code is detected by the code detector 56 and controls the frequency of the signal from the dividers 66 and 42 it should be noted that a potentiometer 74 is shown in the output circuit of the phase locked loop 40 to permit manual control of the amplitude of the locally generated jamming signal applied to the summer 38 such a control may be automated as was previously mentioned in addition a suitable conventional amplifier 76 is illustrated in the audio signal path in the event that amplification of the unscrambled audio signal aud is necessary or desirable before modulation fig5 illustrates an embodiment of the video channel of a decoder or unscrambler in which command signals are inserted in the television program signal to permit control over the programming available to individual subscribers for example category control or tiering signals identifying individual programs or groups of programs may be transmitted in the program signal and detected by a suitable code detector 80 as described in greater detail in u s pat no 4 225 884 the detected code may be compared by a conventional comparator 82 with a code stored in a category or tiering controller 84 that stored code may be for example a code indicative of a service for which the subscriber has paid or a code indicative of a category or programming the subscriber wished to view if the comparison is positive indicating that the subscriber is authorized to use the incoming service and or has selected it the comparator 82 causes a conventional multivibrator or other gating signal generator 84 to produce a gating signal which enables a gate 86 and passes the locally generated jamming signal to the summer 38 thus the video signal is unscrambled as.