The MX375 is a CMOS LSI microcircuit which combines CTCSS Encode/Decode operation with voice band frequency inversion. Frequency inversion is achieved by modulating the input audio with a 3333 Hz carrier frequency. Higher voice band frequencies are translated downward, and lower frequencies upward, resulting in a "mirror image" voice transmission. PvtSQUELCHTM combines CTCSS with inverted speech to prevent users from understanding each other's communications unless the transmissions are accompanied by the group's assigned tone. Its net effect is to eliminate casual eavesdropping and give mobile radio users a certain degree of privacy at a minimal price. Up to 38 PvtSQUELCH user groups (one per CTCSS tone) can share a single radio channel. With PvtSQUELCH, competing businesses can share a radio channel without compromising communications security. Device features: 1) serial or parallel tone programming capability (serial or parallel offered on MX375J, P & LH8), 2) the ability to operate under NOTONE conditions, 3) on-chip Tx and Rx audio filtering, 4) pin-selectable Private/Clear operation, and 5) pre/deemphasis filters in the Tx path, for optimal recovered audio quality. A low-cost 4 MHz crystal/clock and a single 5V supply are required for operation. The MX375 is available in the following package styles: 28-pin PDIP (MX375P), 28-pin CDIP (MX375J) 28-pin PLCC (MX375LH8), and 24-pin PLCC (MX375LH).