to a hybrid 25 hybrid 25 is coupled to telephone 15 which receives the analog signal from telephone 13 telephone 15 transmits voice signals across network 21 back to telephone 13 the analog voice signals from telephone 15 are coupled to hybrid 25 which in turn is coupled to an a d converter 27 a d converter 27 converts the analog signals received from hybrid 25 to digital signals which are transmitted across network 21 in the digital domain to a d a converter 29 located at the echo controlling end d a converter 29 is coupled to hybrid 17 which in turn is coupled to telephone 13 echos are caused by impedance mismatches at hybrid 25 a portion of the analog signal from telephone 13 received on the inbound leg at hybrid 25 may be coupled back onto the outbound leg because of signal processing and transmission delays any incoming signal that is coupled through hybrid 25 is received back at telephone 13 delayed in time as an echo this echo is annoying and objectionable the present invention achieves echo cancellation by performing signal processing in the digital domain the digital signal received from a d converter 19 is processed by an alternate sample overwrite unit 31 the digital signal from a d converter 19 is a stream of eight bit bytes with values ranging from u2212 255 to 255 alternate sample overwrite unit 31 replaces every other byte of the digital signal with a fixed value signature byte which in the preferred embodiment has a value of 255 alternate sample overwrite unit effectively imposes a 4 khz high amplitude signal on top of the analog signal received at telephone 13 the.