It has long been appreciated that FM reception is vulnerable to multipath propagation. The effect of an echo — for instance a reflection off a building — is to create high-order harmonic and intermodulation products on the audio output of the receiver. Such distortion is particularly unpleasant, and it follows that only low levels of multipath are tolerable. The situation is even worse when stereo signals are being broadcast.

This White Paper describes a simple model that was developed to quantify the harmonic distortion caused by a single echo. The model confirms that stereo signals suffer more as a result of multipath, although the distortion varies greatly as a function of echo delay. It would be straightforward to extend the model to include multiple echoes and audio intermodulation products.

The predictions of the model have been compared with the measurements made on actual receivers. Agreement is generally good for audio frequencies below 3 kHz. Higher frequencies can cause strange effects as a result of high-order harmonics entering the receiver’s stereo decoder. Since the extent of this problem depends on the detailed design of the stereo decoder, a universal model cannot make accurate predictions.