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2920 SUPPORT TOOLS
sawtooth oscillator of varying slope. When such code is
needed, it can be generated with one call to the macro
along with the appropriate parameters.
The Compiler makes it easy to specify signal processing
functions in a modular manner and to join the resulting
code modules 'into larger programs. One important
application is the development of digital filters, and the
SP AS20 Compiler contains a number of features rele-
vant to their design.
The Compiler operates interactively, permitting the
designer to
•
set sampling rate, place poles and zeros at chosen
S-plane or Z-plane coordinates, and set error
bounds on gain
•
see immediately, on a graph or list, the frequency
or time response of existing poles and zeros at
specified frequencies, or display any design
parameter
•
generate, store, and review 2920 assembly
language code for each filter or stage, or other
signal processing functions
•
use diskette files as scratchpads to store, review,
modify, and retrieve files of parameters, code, or
commentary
•
create sequences of commands pertinent to
complex or frequently used functions as macros,
naming and storing them on diskette files for
ease of future use, facilitating later interactive
design sessions or unattended test sessions.
The Signal Processing Applications Software/Compiler
accepts high-level language input and produces 2920
assembly language, as described above. The Compiler is
also a filter design aid which permits substantial interac-
tive manipulation of a wide variety of parameters and
constraints, both in design of digital filter stages and in
optimization of the 2920 code in terms of size and error
limits. One principal feature allows specification of the
poles and zeros of a desired filter and, after designer
review of the pertinent graphs, the automatic implemen-
tation of that filter in 2920 assembly code, without hav-
ing to write each of the detailed steps that are required
in assembly language.
Concepts of Filter Design-Designing a filter
involves choosing operations to perform on signals in
order to produce modified signals. These operations are
9-4
usually (but not always) linear. The theory relating con-
tinuous analog filters to sampled digital filters appears
in Chapter 5.
Filters are usually designed to achieve certain gain anc;l
phase characteristics, which can be viewed as resulting
from the location of the filter's poles and zeros. The
desired output amplitude and phase can be approached
in an interactive design session by placing poles and
zeros in the S-plane (for continuous fitlers) or the
Z-plane (for sampled filters), and viewing the resultant
output. Moving these poles and zeros can then change
that output to more closely approximate what is needed.
The Compiler capabilities facilitate this interactive
process of specification, modification, and review by
providing simple commands and graphs for these
functions.
Displaying
Responses-Frequency
and
time
responses of the filter can be examined as the positions
of poles and zeros are manipulated. The gain, phase,
group delay, and step or impulse responses can be
graphed or listed. The frequency range of interest for
these outputs can be specified. While emphasis is placed
on the gain versus frequency response of the filter,
designers can take advantage of the compound com-
mand capability to use the phase, impulse, or step
responses.
The graphs do not require the console device to have
any cursor controls, e.g., the ability to move the beam
directly by pressing a buttom for up, down, left, or
right. The user specifies the X-axis and the Y-axis
ranges. The last curve plotted is always available for
redisplay upon entering the command GRAPH, regard-
less of the effect of intervening commands. It is also
possible to superimpose the last curve plotted and a new
curve, regardless of intervening scale changes. The
graphs can be sent to a diskette file, or hard copies can
be produced on a line printer, since no special cursor
control charcters are assumed.
Generating Code-Once the filter characteristics
(e.g., gain, error, phase) look adequate to meet the
design specifications, the user can generate the code for
each pole or zero with a single command. The Compiler
enables implementation of the filter as cascaded series
of first and second order stages. It also supports the
generation of code to compute independent variables of
the form Y=C*X or Y=C*Y, where C is a constant, and
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