| 摘要 |
A line card integrates subscriber line interface circuitry, A/D and D/A converters, and digital signal processing technology. The digital signal processing technology performs many line card tasks such as switch hook detection, ground key detection, DC feed control, polarity reversal, ringing tests, fault detection, power cross detection, and ring trip detection. Silent polarity reversal is achieved by modulating a digital DC feed current waveform in accordance with a programmable predetermined waveform to smoothly transition the DC voltage of subscriber loop conductors A and B between approximately ground and central office battery voltage. The modulated waveform includes little if any energy in audio frequencies. Additionally, the functionality of the line cards 308[m:1] may be implemented partially or completely in hardware of software.
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