DC level control for an electronic telephone line card
First Claim
1. A DC control circuit for cancelling DC voltage shifts occurring on the input signal of a telephone line card, the telephone line card including an AC input amplifier and a transmit path for detecting and converting the input signal to digital values, said DC control circuit comprising:
- a filter for receiving the transmit path digital values and for filtering and detecting DC voltage shifts of the digital values to provide a DC shift signal indicative thereof;
an adjust circuit coupled to said low pass filter and receiving said DC shift signal for providing a quantized signal;
an accumulator coupled to said adjust circuit and receiving said quantized signal for developing a DC adjust voltage; and
an adder coupled to said accumulator and for coupling to the AC input amplifier for subtracting said DC adjust voltage from the input signal.
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0 Petitions
Accused Products
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.
75 Citations
24 Claims
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1. A DC control circuit for cancelling DC voltage shifts occurring on the input signal of a telephone line card, the telephone line card including an AC input amplifier and a transmit path for detecting and converting the input signal to digital values, said DC control circuit comprising:
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a filter for receiving the transmit path digital values and for filtering and detecting DC voltage shifts of the digital values to provide a DC shift signal indicative thereof; an adjust circuit coupled to said low pass filter and receiving said DC shift signal for providing a quantized signal; an accumulator coupled to said adjust circuit and receiving said quantized signal for developing a DC adjust voltage; and an adder coupled to said accumulator and for coupling to the AC input amplifier for subtracting said DC adjust voltage from the input signal. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12)
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13. A telephone line card for receiving an audio signal from a telephone, comprising:
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an input detection amplifier for separating the metallic component of the audio signal; an analog to digital converter for receiving and converting said analog metallic signal to a digital signal; a decimator for converting said digital signal to a decimated digital signal; a transmit path amplifier receiving and amplifying said decimated digital signal at a programmed gain; a low pass filter receiving said decimated digital signal for filtering and detecting DC voltage shifts for providing DC shift signals indicative thereof; an adjust circuit receiving said DC shift signal for providing a quantized signal; an accumulator receiving said quantized signal for developing a DC adjust voltage; and an adder coupled to said input detection amplifier, receiving said DC adjust voltage and for receiving the audio signal for subtracting said DC adjust voltage from the audio signal. - View Dependent Claims (14, 15, 16, 17, 18, 19, 20, 21)
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22. A telephone system, comprising:
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a telephone including a DTMF generator, wherein said telephone asserts audio signals and dial tones from said DTMF generator onto a two-wire interface; a telephone line card coupled to said two-wire interface for receiving said audio signals, said telephone line card comprising; an input detection amplifier for separating the metallic component of the audio signal, and for providing an analog metallic signal, an analog to digital converter for receiving and converting said analog metallic signal to a digital signal; a decimator for decimating said digital signal for providing a decimated digital signal; a transmit path amplifier receiving and amplifying said decimated digital signal at a programmed gain; a filter receiving said decimated digital signal for filtering and detecting DC voltage shifts for providing DC shift signals indicative thereof; an adjust circuit receiving said DC shift signal for providing a quantized signal; an accumulator receiving said quantized signal for developing a DC adjust voltage; and an adder coupled to said input detection amplifier, receiving said DC adjust voltage and for receiving the audio signal for subtracting said DC adjust voltage from the audio signal; and a compression unit coupled to said transmit path amplifier for converting amplified decimated digital signals to PCM codes; and a PCM highway coupled to said telephone line card for transmitting and receiving PCM codes. - View Dependent Claims (23, 24)
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Specification