US Patent 7555117 Path change detector for echo cancellation

A path change is detected by multiplying the energy of a signal on the output of a summation circuit in one channel of a telephone by a constant to produce a product. The product is compared with the energy of a signal on the input of the summation circuit. A path change is indicated when the energy of the product exceeds the energy of a signal on the input of the summation circuit. The comparison is made in successive frames of an audio signal and a path change is indicated when the energy of a signal on the input of the summation circuit is exceeded in two or more successive frames.