8

5/90, Rev. B1

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

FIGURE 10:

Companding Noise Reduction System

FIGURE 11:

Companding Noise Reduction with 2:1 Compres-

sion/Expansion Ratio

INPUT SIGNAL

INCREASE (dB)

CONTROL

–

(mV/dB)

R

COMPRESSION/

EXPANSION RATIO

EXPANDER

ONLY

OUTPUT SIGNAL

INCREASE (dB)

COMPRESSOR

ONLY

OUTPUT SIGNAL

INCREASE (dB)

GAIN

(REDUCTION

OR INCREASE)

(dB)

20

20

20

20

20

20

20

20

6.67

10.00

13.33

15.00

16.00

17.33

18.00

20.00

22.67

30.00

33.33

35.00

36.00

37.33

38.00

40.00

13.33

10.00

6.67

5.00

4.00

2.67

2.00

0

1.5:1

2:1

3:1

4:1

5:1

7.5:1

10:1

AGC*/Limiter

11,800

7,800

5,800

5,133

4,800

4,415

4,244

3,800

2.0

3.0

4.0

4.5

4.8

5.2

5.4

6.0

*AGC for Compression Only

TABLE 1

COMPANDING NOISE REDUCTION SYSTEM

A complete companding noise reduction system is shown in Fig-

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

R

C

is equal to R

E

. The values of R

C/E

will determine the compres-

sion/expansion ratio.

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

to full limiting with the corresponding values of R

C/E

.

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 be-

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

The compression/expansion ratio needed depends on the me-

dium 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

–20

–40

–60

–80

–80

–60

–40

–20

0

20

25dB

2:1

COMPRESSION

OVERALL

RESPONSE

2:1

EXPANSION

~

~

10k

Ω

200

Ω

2200pF

SIGNAL

INPUT

47

Ω

200

Ω

+

–

36

Ω

10pF

V+

+

–

10k

Ω

V+

39k

Ω

1k

Ω

V–

4.7M

Ω

1

µF

TRANSMISSION

OR

STORAGE

MEDIUM

47

Ω

2200pF

200

Ω

200

Ω

200

Ω

200

Ω

200

Ω

+

–

SIGNAL

OUTPUT

10pF

36k

Ω

REC

IN

+

–

39k

Ω

1k

Ω

4.7M

Ω

1

µF

10k

1k

Ω

4.7M

Ω

1

µF

39k

Ω

10k

Ω

V+

+

–

LOG AV

LOG AV

V+

36k

Ω

+

1

µF

+

1

µF

36k

Ω

V–

V–

V–

OBSOLETE