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Automatic disconnect circuit

  • US 4,156,799 A
  • Filed: 08/16/1978
  • Issued: 05/29/1979
  • Est. Priority Date: 08/16/1978
  • Status: Expired due to Term
First Claim
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1. In a telephone communications systems in which a telephone line couples a remote telephone to a local telephone, and wherein a substantial D.C. level shift appears on the telephone line when the local telephone goes on-hook, an automatic disconnect circuit for disconnecting the remote telephone in response to the remote telephone going on-hook, comprising:

  • signal discrimination means having an input coupled to the telephone line receiving the signals thereon, including voice signals and hang-up clicks generated by the remote and local telephones going on-hook, and adapted to substantially attenuate the voice signals and to pass the hang-up clicks to an output;

    comparator means coupled to the output of said signal discriminator means for generating a control signal in response to each signal which is received from said signal discrimination means and which has an amplitude in excess of a predetermined level;

    a disconnect signal generator responsive to control signals which have a duration greater than a first predetermined interval but less than a second predetermined interval for generating a disconnect signal, said first predetermined interval corresponding to the minimum duration of a control signal generated in response to the remote telephone going on-hook, and said second predetermined interval being shorter than the duration of a control signal generated in response to the D.C. level shift which occurs when the local telephone goes on-hook; and

    means receiving the disconnect signal for disconnecting the telephone line,whereby the attenuation of the voice signals inhibits the latter from generating a control or disconnect signal, signals having an amplitude less than the amplitude of a hang-up click fail to generate a disconnect signal, and said disconnect signal generator is insensitive to any signal on the telephone line generating a control signal whose duration is a typical of that generated by a remote hang-up click, thus preventing a control signal generated either by voice or a local hang-up from disconnecting the telephone line.

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