The Model 517B is a notch filter designed to work with the supervisory detector module. The normal frequency of the detector tone is 25Hz. This frequency is sub-audible in most distributed systems. However, telco lines usually cannot pass this low frequency. In systems using telco lines, the detector frequency is shifted to allow it to pass through the telco system. The notch filter module is used to remove the higher detector frequency, prior to distributing the paging signal.