SSM-2122 Просмотр технического описания (PDF) - Analog Devices

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SSM-2122

Dynamic Range Processors/Dual VCA

Analog Devices 

SSM2120/SSM2122

SIGNAL

INPUT

V+

THRESHOLD

RIN1

RECIN

IN |IIN|

FC ≤ 20Hz

CAV1

V+

LOG AV

1.5MΩ

RIN2

RECIN

IN |IIN|

FC = 5Hz

CAV2

V–

V+

LOG AV

1.5MΩ

V–

160kΩ

1kΩ

V–

39kΩ

CONOUT

12kΩ

V–

160kΩ

1kΩ

39kΩ CONOUT 5.6kΩ

36kΩ

V–

47Ω

2200pF

12kΩ

200Ω

+VC

36kΩ

47Ω

2200pF

200Ω

–VC

200Ω

SIGNAL

OUTPUT

DOWNWARD EXPANDER

+VC

36kΩ

36kΩ

36kΩ

100pF

36kΩ

–VC

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 +VC 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

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