11

5/90, Rev. B1

SSM-2120/SSM-2122 DYNAMIC RANGE PROCESSOR/DUAL VCA

DYNAMIC FILTER WITH DOWNWARD EXPANDER

A composite single-ended noise reduction system can be real-

ized by a combination of dynamic filtering and a downward ex-

pander. 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 con-

tribute distinct advantages while compensating for each other’s

deficiencies.

Downward expansion uses a VCA controlled by the level detec-

tor. 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 in-

creasing 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 predomi-

nantly low frequency content such as a bass solo where wide-

band frequency noise would be heard at full level. Also, since

the level detector has a time constant for signal averaging, per-

cussive material can modulate the noise floor causing a “pump-

ing” or “breathing” effect.

The dynamic filter and downward expander techniques used to-

gether can be employed more subtly to achieve a given level of

noise reduction than would be required if used individually. Up

to 30dB of noise reduction can be realized while preserving the

crisp highs with a minimum of transient side effects.

FIGURE 14:

Dynamic Filter with Downward Expander

FIGURE 15:

Typical Downward Expander I/O Characteristics

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

+20

+20

–30

–30

–40

–45

–60

–75

–50

-60

+

–

REC

IN

LOG AV

1.5M

Ω

+

1k

Ω

V–

+

–

V+

LOG AV

V–

1.5M

Ω

+

1k

Ω

V–

39k

Ω

5.6k

Ω

CON

OUT

39k

Ω

12k

Ω

200

Ω

200

Ω

200

Ω

200

Ω

–

+

36k

Ω

SIGNAL

OUTPUT

36k

Ω

36k

Ω

36k

Ω

DOWNWARD EXPANDER

100pF

NOISE FILTER

THRESHOLD

V+

V–

REC

IN

V+

V–

CON

OUT

160k

Ω

160k

Ω

12k

Ω

–

+

–

+

47

Ω

2200pF

SIGNAL

INPUT

36k

Ω

47

Ω

36k

Ω

2200pF

OBSOLETE