SSM2120 Datasheet, PDF (6/12 Pages) Analog Devices – Dynamic Range Processors/Dual VCA

SSM2120/SSM2122
In the noise gate, downward expander and compressor/limiter
applications, this potentiometer will establish the onset of the
control action. The sensitivity of the control action depends on
the value of R_T.
For a positive unipolar control output add two diodes as shown
in Figure 7a. This is useful in compressor/limiter applications.
Figure 7b shows a typical response.
Bipolar control outputs can be realized by adding a resistor from
the op amp output to V+. This is useful in compandor circuits
as shown in Figure 8a, with its response in Figure 8b. The value
of the resistor R_PV will determine the maximum output from the
control amplifier.
STEREO COMPRESSOR/LIMITER
The two control circuits of Figures 6 and 7 can be used in
conjunction to produce composite control voltages. Figures 9a
and 9b show this type of circuit and transfer function for a
stereo compressor/limiter which also acts as a downward
expander for noise gating. The output noise in the absence of a
R_IN
L
R_IN
MONO
OR R
REC IN
2V
MONO – R_IN = 10kΩ
STEREO – R_IN = 20kΩ
THRESHOLD V+
CONTROL
|I_IN|
V_T
V+
R_T
39kΩ
1kΩ
R_{PV RCON}
CON
OUT
TO –V_C
200Ω
LOG AV
THRESH
C_AV
1.5MΩ
V–
THRESHOLD
+
*
–
*UPPER LIMIT CAN BE FIXED BY
VALUE OF PULL UP RESISTOR (R_PV)
CONNECTED TO POSITIVE SUPPLY
V–
V_IN – dB
a. Control Circuit
b. Typical Compressor/Limiter Control
Curve
Figure 7. Compressor/Limiter Control Circuit and Typical Response
R_IN
L
R_IN
MONO
OR R
REC_IN
R_LL
2V
MONO – R_IN = 10kΩ
STEREO – R_IN = 20kΩ
V+
GAIN
|I_IN|
V+ V–
V_T
R_T
R_PV
39kΩ
1kΩ
CON_OUT
LOG AV
THRESH
C_AV
1.5MΩ
V–
V–
TO +V_C
OR –V_C
200Ω
*
+
V_T < 0 V_T = 0 V_T > 0
–
*UPPER AND
LOWER LIMITS CAN
BE ESTABLISHED BY
* ^{VALUES OF R}PV ^AND
R_LL, RESPECTIVELY
V_IN – dB
a. Control Circuit
b. Typical Compandor Control Curves
Figure 8. Compandor Control Circuit and Typical Curves
THRESHOLD EXP.
EXPANSION
THRESHOLD
COMPRESSION
THRESHOLD
L
MONO
OR R
FIGURE 6
+V_C
200Ω
THRESHOLD COM.
FIGURE 7
–V_C
200Ω
*
V_IN – dB
a. Control Circuit
b. Input/Output Curve
Figure 9. Control Circuit for Stereo Compressor/Limiter with Noise Gating and Input/Output Curve
–6–
REV. C