Patent GB-1360038-A (GB)
Title: Apparatus for blocking transmission of dialing pulses from subscriber telephone sets
IPC: H04M1/677
Filed: 1971-09-17
Abstract: 1360038 Automatic exchange systems SOC ITALIANA TELECOMUNICAZIONI SIEMENS SpA 17 Sept 1971 [6 Oct 1970] 43487/71 Heading H4K A substation includes an impulse counting facility which, in the event of a prohibited first digit being dialled or of dialling being attempted before the arrival of dial tone or in the face of engaged tone, causes the transmission of subsequent digits to be inhibited. The inhibition may take the form of an impedance connected across the loop so as to cause the impulses to have too small an amplitude to be registered at the exchange. The handset microphone is shortcircuited pending the arrival of dial tone so as to prevent the tone receiver operating incorrectly to speech picked up by the microphone. A pulse shaper RI which includes an F.E.T., tr5, is connected across the line wires a, b so as to respond to impulses generated by the subset AT before they are applied to a pulse counter C/O. In addition the enveloper of each pulse train is determined by integrator a8, c9, r12, 15 provided that NOR gate N1 has been opened by the setting of bi-stable BS3. This normally occurs in response to the initial closing of the loop when the handset is lifted, BS3 being insensitive to dialling impulses by virtue of the charge/discharge time of capacitor C10. After integration the waveform is differentiated C13, r14, 17 so as to produce a pulse at the trailing edge of the waveform. This corresponding to an end-of-digit signal. The pulse is used to set bistable BS5 in the event that the counter has determined that the digit is inadmissible (C/O's output is applicable to BS5 via OR gate O 2 ). A further differentiator c14, r17, I8 in effect provides a delay for the end-of-digit pulse so that it may be used to reset the counter after its contents have been utilized. The set bi-stable BS5 turns-on busy tone generator Gt which injects a busy tone into the line and additionally it shunts the line with a high impedance by turning-on transistor tr1. BS5 may be set in an alternative manner in the event that dial tone does not precede dialling. For this purpose tone detector A which is A.C. coupled to the line provides an integrated pulse output from bistable BS1 upon receipt of dial tone. This output is effective via bi-stable BS4 to block NAND gate N4 thereby preventing dialling pulses from 13 and N1 affecting bi-stable BS5 and additionally it resets bi-stable BS2 so as to remove a short circuit across the handset microphone connected to terminals M and N in the subset AT. The short circuit is applied as soon as the handset is lifted (NAND gate N2 activated by the output from I3 and set BS3) so as to prevent the subscriber'sspeech from being accidentally interpreted as dial tone by detector A. At the end of the call. The replacement of the handset is detected via the pulse shaper so as to reset all the operative bi-stables. Power for the calls-barring arrangement is derived from line when the handset is on-hook via the charge on capacitor C6 as determined by Zener diode 2 and when the handset is off-hook again via capacitor C6 but this time as determined by the potential across a capacitor C15 which is detected by a direct current converter CONV. so as to produce a charging potential for C6. Switch In serves to disable the arrangement when actuated to the open state by an authorized subscriber.