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 a reduction of reduction the bandpass of the 421 to minimize feedback or amplifier power waste caused by spurious out-of-band signals.