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Microphone Preamplifier with

Variable Compression and Noise Gating

SSM2166

*

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鈥擯revents ADC Overload

Adjustable Release Time

Low Noise and Distortion

Power-Down Feature

20 kHz Bandwidth ( 1 dB)

Low Cost

APPLICATIONS

Microphone Preamplifier/Processors

Computer Sound Cards

Public Address/Paging Systems

Communication Headsets

Telephone Conferencing

Guitar Sustain Effects Generators

Computerized Voice Recognition

Surveillance Systems

Karaoke and DJ Mixers

GENERAL DESCRIPTION

this gain is in addition to the variable gain in other compression

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 a 14-lead SOIC package, and is guar-

anteed for operation over the extended industrial temperature

range of 鈥?0擄C to +85擄C. For similar features and performance

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

10

0

鈥?0

RATIO = 10:1

RATIO = 2:1

OUTPUT 鈥?dBu

鈥?0

鈥?0

鈥?0

鈥?0

鈥?0

鈥?0

RATIO = 1:1

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 鈥渞otation point鈥?

signals above the rotation point are limited to prevent overload

and eliminate 鈥減opping.鈥?In the 1:1 compression setting, the

SSM2166 can be programmed with a fixed gain of up to 20 dB;

R1 = 10k

5

BUFOUT

鈥揑N

AUDIO

+IN

6

7

0.1 F

LEVEL

DETECTOR

R2 = 10k

+

1 F

BUFFER

1k

10 F

+

3

VCA

IN

4

鈥?0

鈥?0

鈥?0

鈥?0

INPUT 鈥?dBu

鈥?0

鈥?0

0

Figure 1. SSM2166 Compression and Gating Char-

acteristics with 10 dB of Fixed Gain (The Gain Adjust

Pin Can Be Used to Vary This Fixed Gain Amount)

10 F

*

+

14

VCA

R

1k

VCA

V+

2

2.3k

V+

VCA GAIN

ADJ

13

OUTPUT

9

500k

NOISE GATE

SET

11

17k

CONTROL

SSM2166

12

POWER

DOWN

1

GND

10

8 +

AVG

25k

CAP 22 F

*Patents

pending.

ROTATION

POINT SET

COMPRESSION

RATIO

SET

*

OPTIONAL

REV. B

Figure 2. Functional Block Diagram and Typical Speech Application

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 that

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

under any patent or patent rights of Analog Devices. Trademarks and

registered trademarks are the property of their respective companies.

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

Tel: 781/329-4700

www.analog.com

Fax: 781/326-8703

漏 2003 Analog Devices, Inc. All rights reserved.

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