SSM2120 Datasheet, PDF (7/12 Pages) Analog Devices – Dynamic Range Processors/Dual VCA

SSM2120/SSM2122
SIGNAL
INPUT
36kΩ
47Ω
200Ω
2200pF
200Ω
1µF
10kΩ
|I_IN|
REC_IN
+V_C
–V_C
V+
10pF
36Ω
TRANSMISSION
OR
STORAGE
MEDIUM
V+
R_C
1kΩ
39kΩ
10kΩ
1µF
LOG AV
4.7MΩ
V–
V–
–V_C
36kΩ
200Ω
2200pF
47Ω
200Ω
1µF
10kΩ
|I_IN|
REC_IN
+V_C
V+
10pF
36kΩ
SIGNAL
OUTPUT
R_E
1kΩ
V+
39kΩ
10kΩ
1µF
LOG AV
4.7MΩ
V–
V–
Figure 10. Companding Noise Rejection System
signal will be dependent on the noise of the current-to-voltage
converter amplifier if the expansion ratio is high enough.
As discussed in the Threshold Control section, the use of the
control circuit of Figure 6, including the R_PV to V+ and two
diodes, yields positive unipolar control outputs.
COMPANDING NOISE REDUCTION SYSTEM
A complete companding noise reduction system is shown in
Figure 10. Normally, to obtain an overall gain of unity, the
value of R_C is equal to R_E. The values of R_C/E will determine the
compression/expansion ratio.
Table II shows compression/expansion ratios ranging from 1.5:1
to full limiting with the corresponding values of R_C/E.
An example of a 2:1 compression/expansion ratio is plotted in
Figure 11. Note that signal compression increases gain for low
level signals and reduces gain for high levels while expansion
does the reverse. The net result for the system is the same as the
original input signal except that it has been compressed before
being sent to a given medium and expanded after recovery. The
compression/expansion ratio needed depends on the medium
being used. As an extreme example, a household tape player
would require a higher compression/expansion ratio than a
professional stereo system.
20
I_REF ≈ 3µA
R_REF = 4.7MΩ
0
OVERALL
RESPONSE
2:1
EXPANSION
–20
2:1
COMPRESSION
–40
25dB
–60
–80
–80
–60
–40
–20
0
20
INPUT SIGNAL LEVEL – dB
Figure 11. Companding Noise Reduction with 2:1
Compression/Expansion Ratio
Input Signal
Increase (dB)
Gain
(Reduction
or Increase)
(dB)
20
6.67
20
10.00
20
13.33
20
15.00
20
16.00
20
17.33
20
18.00
20
20.00
*AGC for Compression Only.
Compressor
Only
Output Signal
Increase (dB)
13.33
10.00
6.67
5.00
4.00
2.67
2.00
0
Table II.
Expander
Only
Output Signal
Increase (dB)
22.67
30.00
33.33
35.00
36.00
37.33
38.00
40.00
Compression/
Expansion Ratio
1.5:1
2:1
3:1
4:1
5:1
7.5:1
10:1
AGC*/Limiter
R_C/E ⍀
11,800
7,800
5,800
5,133
4,800
4,415
4,244
3,800
⌬V_CONTROL –
(mV/dB)
2.0
3.0
4.0
4.5
4.8
5.2
5.4
6.0
REV. C
–7–