The SSM2165 is 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 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 “popping.” A downward expander (noise gate) prevents amplification 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-pin SOIC and P-DIP packages, and guaranteed for operation over the extended industrial ture range of -40°C to +85°C. As shown in Figure 1a, the SSM2165-1 has a rotation point of -25.7 dBu (40 mV)1 , a VCA gain of 18 dB, and gives -7.7 dBu (320 mV) before limiting. As shown in Figure 1b, the SSM2165-2 has a rotation point of -17.8 dBu (100 mV), a VCA gain of 8 dB and gives -9.8 dBu (250 mV) before limiting. Both have a noise gate threshold of -64 dBu (500 µV), below which downward expansion reduces the gain with a ratio of approximately 1:3. That is, a -3 dB reduction of output signal occurs with a -1 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.