DC level control for an electronic telephone line card
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United States of America Patent
Stats
-
Dec 10, 1996
Grant Date -
N/A
app pub date -
Mar 3, 1995
filing date -
Mar 3, 1995
priority date (Note) -
In Force
status (Latency Note)
Importance
Abstract
DC level control for an electronic telephone line card which filters the DC component from the input audio signal, determines the gain setting of the transmit path, develops a DC adjust voltage opposite to that of the DC shift component and subtracts the DC adjust voltage to the input voltage to cancel the DC shift component. A quantized, discontinuous feedback path is implemented to respond only to DC shifts so the AC operation remains substantially unaffected. Such discontinuous feedback eliminates stability and impedance matching problems introduced with continuous feedback solutions. A DC control circuit according to the present invention includes a low pass filter for detecting DC shifts, a quantizer for asserting an incremental signal, a threshold detector for activating adjustment functions, and an accumulator for developing the DC adjust voltage by incremental steps. A long term low pass filter and reset circuit resets the accumulator to cancel residual DC adjust voltages which might otherwise reduce the dynamic range of the telephone line transmit path.
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