Arrangement for deriving a local clock in a packet cable telephony modem
AssigneeGeneral Instrument Corporation
InventorsDavid Beryl Lazarus, Yucheng Jin
Life of this patent
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Abstract
A broadband terminal interface unit for use in an HFC communication network includes a phase-locked loop for providing a smooth transition between a CMTS-provided “national” clock and a local clock when the CMTS clock is lost (or before it is required). By controlling the tracking range of the PLL, the sensitivity of the circuit is controlled so that some drift in the national clock can be accommodated without unnecessarily reducing the performance of the components within the broadband terminal interface unit.
Claims7
A broadband terminal interface unit for use in a bi-directional HFC communication network, said broadband terminal interface unit for supporting the transmission of packetized audio signals between an end-user and a common network element in said HFC communication network and comprising
- a codec element responsive to audio signals originating at an end-user and generating as an output a coded representation thereof at a predetermined clocking frequency and also responsive to coded signals received from said HFC communication network and generating as an output a decoded audio signal to said end-user;
- a digital signal processor, responsive to the coded output from said codec element for assembling said coded output into packets, and responsive to the coded input from said HFC communication network for dis-assembling said coded input at the predetermined clocking frequency;
- and
- a phase-locked loop responsive to both a national clock signal generated in said HFC network and a local clock source for generating the predetermined clocking frequency and providing said predetermined clocking frequency as an input to said codec and digital signal processor, said phase-locked loop locking onto the national clock signal as long as said national clock signal is present and within a predetermined tracking range of ±n%.
Dependencies are inferred from the claim text.