10

5/90, Rev. B1

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

FIGURE 13:

3dB Filter Response

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

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

by applying a 300Hz signal to the wideband detector and a 20kHz

signal to the high-frequency band detector simultaneously. These

figures correspond to filter characteristics for 50dB, 70dB and

90dB 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 1kHz. This occurs as the con-

trol circuit detects the noise floor of the source material.

Dynamic filtering limits the signal bandwidth to less than 1kHz

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 50kHz with a nominal signal

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

content, the filter will close to the required bandwidth. 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.”

a) V

THRESH

at V+

c) V

THRESH

at V–

WIDEBAND SIGNAL LEVEL (dB)

–50

0

10

20

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

–50

–40

–30

–20

–10

0

10

20

–40

–30

–20

–10

b) V

THRESH

Centered

WIDEBAND SIGNAL LEVEL (dB)

–50

0

10

20

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

–50

–40

–30

–20

–10

0

10

20

–40

–30

–20

–10

WIDEBAND SIGNAL LEVEL (dB)

–50

1.1

1.1

1.1

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

5.5

11.7

11.7

17.8

26

26

26

50.6 50.6 50.6

50.6 50.6

50.6

–50

0

10

20

–40

–30

–20

–10

–40

–30

–20

–10

0

10

20

1.0

1.0

1.0

1.0

1.0

1.0

OBSOLETE