SSM2120/SSM2122

REV. C

–10–

V+

LOG AV

V–

SIGNAL

INPUT

SIGNAL

OUTPUT

160k

Ω

39k

Ω

1.5M

Ω

1k

Ω

V–

2200pF

47

Ω

36k

Ω

36k

Ω

100pF

12k

Ω

V–

200

Ω

200

Ω

THRESHOLD

160k

Ω

39k

Ω

1k

Ω

V–

36k

Ω

5.6k

Ω

36k

Ω

V+

V+

LOG AV

1.5M

Ω

V–

36k

Ω

36k

Ω

200

Ω

200

Ω

12k

Ω

2200pF

47

Ω

DOWNWARD EXPANDER

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

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

–30

–40

–50

–60

+20

–30

–45

–60

–75

Figure 15. Typical Downward Expander I/O Characteristics

at –30 dB Threshold Level (1:1.5 Ratio)