SSM2120/SSM2122

REV. C

–5–

The decay rates are linear ramps that are dependent on the

the formula:

Decrementation Rate (in dB/s) =

5 dB

3 dB

2 dB

1 dB

10 dB Step

11.28 ms

21.46

30.19

46.09

20 dB Step

16.65

26.83

35.56

51.46

30 dB Step

18.15

28.33

37.06

52.96

40 dB Step

18.61

27.79

37.52

53.42

50 dB Step

(+144

µs)

60 dB Step

(+46

µs)

APPLICATIONS

The following applications for the SSM2120 use both the VCAs

and level detectors in conjunction to assimilate a variety of

functions.

The first section describes the arrangement of the threshold

control in each control circuit configuration. These control

circuits form the foundation for the applications to follow which

include the downward expander, compressor/limiter and

compandor.

THRESHOLD CONTROL

Figure 6a shows the control circuit for a typical downward

expander while Figure 6b shows a typical control curve. Here,

unipolar control output. This is typically used in noise gate,

downward expander, and dynamic filter applications. This

potentiometer is used in all applications to control the signal

level versus control voltage characteristics.

10

0%

100

90

1s

2V

Figure 4. Detector Output

10

0%

100

90

2V

50ms

Figure 5. Overlayed Detector Output

DYNAMIC ATTACK AND DECAY RATES

Figure 5 shows the output levels overlayed using a storage

scope. The attack rate is determined by the step size and the

step size increase. Table I shows the values of total settling

times to within 5 dB, 3 dB, 2 dB and 1 dB of final value with

settling time for larger steps is negligible. To calculate the attack

+

–

*LOWER LIMIT CAN BE FIXED

BY CONNECTING A RESISTOR

THRESHOLD

*

2V

V+

V–

39k

Ω

200

Ω

MONO

OR R

THRESHOLD

CONTROL

LOG AV

L

V–

1.5M

Ω

1k

Ω

V–

a. Control Circuit

b. Typical Downward Expander

Control Curve

Figure 6. Noise Gate/Downward Expander Control Circuit and Typical Response