This invention permits the upstream transmission of short packets of information in a cable TV 
system, and blocking upstream 
noise at all other times, and does so without interfering with 
image quality of normal 
TV viewing. A remotely operable ingress 
noise blocking filter is placed at the terminating junction between a subscriber's coaxial drop cable and a corresponding feeder tap in a cable TV 
system. The ingress 
noise blocking filter contains a 
high pass filter to pass the normal TV band. This 
high pass filter is bypassed by a section containing low pass filters and a switch operated when receiving a 
control signal from a 
cable modem during those short durations the 
cable modem is authorized to transmit an upstream 
signal. Low pass filters isolate the switching elements so that switching transients cannot occur in the downstream TV band. The level of the amplitude of the downstream 
signal is unaffected whether the switching elements are open or closed. Further, the switching arrangement provides a relatively 
constant impedance, both when the switching element is on and off to provide a high 
return loss and avoid 
signal reflections and maintain 
signal quality. In a diagnostic mode, the location of each noise source is determined by correlating the energy measured at each a set of frequencies across the band at the headend during those periods when the 
cable modem is allowed to transmit versus the energy received during those periods when the same cable modem is not authorized to transmit.