SSM2120 Datasheet, PDF (10/12 Pages) Analog Devices – Dynamic Range Processors/Dual VCA

SSM2120/SSM2122
SIGNAL
INPUT
V+
THRESHOLD
R_IN1
REC_IN
IN |I_IN|
F_C ≤ 20Hz
C_AV1
V+
LOG AV
1.5MΩ
R_IN2
REC_IN
IN |I_IN|
F_C = 5Hz
C_AV2
V–
V+
LOG AV
1.5MΩ
V–
160kΩ
1kΩ
V–
39kΩ
CON_OUT
12kΩ
V–
160kΩ
1kΩ
39kΩ CON_OUT 5.6kΩ
36kΩ
V–
47Ω
2200pF
12kΩ
200Ω
+V_C
36kΩ
47Ω
2200pF
200Ω
–V_C
200Ω
SIGNAL
OUTPUT
DOWNWARD EXPANDER
+V_C
36kΩ
36kΩ
36kΩ
100pF
36kΩ
–V_C
200Ω
Figure 14. Dynamic Filter with Downward Expander
DYNAMIC FILTER WITH DOWNWARD EXPANDER
A composite single-ended noise reduction system can be
realized by a combination of dynamic filtering and a downward
expander. As shown in Figure 14, the output from the wideband
detector can also be connected to the +V_C control port of the
second VCA which is connected in series with the sliding filter.
This will act as a downward expander with a threshold that
tracks that of the filter. Although both of these techniques are
used for noise reduction, each alone will pass appreciable
amounts of noise under some conditions. When used together,
both contribute distinct advantages while compensating for each
other’s deficiencies.
Downward expansion uses a VCA controlled by the level
detector. This section maintains dynamic range integrity for all
levels above the user adjustable threshold level. As the input
level decreases below the threshold, gain reduction occurs at an
increasing rate (see Figure 15). This technique reduces audible
noise in fade outs or low level signal passages by keeping the
standing noise floor well below the program material.
This technique by itself is less effective for signals with
predominantly low frequency content such as a bass solo where
wideband frequency noise would be heard at full level. Also,
since the level detector has a time constant for signal averaging,
percussive material can modulate the noise floor causing a
“pumping” or “breathing” effect.
The dynamic filter and downward expander techniques used
together can be employed more subtly to achieve a given level of
noise reduction than would be required if used individually. Up
to 30 dB of noise reduction can be realized while preserving the
crisp highs with a minimum of transient side effects.
+20
+20
–30
–30
–40
–45
–50
–60
–60
–75
Figure 15. Typical Downward Expander I/O Characteristics
at –30 dB Threshold Level (1:1.5 Ratio)
–10–
REV. C