Philips 32PW9618 Service Manual page 141

Circuit Descriptions, Abbreviation List, and IC Data Sheets
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 (Vin) 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.
Remember that for an OpAmp Vout= Vin*((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 µs. 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
frequency domain:
7118
F125
BC847B
5133
+8V +8b
BLM21
4106
F124
+8V
F126
7119
BC847B
+5V
3150 3152
18K 18K
2151
22p
4 F145
2
3160
1
680R
7113-A
8
3
2165
NJM4565M
+8V
22n
+8V
2176
470p
5 F144
8
3161
7
680R
6
7113-B
4
2166
NJM4565M
22n
2152
3164
22p
3K3
3151 3153
3165
18K 18K
3K3
Note that the other two switches of 7111 (respectively 7111A
and 7111B) are not used.
+5V
2137 2136
100n 100n
16
7111-A
Vdd
74HC4053D
12 11
Y0 S
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
16
7111-C
Vdd
74HC4053D
5 Y0 S 9
F146
3 Y1 Z 4
Vss Vee E
8 7 6
+8V
+5V
5
8
7
6
7116-B
4
NJM4565M
2169
33p
3173
27K
7120 7121
BC857B BC847B
Figure 9-16 Stereo encoding frequency spectrum
L
R
Subcarrier freq.
Figure 9-15 Stereo encoder circuitry
+5V
2135
7110
100n
74HC4060
16
VDD
7
3
5
4
+
1
6
14
CT
13
15
9
12 1
CT=0 11
2
3
13
10
&
2133
11 9
Z1
33p
GND
8
3130
270K
+8V
1106
AT-51
4M
F148
+8V
3
8 F149
F147 1
3174 3177 2
7116-A
4
8K2 3K9
NJM4565M
3178
8K2
EM6E 9.
Multiplex without pilot
L+R
L-R L-R
20kHz 46.875kHz
STEREO
CODER
F131
F130
3131 2134
2K2
33p
MODULATION
LEVEL
4105
2175
F150 3188
2195
4K7
1u
1n2
3187 5130
220u
22K
CL 36532008_077.eps
EN 141
66.875kHz
CL 36532008_076.eps
090503
subc
pilot
MPX
290403