EE 451
Lab 2: Sampling and Aliasing in Real Time with the
Motorola DSP56002 EVM
For experimentation in real-time signal processing, we will use the
following equipment:
- Motorola DSP56002 EVM, with a sampling rate of 48 kHz.
- An oscilloscope.
- A function generator.
- A source of audio (e.g., a CD player with headphones).
- Use the 56000 assembly-language program
play.asm.
To assemble this, first copy the files
ada_init.asm
and
txrx_isr.asm
from the C:\EVM
directory to your
directory. The codec is a complicated chip, and these pre-written
routines do the work necessary to sample the input signal. This will be
discussed further in lab.
Assemble the program using the following command:
ASM56000 -a -b -l lab2.asm
- Set the function generator to give a 2-volt peak-to-peak, 5 kHz
sine wave. Connect the output with the mono cable to the input of the
A/D converter. Connect the outputs of the D/A converter to two channels
of an oscilloscope. Run the program EVM56K on the PC. Give the
command load lab2, which will load the attached program into the
56002. The command disassemble $40 should show you the
instructions as loaded into the 56002. go $40 will run the
program. The two outputs should look the same. Be sure to sketch the signals
in your lab book.
- Vary the frequency of the function generator, and observe the
response of the D/A converter.
- Connect your audio source to the input of the A/D. Listen to the
output through headphones. Note the quality of the sound.
- Stop the 56002. Modify the program to put out the left channel
every sample interval, but put out the right channel only every 12th
sample interval. This output of the right channel will then have a
sampling rate of 4 kHz. The easiest way to do this is with a DO
instruction. Set up the DO instruction to read execute the
sampling code 11 times without putting the input value into Accumulator
B, then add sampling code to put the input value into both A and B, and
finally loop back to the DO instruction. Be sure to sketch the
signals in your lab book.
Connect the function generator back to the A/D, and run this modified
program. Vary the frequency of the function generator, and compare the
two outputs of the D/A converter. Observe aliasing for high
frequencies. At what frequency does aliasing begin? Be sure to document what
the aliasing does at high frequencies.
- Modify the above program so both channels put out every 48th sample,
giving a sampling frequency of 1 kHz.
Connect your audio source to the input of the A/D. Listen to the
output through headphones. Note the quality of the sound.
Bill Rison,
<rison@ee.nmt.edu >
Thu Sep 5 1996
Copyright © 1996, New Mexico Tech