If the delay is very small (10s of milliseconds or less ), the phenomenon is called sidetone. If the delay is fairly significant (more than a few hundred milliseconds), it is considered annoying. In telephony, echo is the reflected copy of one's voice heard some time later. ITU standards G.168 and P.340 describe requirements and tests for echo cancellers in digital and PSTN applications, respectively. This technique is generally implemented digitally using a digital signal processor or software, although it can be implemented in analog circuits as well. Once the echo is recognized, it can be removed by subtracting it from the transmitted or received signal.