Abstract: In a borehole telemetry system for acoustically transmitting data over a pipe suspended in a borehole, the level of noise in the data stream is inherently high, making the use of discrete frequencies advantageous to provide useful data. Any drift in electrical circuits which might affect or be affected by the use of precise frequencies is undesirable. A synchronous detector facilitates the removal of noise components from the data stream by comparing in a commutative filter the phase difference between a switching signal taken from the incoming data signal path, after the second stage of commutative filtering, with the data signal emerging from the last stage of filtering. The synchronous detector also compares a data signal having two stages of filtering with another data signal having six stages of filtering with the additional filtering, causing a phase shift in frequencies outside the precise frequency window, which the synchronous detector also rejects.