Processing Approach
Reference: Digital Signal Processing: Principles, Algorithms, and Applications.
John G. Proakis, Dimitris G. Manolakis. Third Edition. 1996. pp. 832-834.
Progress/Project Changes
Block Diagram
Current Selection
• Parallel Bandpass FIR Filter Approach
(1st approach) is the best approach due
to the nonlinear transfer characteristic
addition that is applied in the time
domain and the large delay inherent to
the Multirate Signal Processing
Approach
Progress/Project Changes
Previous EE452 Schedule
Approach
• Weeks 1-4: Complete and simulate model of Fender
Champ in MATLAB from obtained 12AX7 and 6V6GT
tube data sheets
• Weeks 5-8: Complete software to program the
actual DSP board and interface the appropriate
hardware to the ADC and DAC
• Weeks 13-14: Senior 2003 Expo Preparation
• Weeks 15-16: Senior Project Presentation
• There is a 4-week window that is intended to allow
for setbacks
Progress/Project Changes
Lab Work
Approach Changes
• Complete and simulate model of 1961 Fender Champ
obtained from nonlinear transfer characteristics of 16bit audio output of 1961 Fender Champ
• Based on similarities and differences of nonlinear
transfer characteristics, take more 16-bit audio output
of 1961 Fender Champ from sinusoidal inputs
• Determine frequency ranges of approximate nonlinear
transfer characteristics from data and guitar
frequency chart
• Record output from 1952 Fender Telecaster directly
for 1961 Fender Champ response simulation verification
• Verify highest frequency input from the guitar
Progress/Project Changes
Lab Work
Reference: http://home.pacbell.net/vaughn44/m3.music.notes.6.pdf
Progress/Project Changes
Lab Work
Nonlinear Transfer Characteristic Determination
from 16-bit Audio Output of 1961 Fender Champ
Volume ‘12’ 523.25 (Hz)
Progress/Project Changes
Lab Work
Nonlinear Transfer Characteristic Determination
from 16-bit Audio Output of 1961 Fender Champ
Progress/Project Changes
Lab Work
Nonlinear Transfer Characteristic Determination
from 16-bit Audio Output of 1961 Fender Champ
• Eight more sinusoidal inputs were used to record 16-bit
audio output of 1961 Fender Champ
• Frequency, time domain, and transfer characteristics
of this data were plotted and analyzed
• ‘polyfit’ in MATLAB used to provide curve fits for eight
selected transfer characteristics
Progress/Project Changes
Lab Work
Highest Frequency from Guitar
Progress/Project Changes
Lab Work
Input to 1961 Fender Champ at Volume ‘6’
(Output of Guitar)
Progress/Project Changes
Lab Work
Fender Champ Response at Volume ‘6’ to
1952 Fender Telecaster
Progress/Project Changes
Lab Work
• Nonlinear transfer characteristic curve fits were
performed for eight frequency ranges where the curve
was selected for one frequency to be approximate to
characteristic curves of surrounding frequencies
• The frequency ranges were the following:
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0 - 250 (Hz)
250 - 450 (Hz)
450 - 700 (Hz)
700 - 900 (Hz)
900 - 1500 (Hz)
1500 - 2000 (Hz)
2000 - 3000 (Hz)
3000 - 4500 (Hz)
Progress/Project Changes
Lab Work
• FIR coefficients were generated for these filters with
FDATool in MATLAB due to the time spent fitting the
nonlinear transfer characteristic curves
• The nonlinear transfer characteristics for Volume ‘6’
were performed on guitar output
Progress/Project Changes
Lab Work
Previous Output of DSP Model of 1961 Fender Champ
at Volume ‘6’
Progress/Project Changes
Lab Work
Output of DSP Model of 1961 Fender Champ
at Volume ‘6’
Clipping seen from gain of 7 FIR filters being applied to nonlinear
transfer characteristics defined for a -1 to 1 input range.
Progress/Project Changes
Lab Work
Current Output of DSP Model of 1961 Fender Champ
at Volume ‘6’
Filter Bank 5 divided into 900-1200 (Hz) and 1200-1500 (Hz)
Progress/Project Changes
Lab Work
Comparison of DSP Model of 1961 Fender Champ at Volume ‘6’
to Actual Amplifier Output
Current Objectives
• Discover source of high frequency with DSP model of
1961 Fender Champ in MATLAB code
• Implement the MATLAB code simulation in Simulink
• If there is no time to get the code ready for the Texas
Instruments TMS320C6711 DSP board or the
Embedded Target cannot be obtained, the processed
output from MATLAB will be sent through the board’s
D/A converter for demonstration
• Otherwise, the code will be generated for the DSP
from the tools available from Simulink