In the figure we have a circuit in which the second tone recognizer remains disabled until the first recognizes the corresponding tone. With this feature, the circuit can recognize two tones in the right sequence, which is not the case in the first case. Past generation paging systems used this system, usually employing three sequential tones, to identify the receiver to be called. There are many applications for this type of signal recognizer, such as the use of data, telephone and other lines for overlapping information.
