Baseband Processor For Mixed Mode Land Mobile Radio
CMX880
CMX880
Carrier
Raised
I
I
MOD_1
Filtered
baseband
signals
D to A
Converter
w
(cos( t))
Cosine, H(f)
Filter
Symbol Code
to I and Q
Levels
Conversion
Table.
Amplitude
Modulator
+
+
FSB
Carrier
Raised
Cosine, H(f)
Filter
Q
Q
MOD_2
D to A
Converter
w
(sin( t))
Amplitude
Modulator
5-level
I and Q
Figure 16 CQPSK Transmitter Processing Path
Processing Voice Signals for APCO-25
If the C4FM or CQPSK transmission is of digitally encoded voice, the voice signal from the microphone inputs is
A-to-D converted, filtered and decimated to 8000S/s. The filter characteristics (including the filtering due to the
recommended external components) meet the output filter template recommended in the ‘Speech Input/Output
Requirements’, section 4 of the ‘APCO Project 25 Vocoder Description, Standard IS102BABA, 1/11/95’, shown in
Figure 17, including the high pass filter H(z) = (1 - z-1) / (1 - 0.99 z-1) at 8000S/s, specified in ‘Speech Analysis’,
section 5 of the same standard. Thus the voice data provided to the DSP over the FSB at 8000S/s corresponds
to signal denoted as s(n) throughout that standard.
10
0
-10
-20
-30
-40
-50
-60
-70
-80
-90
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
10000
Frequency (Hz)
Figure 17 Voice Filter Template for C4FM and CQPSK Transmission
ã 2001 Consumer Microcircuits Limited
35
D/880/1
CMLMICRO [ CML MICROCIRCUITS ]
