The longer the expected delay, the larger the required FIFO buffer. Detecting 'double talk' The echo cancellation performed in the gateway is referred to as near-end echo cancellation because the echo is canceled at the end nearest the echo source. It is possible to perform echo cancellation at the far end, Gateway A, but the echo path will be much longer, normally requiring more processing power. The echo canceler compares the signal similarity and power level coming from the hybrid switch to the previously stored signals passed from the packet network. The comparison is made using a model of the hybrid circuit which was created by an adaptive algorithm inside the echo canceler.
Double talk occurs when both sides of a conversation attempt to speak simultaneously.
The near-end speaker confuses the echo identification process, thus limiting the effectiveness of the echo canceler. In addition, the echo canceler must not clip the beginning of the double-talk session. The double-talk detector works in most cases, but when it fails to detect double talk, some echo is still noticeable. The echo canceler also must identify and remove background noise. Once an echo is identified, the echo canceler subtracts the echo from the returning signal.