9

5/90, Rev. B1

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

FIGURE 12:

Dynamic Noise Filter Circuit

DYNAMIC FILTER

Figure 12 shows a control circuit for a dynamic filter capable of

single ended (non-encode/decode) noise reduction. Such circuits

usually suffer from a loss of high-frequency content at low signal

levels because their control circuits detect the absolute amount

of highs present in the signal. This circuit, however, measures

wideband level as well as high-frequency band level to produce

a composite control signal combined in a 1:2 ratio respectively.

The upper detector senses wideband signals with a cutoff of 20Hz

while the lower detector has a 5kHz cutoff to sense only high-

frequency band signals. This approach allows very good noise

masking with a minimum loss of “highs” when the signal level

goes below the threshold.

AUDIO

INPUT

10k

Ω 2.2µF REC

IN

V+

+

–

THRESHOLD

CONTROL

V–

160k

Ω

3.3

µF

2

LOG AV

1.5M

Ω

V–

160k

Ω

1k

Ω

THRESH

1

39k

Ω

CON

3

10k

Ω 3300pF

3.3

µF

13

1.5M

Ω

V–

V+

+

–

1k

Ω

9

15

LOG AV

THRESH

39k

Ω

14

12

5.6k

Ω

200

Ω

5

+

–

SIG

OUT

+

–

5

36k

Ω

36k

Ω

100pF

AUDIO

OUTPUT

SIG

IN

8

36k

Ω

47

Ω

2200pF

36k

Ω

(WIDEBAND)

(HIGH FREQUENCY)

V–

V–

CON

OUT

REC

IN

7

200

Ω

OBSOLETE