SSM-2120 Analog, SSM-2120 Datasheet - Page 10

• Manufacturer Part Number: SSM-2120
• Description: (SSM-2122) Dynamic Range Processors/Dual VCA
• Manufacturer: Analog

SSM2120/SSM2122
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
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.
SIGNAL
INPUT
R
R
F
IN1
IN2
F
C
C
= 5Hz
20Hz
REC
REC
IN
IN
IN |I
IN |I
C
C
AV1
AV2
I N
I N
|
|
Figure 14. Dynamic Filter with Downward Expander
V–
V–
V+
V+
C
LOG AV
LOG AV
1.5M
1.5M
control port of the
160k
V+
1k
36k
160k
THRESHOLD
1k
39k
39k
V–
CON
–10–
V–
2200pF
V–
OUT
CON
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.
Figure 15. Typical Downward Expander I/O Characteristics
at –30 dB Threshold Level (1:1.5 Ratio)
47
5.6k
OUT
12k
+20
–30
–40
–50
–60
+V
–V
200
200
C
C
2200pF
47
12k
36k
36k
DOWNWARD EXPANDER
+V
–V
200
200
C
C
36k
100pF
36k
36k
SIGNAL
OUTPUT
+20
–30
–45
–60
–75
REV. C