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Abstract

A system and method for adjusting the phase of the analog signal produced by an analog modem connected to a digital modem over a telephone network. The digital portion of the telephone network is locked to the network clock, and the modems have no control over the sampling timing or rate as the analog signal is sampled and quantized by a codec. If the analog signal is shifted in phase relative to the network clock, then the codec may be sampling at unresolvable points on the analog signal, thereby causing errors and a decrease in the usable bandwidth for transmitting data. The analog modem symbol frequency is locked to the clock of the digital network using loop-back timing. A phase estimate is computed using the quantized samples of a known reference signal. Next, a 'phase offset' is calculated by comparing the phase estimate to an optimum phase value. Then the digital modem sends the calculated phase offset information to the analog modem. The analog modem then delays its transmitted signal by the phase offset. After the phase of the analog modem's transmitter is adjusted, the analog signal reaches the codec at the phase desired by the digital modem.

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First Claim