The Symetrix 421 is a wide-range AGC-Leveler, a peak limiter, and a downward expander. The 421 reduces the dynamic range of its input signal by the amount of its ratio setting. That is, if the ratio setting is 2:1, then 40 dB of input range turns into 20 dB at the output. A fast peak limiter puts an absolute ceiling on the output level so you know that peaks are held to the level that you set. A downward expander reduces the gain when signal levels fall too low to process, reducing the noise buildup at low signal levels. Speech filters allow reducing the bandpass of the 421 to minimize feedback or amplifier power waste caused by spurious out-of-band signals. AGC-Levelers can be used anywhere that you might need to reduce the dynamic range of an audio signal over a wide range of signal levels. Possible applications include: tape duplication (especially cassette), driving telephone lines, driving broadcast or STL transmitters, podium or lectern microphones，paging systems, unattended sound systems, and music recording. The 421 is Listed by Underwriters Laboratories Inc. (UL). Samples of this product have been evaluated by UL and meet the applicable UL Standards for Safety.