1.   Digital Signal Processing Implementation of a 1961 Fender Champ Amplifier

2. Outline Background Progress/Project Changes Objectives Restatement
Functional Description
Block Diagram
Previous EE452 Schedule
Lab Work
Current Objectives

3. Solid-State Amplifiers
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.

4. Solid-State Amplifiers
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.

5. 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.

6. Background
Tube Amplifiers
Lower harmonics have the most presence and thus produce a louder sound than solid- state amplifiers at high volumes.
Reference: Barbour, Eric.
"The Cool Sound of Tubes.”
Ed., Michael J. Riezenman.
IEEE Spectrum August 1998.

7. Background Tube disadvantages: short life time fragility
Tube Amplifiers
Tube disadvantages:
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)

8. 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

9. Progress/Project Changes
Objectives

10. 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

11. 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:

12. 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

13. Progress/Project Changes
Functional Description
Analog Audio Signal
from Guitar
DSP with C/C++
or Assembly
Digital Filters
Audio Output with
Tube Amplifier Sound
Interfacing Circuitry
to Guitar Cable
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

14. Progress/Project Changes
Functional Description
Analog Audio Signal
from Guitar
DSP with C/C++
or Assembly
Digital Filters
Audio Output with
Tube Amplifier Sound
Interfacing Circuitry
to Guitar Cable
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

15. Block Diagram ... ... Parallel Bandpass FIR Filter Approach BP BP BP
Analog Audio Signal Input
from Guitar or File
Mode of Operation (Software) BP BP BP BP BP BP
...
Nonlinear Transfer Characteristics BP BP BP BP BP BP
...
Summer
Parallel Bandpass FIR Filter Approach
Final BP
Equivalent Tube Amplifier
Signal Output

16. Progress/Project Changes
Block Diagram
...
Analog Audio Signal Input
from Guitar or File
External Volume Selection 2 LP 2 LP
... 2
Mode of Operation (Software) HP
... LP 2 HP 2
...
...
...
... HP
... 2
Nonlinear Transfer Characteristics 2 LP 2 LP 2 HP
Equivalent Tube Amplifier
Signal Output 2 LP 2 HP 2 HP
Multirate Signal Processing Approach
Reference: Digital Signal Processing: Principles, Algorithms, and Applications.
John G. Proakis, Dimitris G. Manolakis. Third Edition pp

17. 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

18. 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

19. Progress/Project Changes
Lab Work
Approach Changes
Complete and simulate model of 1961 Fender Champ obtained from nonlinear transfer characteristics of 16-bit 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

20. Progress/Project Changes
Lab Work
Reference:

21. Progress/Project Changes
Lab Work
Nonlinear Transfer Characteristic Determination
from 16-bit Audio Output of 1961 Fender Champ
Volume ‘12’ (Hz)

22. Progress/Project Changes
Lab Work
Nonlinear Transfer Characteristic Determination
from 16-bit Audio Output of 1961 Fender Champ

23. 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

24. Progress/Project Changes
Lab Work
Highest Frequency from Guitar

25. Progress/Project Changes
Lab Work
Input to 1961 Fender Champ at Volume ‘6’
(Output of Guitar)

26. Progress/Project Changes
Lab Work
Fender Champ Response at Volume ‘6’ to
1952 Fender Telecaster

27. 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:
(Hz)
(Hz)
(Hz)
(Hz)
(Hz)
(Hz)
(Hz)
(Hz)

28. 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

29. Progress/Project Changes
Lab Work
Previous Output of DSP Model of 1961 Fender Champ
at Volume ‘6’

30. 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.

31. Progress/Project Changes
Lab Work
Current Output of DSP Model of 1961 Fender Champ
at Volume ‘6’
Filter Bank 5 divided into (Hz) and (Hz)

32. Progress/Project Changes
Lab Work
Comparison of DSP Model of 1961 Fender Champ at Volume ‘6’
to Actual Amplifier Output

33. 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