a

SSM2166*

Microphone Preamplifier with

Variable Compression and Noise Gating

Information furnished by Analog Devices is believed to be accurate and

reliable. However, no responsibility is assumed by Analog Devices for its

use, nor for any infringements of patents or other rights of third parties

which may result from its use. No license is granted by implication or

otherwise under any patent or patent rights of Analog Devices.

FEATURES

Complete Microphone Conditioner in a 14-Lead Package

Single +5 V Operation

Adjustable Noise Gate Threshold

Compression Ratio Set by External Resistor

Automatic Limiting Feature—Prevents ADC Overload

Adjustable Release Time

Low Noise and Distortion

Power-Down Feature

20 kHz Bandwidth ( 1 dB)

Low Cost

APPLICATIONS

Microphone Preamplifier/Processor

Computer Sound Cards

Public Address/Paging Systems

Communication Headsets

Telephone Conferencing

Guitar Sustain Effects Generator

Computerized Voice Recognition

Surveillance Systems

Karaoke and DJ Mixers

20 dB; this gain is in addition to the variable gain in other com-

pression settings. The input buffer can also be configured for front-

end gains of 0 dB to 20 dB. A downward expander (noise gate)

prevents amplification of noise or hum. This results in opti-

mized signal levels prior to digitization, thereby eliminating the

need for additional gain or attenuation in the digital domain

that could add noise or impair accuracy of speech recognition

algorithms. The compression ratio and time constants are set

externally. A high degree of flexibility is provided by the VCA

Gain, Rotation Point, and Noise Gate adjustment pins.

The SSM2166 is an ideal companion product for audio codecs

used in computer systems, such as the AD1845 and AD1847.

The device is available in 14-lead SOIC and P-DIP packages,

and guaranteed for operation over the extended industrial tempera-

ture range of –40

°C to +85°C. For similar features/performance

in an 8-lead package, please refer to the SSM2165.

INPUT – dBu

–70

0

–60

–50

–40

–30

–20

–10

10

0

–30

–40

–50

–60

–10

–20

RATIO = 10:1

RATIO = 2:1

RATIO = 1:1

Figure 1. SSM2166 Compression and Gating Characteris-

tics with 10 dB of Fixed Gain (The Gain Adjust Pin Can Be

Used to Vary This Fixed Gain Amount)

REV. A

*Patents pending.

Figure 2. Functional Block Diagram and Typical Speech Application

GENERAL DESCRIPTION

The SSM2166 integrates a complete and flexible solution for

conditioning microphone inputs in computer audio systems. It

is also excellent for improving vocal clarity in communications

and public address systems. A low noise voltage controlled

amplifier (VCA) provides a gain that is dynamically adjusted by

a control loop to maintain a set compression characteristic. The

compression ratio is set by a single resistor and can be varied

from 1:1 to over 15:1 relative to a user defined “rotation

point;” signals above the rotation point are limited to prevent

overload and eliminate “popping.” In the 1:1 compression set-

ting the SSM2166 can be programmed with a fixed gain of up to

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.

Tel: 781/329-4700

World Wide Web Site: http://www.analog.com

Fax: 781/326-8703

© Analog Devices, Inc., 1999

CONTROL

AUDIO

+IN

0.1 F

6

53

10 F

*

13

9

NOISE GATE

SET

V+

11

ROTATION

POINT SET

10

8

1

GND

22 F

COMPRESSION

RATIO

SET

25k

2

2.3k

14

500k

17k

1k

7

LEVEL

DETECTOR

BUFFER

VCA

1k

SSM2166

4

10 F

R1 = 10k

R2 = 10k

1 F

+

12

POWER

DOWN

AVG

CAP

+

++

V+

VCA GAIN

ADJ