SSM2120/SSM2122

REV. C

–7–

V+

LOG AV

V–

SIGNAL

INPUT

SIGNAL

OUTPUT

200

Ω

2200pF

47

Ω

36k

Ω

36

Ω

10pF

TRANSMISSION

OR

STORAGE

MEDIUM

1µF

10k

Ω

39k

Ω

10k

Ω

V+

1k

Ω

1µF

4.7M

Ω

V+

LOG AV

V–

2200pF

47

Ω

36k

Ω

36k

Ω

10pF

1µF

10k

Ω

39k

Ω

10k

Ω

V+

1k

Ω

1µF

4.7M

Ω

200

Ω

200

Ω

200

Ω

V–

V–

Figure 10. Companding Noise Rejection System

signal will be dependent on the noise of the current-to-voltage

converter amplifier if the expansion ratio is high enough.

As discussed in the Threshold Control section, the use of the

diodes, yields positive unipolar control outputs.

COMPANDING NOISE REDUCTION SYSTEM

A complete companding noise reduction system is shown in

Figure 10. Normally, to obtain an overall gain of unity, the

compression/expansion ratio.

Table II shows compression/expansion ratios ranging from 1.5:1

An example of a 2:1 compression/expansion ratio is plotted in

Figure 11. Note that signal compression increases gain for low

level signals and reduces gain for high levels while expansion

does the reverse. The net result for the system is the same as the

original input signal except that it has been compressed before

being sent to a given medium and expanded after recovery. The

compression/expansion ratio needed depends on the medium

being used. As an extreme example, a household tape player

would require a higher compression/expansion ratio than a

professional stereo system.

INPUT SIGNAL LEVEL – dB

20

0

–80

–80

20

–60

–40

–20

0

–20

–40

–60

OVERALL

RESPONSE

2:1

EXPANSION

2:1

COMPRESSION

25dB

Figure 11. Companding Noise Reduction with 2:1

Compression/Expansion Ratio

Table II.

Gain

Compressor

Expander

(Reduction

Only

Only

Input Signal

or Increase)

Output Signal

Output Signal

Compression/

Increase (dB)

(dB)

Increase (dB)

Increase (dB)

Expansion Ratio

(mV/dB)

20

6.67

13.33

22.67

1.5:1

11,800

2.0

20

10.00

10.00

30.00

2:1

7,800

3.0

20

13.33

6.67

33.33

3:1

5,800

4.0

20

15.00

5.00

35.00

4:1

5,133

4.5

20

16.00

4.00

36.00

5:1

4,800

4.8

20

17.33

2.67

37.33

7.5:1

4,415

5.2

20

18.00

2.00

38.00

10:1

4,244

5.4

20

20.00

0

40.00

AGC*/Limiter

3,800

6.0

*AGC for Compression Only.