Digital Signal Processing Reference
In-Depth Information
10
/* Start loop for number of quad factors */
11
for(j = 0; j < numb_quads ;j++)
12
{ w = o - *m1 * *c++;
C:
0.20,1.0,
-0.80
,-2.0,-1.0,0.05,-0.20,-2.0,-1.0
M
:
1.0
,3.0,
2.0
,4.0
w:
-0.40
13
w -= *m2 * *c++;
C:
0.20,1.0,-0.80,
-2.0
,-1.0,0.05,-0.20,-2.0,-1.0
M
:
1.0
,3.0,
2.0
,4.0
w:
1.20
14
o = w + *m1 * *c++;
C:
0.20,1.0,-0.80,-2.0,
-1.0
,0.05,-0.20,-2.0,-1.0
M
:
1.0
,3.0,
2.0
,4.0
o:
-0.80
15
o += *m2 * *c++;
C:
0.20,1.0,-0.80,-2.0,-1.0,
0.05
,-0.20,-2.0,-1.0
M
:
1.0
,3.0,
2.0
,4.0
o:
-2.8
16
*m2++ = *m1;
M
:
1.0
,3.0,1.0,
4.0
17
*m1++ = w;
M
: 1.2,
3.0
,1.0,
4.0
18
}
19
/* Convert output to int and store */
20
*y++ = (int)ceil(o-0.5);
y:
-3,
0
,0,0,0,...
21
}
}
Listing 8.2
Continued.
Notice that the method used to access the gain constant is by using the
*c
notation, which can be read as “the value at which
c
is pointing.” Since
c
has just
been initialized to the start of the
C
array, and since the first element in
C
is the
gain constant, then
c
is pointing at the gain constant. The “++” notation after
c
instructs the computer to increment the value of
c
by one
after
the equation has
been evaluated. (This is referred to as post-incrementing as opposed to
++c
,
which is called preincrementing.) At this point, we are performing pointer
arithmetic, which is a special type of arithmetic. Remember that the variable
c
(and
C
) contains a number that is an address of where coefficients are stored in
memory. When we increment a pointer we are incrementing an address and
therefore must do so in a meaningful way. In this case, the coefficients are stored
as
double
s, which take up 8 bytes of memory on a PC. It would be meaningless
to increment the address of an array containing
double
s by 1 byte, or 2, or
anything but 8 bytes. Therefore, when we tell the machine to increment
c
, it
increments the number stored in
c
by 8. (This is what is meant by special
arithmetic.) The compiler keeps track of how to increment pointer variables based
