stereo noise filter

The signal-to-noise ratio of an FM broadcast received in stereo is considerably worse than that of the same broadcast received in mono. This is most noticeable on weak trans- missions, when switching over from stereo to mono will considerably reduce the noise level. This noise reduction occurs because the left-channel noise is largely in anti-phase to the right-channel noise. Switching to mono sums the two channels and the anti- phase noise signals cancel. By summing only the high-frequency com- ponents of the signal it is possible to elim- inate the annoying high-frequency noise without destroying the stereo image since channel separation is still maintained at middle and low frequencies.