Before implementing the entire system shown in Multirate Processing:
Introduction, we recommend you design a system that
consists of a cascade of filters FIR 1 and FIR 2 without the
sample-rate compressor or expander. After verifying that the
response of your two-filter system is correct, proceed to
implement the complete multirate system and verify its total
response. At first, use fixed compression and expansion
factors of
Compressed-rate processing
In order to perform the processing at the lower sample rate, implement a counter in your code. Your counter will determine when the compressed-rate processing is to occur, and it can also be used to determine when to insert zeros into FIR 3 to implement the sample-rate expander.
Some instructions that may be useful for implementing your
multirate structure are the addm (add to
memory) and bc (branch conditional)
instructions. You may also find the banz
(branch on auxiliary register not zero) and the b
(branch) instruction useful.
Real-time rate change and MATLAB interface
A simple graphical user interface (GUI) is available (as mrategui.m, which requires ser_snd.m) that sends a number between 1 and 10 to the DSP via the serial port. This can be used to change the compression and expansion factor in real time.
Run the GUI by typing mrategui at the MATLAB
prompt. A figure should automatically open up with a slider
on it; adjusting the slider changes the compression and
expansion factor sent to the DSP.
The assembly code that you have been given stores the last
number that the DSP has received from the computer in the
memory location labeled hold. Therefore,
unless you have changed the serial portion of the given
code, you can find the last compression and expansion factor
set by the GUI in this location. You need to modify your
code so that each time a new number is received on the
serial port, the compression and expansion factor is
changed. If a "1" is received on the serial port, the
entire system should run at the full rate; if a "10" is
received, the system should discard nine samples between
each sample processed at the lower rate.
Note that the READSER and WRITSER
macros, which are used to read data from and send data to
the serial port, overwrite AR0,
AR1, AR2, and AR3
registers, as well as BK and the condition flag
TC. You must therefore ensure that these
registers are not used by your code, or that you save and
restore their values in memory before you call the
READSER and WRITSER macros. This
can be done using the mvdm and
mvmd instructions. The serial macros set up
the AR1 and AR3 each time they are
called, so there is no need to change these registers before
the macros are called.
More detail about the READSER and
WRITSER macros can be found in Core File: Serial Port Communication
Between MATLAB and TI TMS320C54x.
