An apparatus and method for monitoring the received pulses of a transmitted pulse code modulation signal to determine if the received pulses lie within a predetermined pulse distortion tolerance zone is disclosed. The received pulses are monitored to determine the number of received pulse edges which are received during first and second consecutive predetermined time zones, the total period of the time zones being equal to the period of the pulse pattern of the received message which would occur in the case of distortionless transmission. The first time zone extends around the position where the pulse edge of the received pulses would be in the case of distortionless transmission. An error recognition signal is generated when either more than one of the received pulse edges arrives during the first time zone or only one of the received pulse edges arrives during the second time zone.