Stereo decoder changes automatically from mono to stereo - by responding to stereo signal presence and not to pilot by use of filters

摘要

Thestereo decoder changes over automatically from mono to stereo independently of the pilot tone and only when stereo signal are actually transmitted. - The control signal for the changeover process switches is obtained using a filter (7) whose passband centre frequency lies within the frequency range occupied by the incoming multiplexed signals difference signal (Sm) and whose input receives at least this difference signal. The filter output is rectified and if necessary amplified and passed to the switching stage (6). The filter centre frequency lies near the subcarrier and includes both the difference signal sidebands.