SSM2120/SSM2122

REV. C

–9–

Figures 13a–c show the filter’s 3 dB frequency response with the

threshold potentiometer at V+, centered, and V–. Data was

taken by applying a 300 Hz signal to the wideband detector and

a 20 kHz signal to the high-frequency band detector simultane-

ously. These figures correspond to filter characteristics for

50 dB, 70 dB and 90 dB dynamic range program source

material, respectively. The system could thus treat signals from

anything ranging from 1/4" magnetic tape to high performance

compact disc players.

Note that in Figure 13a the control circuit is designed so that

the minimum cutoff frequency is about 1 kHz. This occurs as

the control circuit detects the noise floor of the source material.

Dynamic filtering limits the signal bandwidth to less than 1 kHz

unless enough highs are detected in the signal to cover the noise

floor in the mid- and high frequency range. In this case the filter

opens to pass more of the audio band as more highs are detected.

The filter’s bandwidth can extend to 50 kHz with a nominal

signal level at the input. At other signal levels with varying high

frequency content, the filter will close to the required band-

width. Here, noise outside the band is removed while the

perceived noise is masked by other signals within the band.

Even in this system, however, a certain amount of mid- and high

frequency components will be lost, especially during transients

at very low signal levels. This circuit does not address low

frequency noise such as “hum” and “rumble.”

WIDEBAND SIGNAL LEVEL – dB

–50

–40

–30

–20

–10

0

10

20

20

–20

–30

–40

–50

0

–10

10

1.0

1.0

1.0

1.0

1.0

1.1

1.1

1.1

1.0

1.0

1.2

1.7

2.4

2.4

2.4

1.9

2.75

3.9

5.5

5.5

5.5

6

8.3

11.7

11.7

11.7

17.8

26

26

26

50.6

50.6

50.6

50.6

50.6

50.6

WIDEBAND SIGNAL LEVEL – dB

–50

–40

–30

–20

–10

0

10

20

20

–20

–30

–40

–50

0

–10

10

1.5

2.2

3.1

4.2

4.2

4.2

4.2

4.2

4.9

7.1

10

10

10

10

10

15.1

22

22

22

22

22

48

49.2

49.2

49.2

49.2

50.6

50.6

50.6

50.6

50.6

50.6

50.6

50.6

50.6

50.6

WIDEBAND SIGNAL LEVEL – dB

–50

–40

–30

–20

–10

0

10

20

20

–20

–30

–40

–50

0

–10

10

12.3

17.3

17.8

17.8

17.8

17.8

17.8

17.8

40

41

41

41

41

41

41

50.6

50.6

50.6

50.6

50.6

50.6

50.6

50.6

50.6

50.6

50.6

50.6

50.6

50.6

50.6

50.6

50.6

50.6

50.6

50.6

50.6

Figure 13. 3 dB Filter Response