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EN 158
9.
EM6E AB
Vref
C1
R1
IN
1k
2.2u
Figure 9-14 Basic compander configuration
The basic compander configuration is given above. From the
diagram, one can understand the factor of two of companding
as follows:
There is a variable gain cell (variable resistor) in the feedback
loop of an OpAmp (NJM4565M) that is controlled by the
rectifier cell that detects the output voltage of the OpAmp and
translates it into a current send to the gain cell. The rectifier has
an attack time constant (C3) and a decay time constant (C4),
which is optimised to give the best auditive result.
If the input (V_in) rises with e.g. 4 times then the output can rise
only 2 times. This is because the feedback resistor formed by
the gain cell is decreased with a factor of two.
7118
F125
BC847B
5133
+8V
BLM21
4106
F124
7119
BC847B
3150
3152
18K
18K
2151
22p
4
F145
2
1
7113-A
8
3
NJM4565M
+8V
+8V
2176
470p
5
F144
8
7
6
7113-B
4
NJM4565M
2152
22p
3151
3153
18K
18K
Note: the other two switches of 7111 (respectively 7111A and
7111B) are not used.
Circuit Descriptions, Abbreviation List, and IC Data Sheets
C2
R2
Gain cell
1k
2.2u
R3
C5
Rectifier
1k
2.2u
C4
C3
10u
1u
+8V
X1
CL36532008_075.eps
+5V
+8b
2137
2136
100n
100n
+8V
16
7111-A
Vdd
74HC4053D
12
Y0
S
11
F126
13
Y1
Z
14
+5V
Vss
Vee
E
8
7
6
16
7111-B
Vdd
74HC4053D
2
Y0
S
10
1
Y1
Z
15
Vss
Vee
E
8
7
6
7111-C
16
Vdd
74HC4053D
3160
5
Y0
S
9
680R
F146
3
Y1
Z
4
2165
Vss
Vee
E
22n
8
7
6
+5V
3161
680R
2166
22n
3164
3K3
3165
3K3
Figure 9-16 Stereo encoding frequency spectrum
Remember that for an OpAmp V_out= V_in*((Rf+R1)/R1),
where R1 is constant (Rf= R2+internal gain cell resistance).
The Pre-Emphasis, Stereo Encoding, and Output Filtering
The two channels that are available from the compander are
now having pre-emphasis with a time constant of 12 us. Too
much time constant would give problems with the voltage rising
too high at maximum modulation frequency, and hence
occupied bandwidth of the modulated signal. The pre-
emphasis consists of a simple RC circuit. The stereo encoder
7111C is in fact just an electronic switch, which is driven by the
sub carrier frequency. With this operation, the frequency
spectrum at the output of the switch is as follows in the
OUT
frequency domain:
060503
L
R
Subcarrier freq.
7110
74HC4060
+5V
12
2133
11
33p
+8V
F148
+8V
3
5
F147
8
7
3174
3177
2
8K2
3K9
6
7116-B
4
NJM4565M
3178
2169
8K2
33p
3173
27K
7120
7121
BC857B
BC847B
Multiplex without pilot
L+R
L-R
20kHz
Figure 9-15 Stereo encoder circuitry
+5V
2135
100n
16
STEREO
VDD
7
CODER
3
5
4
+
1
6
F131
14
CT
13
15
9
F130
1
CT=0
11
2
3
13
2134
10
3131
&
2K2
33p
9
Z1
GND
8
3130
270K
1106
AT-51
4M
MODULATION
LEVEL
4105
+8V
F150
2195
8
F149
1n2
3187
5130
1
220u
22K
7116-A
4
NJM4565M
L-R
46.875kHz
66.875kHz
CL 36532008_076.eps
090503
subc
pilot
2175
3188
MPX
4K7
1u
CL 36532008_077.eps
290403
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