Telephonic communications – Substation or terminal circuitry – Hold circuit
Patent
1996-02-28
1997-02-04
Zele, Krista M.
Telephonic communications
Substation or terminal circuitry
Hold circuit
379377, 379379, 379380, 379387, 379162, 379163, H04M 322
Patent
active
056007159
ABSTRACT:
An integrated loop current detector for a public switched telephone network modem. The present invention solves the loop current detect problem in general, and is especially useful in modems employing echo cancellation techniques due the linearity afforded by the invention. In one advantageous embodiment of the present invention, the current detector includes a solid-state loop hold circuit coupled to terminals of the PSTN. A current mirror circuit is coupled in parallel with the loop holding circuit, wherein the current mirror circuit includes a transistor having a control terminal coupled to the DC operating point of the loop holding circuit. The transistor is operable to detect interruptions in loop current. An opto-coupler device having an input coupled to the switching device has an output coupled to a current detect input line of said modem thereby providing an isolated output for quick and reliable indication of current interruptions in said loop hold circuit.
REFERENCES:
patent: Re29499 (1977-12-01), Spencer
patent: 4454385 (1984-06-01), Grantland et al.
patent: 4524246 (1985-06-01), Meza
patent: 4841566 (1989-06-01), Lott
patent: 5315644 (1994-05-01), Lester et al.
Lucent Technologies - Inc.
Tebcherani Gloria
Zele Krista M.
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