SSM2120/SSM2122

REV. C

–8–

DYNAMIC FILTER

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

single ended (nonencode/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

20 Hz while the lower detector has a 5 kHz 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.

V+

LOG AV

V–

AUDIO

INPUT

AUDIO

OUTPUT

160k

Ω

2.2µF

10k

Ω

39k

Ω

1.5M

Ω

1k

Ω

3.3µF

2200pF

47

Ω

36k

Ω

36k

Ω

100pF

12k

Ω

V–

200

Ω

200

Ω

9

THRESHOLD

CONTROL

THRESH

3

2

160k

Ω

39k

Ω

1k

Ω

V–

THRESH

14

V+

LOG AV

3300pF

10k

Ω

1.5M

Ω

3.3µF

15

13

(HIGH FREQUENCY)

(WIDEBAND)

36k

Ω

5

7

8

5.6k

Ω

5

36k

Ω

12

1

V–

V–

Figure 12. Dynamic Noise Filter Circuit