a
Microphone Preamplifier with
Variable Compression and Noise Gating
SSM2166*
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 鈥?0擄C to +85擄C. For similar features/performance
in an 8-lead package, please refer to the SSM2165.
10
0
鈥?0
OUTPUT 鈥?dBu
鈥?0
鈥?0
鈥?0
鈥?0
鈥?0
鈥?0
RATIO = 1:1
RATIO = 10:1
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/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
GENERAL DESCRIPTION
RATIO = 2:1
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 set-
ting the SSM2166 can be programmed with a fixed gain of up to
R1 = 10k
5
BUF
OUT
6
AUDIO
+IN
7
0.1 F
LEVEL
DETECTOR
R2 = 10k
+
1 F
BUFFER
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 Characteris-
tics 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
1k
VCA
V+
2
2.3k
V+
VCA GAIN
ADJ
13
V
OUT
9
500k
NOISE GATE
SET
11
17k
CONTROL
SSM2166
12
POWER
DOWN
1
GND
8
AVG
CAP 22 F
+
10
25k
ROTATION
POINT SET
COMPRESSION
RATIO
SET
*
OPTIONAL
*Patents
pending.
Figure 2. Functional Block Diagram and Typical Speech Application
REV. A
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.
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
漏 Analog Devices, Inc., 1999
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