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S
Ñ
Offset + 0
S
0
Offset + 2
36.14 DEMODÐReal Sin; Real Cos, Real X, and
Complex Y
Using the values provided in the parameter packet, the DEMOD implements a basic
demodulator function, shown in Figure 36-27, with a modulation table composed of (cos
wnT, sin wnT) pairs, real input samples, and complex outputs. The input data is in a circular
buffer with size (M+1) and the output data is in a circular buffer with size (N+1). The AGC
parameter controls the demodulator gain.
36.14.1 Modulation Table, Input and Output Buffers, and AGC
Constant
The modulation table consists of 16-bit cosine and sine pairs that occupy (K+1) bytes in
memory. The input sample buffer is a circular buffer containing (M+1) bytes. The next
16-bit sample is stored in the address that follows the previous sample. The output buffer is
a circular buffer that contains (N+1) bytes and the next output is stored in the address that
follows the previous output. The AGC constant is in the range -1£ AGC£1. See Table 36-21.
Table 36-21. DEMOD Modulation Table, Input, and Output Buffers
Modulation Table
MOTOROLA
W
I
X
Ñ
W
I
0
0
Figure 36-26. MOD Modulation Example
X(n)
{Real}
cos wnT, sin wnT, AGC
Real Y n ( )
{
}
(
=
Image Y n ( )
{
}
(
1 AGC
=
Figure 36-27. DEMOD Function
Input Samples
sin q
1
cos q
1
sin q
2
cos q
2
*
*
sin q
n
Chapter 36. Digital Signal Processing
Part V. The Communications Processor Module
Ñ
Ñ
Ñ
Ñ
0
0
0
0
I=3 (Three Iterations)
Y(n)
{Complex}
{Real}
) X
´
n ( )
´
wnT
1 AGC
+
cos
) X
´
n ( )
´
wnT
Ð (
+
sin
Output (Complex)
*
*
x(n-k+1)
imaginary{Y(n-k+1)}
*
real{Y(n-k+1)}
*
x(n-2)
x(n-1)
imaginary{Y(n-2)}
Ñ
OPCODE
0
01000
)
*
*
*
*
36-23
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