a
Microphone Preamplifier with
Variable Compression and Noise Gating
SSM2165*
FUNCTIONAL BLOCK DIAGRAM
C2
10 F
+
VCA
IN
R
A
2
VCA
BUFFER
LEVEL
DETECTOR
CONTROL
FEATURES
Complete Microphone Conditioner in an 8-Lead Package
Single +5 V Operation
Preset Noise Gate Threshold
Compression Ratio Set by External Resistor
Automatic Limiting Feature Prevents ADC Overload
Adjustable Release Time
Low Noise and Distortion
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
V+
V+ BUF
OUT
+1
AUDIO
IN+
C1
0.1 F
R
A
2
V
OUT
SSM2165
AVG CAP
C3
22 F
+
R1
25k
COMPRESSION
RATIO SET
GND
The SSM2165 is a complete and flexible solution for condition-
ing 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 compres-
sion ratio is set by a single resistor and can be varied from 1:1 to
over 15:1 relative to the fixed rotation point. Signals above the
rotation point are limited to prevent overload and to eliminate
鈥減opping.鈥?A downward expander (noise gate) prevents amplifi-
cation of noise or hum. This results in optimized 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 flexibility
of setting the compression ratio and the time constant of the
level detector, coupled with two values of rotation point, make
the SSM2165 easy to integrate in a wide variety of microphone
conditioning applications.
The SSM2165 is an ideal companion product for audio codecs
used in computer systems, such as the AD1845 and AD1847.
The device is available in 8-lead SOIC and P-DIP packages, and
guaranteed for operation over the extended industrial temperature
range of 鈥?0擄C to +85擄C. As shown in Figure 1a, the SSM2165-1
has a rotation point of 鈥?5.7 dBu (40 mV)
1
, a VCA gain of 18 dB,
and gives 鈥?.7 dBu (320 mV) before limiting. As shown in Figure
1b, the SSM2165-2 has a rotation point of 鈥?7.8 dBu (100 mV),
*
Patents pending.
1
All signals are in rms volts or dBu (0 dBu = 0.775 V rms).
a VCA gain of 8 dB and gives 鈥?.8 dBu (250 mV) before limiting.
Both have a noise gate threshold of 鈥?4 dBu (500
碌V),
below
which downward expansion reduces the gain with a ratio of
approximately 1:3. That is, a 鈥? dB reduction of output signal
occurs with a 鈥? dB reduction of input signal. For applications
requiring adjustable noise gate threshold, VCA gain up to 18 dB,
and adjustable rotation point, please refer to the SSM2166.
0
鈥?0
鈥?0
鈥?0
鈥?0
鈥?0
鈥?0
鈥?0
OUTPUT 鈥?dBu
鈥?0
鈥?0
鈥?0
鈥?0
INPUT 鈥?dBu
鈥?0
鈥?0
鈥?0
Figure 1a. SSM2165-1 Compression and Gating Characteristics
0
鈥?0
鈥?0
OUTPUT 鈥?dBu
鈥?0
鈥?0
鈥?0
鈥?0
鈥?0
鈥?0
鈥?0
鈥?0
鈥?0
鈥?0
INPUT 鈥?dBu
鈥?0
鈥?0
鈥?0
Figure 1b. SSM2165-2 Compression and Gating Characteristics
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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