Network Termination Device Hum Signal Compensation Circuit
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Solution Overview
Problem
Network termination devices in analog telephony systems experience audible hum due to mains hum interference from high-frequency switching power supplies, which compromises the quality of voice transmission.
Innovation Solution
A compensation circuit is implemented in the network termination device to detect and generate a signal opposite in phase with the mains hum signal, which is then combined with the analog telephony signal to suppress the hum, thereby improving voice transmission quality.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Volume of moving object
If a switched power supply is used to generate DC voltage, then the power supply can be realized with a compact design, but high frequency voltage spikes cause common-mode interference and mains hum in the POTS signal
Solution Approach 1:
The patent detects the mains hum interference caused by the switched power supply and generates a compensation signal that is opposite in phase to the hum signal. By adding this compensation signal to the POTS signal, the harmful hum is canceled out, converting the power supply's interference problem into a solvable signal processing task.
Solution Approach 2:
The patent introduces a Y-capacitor as an intermediary element between the primary and secondary sides of the transformer. This capacitor provides a low-impedance path for high-frequency common-mode currents, preventing them from coupling into the POTS signal and causing hum interference.
2Object-affected harmful factors
If a Y-capacitor is used to short-circuit high frequency signals, then common-mode interference is reduced, but the Y-capacitor must comply with safety class Y and size limitations of 2-3 nF
Solution Approach 1:
The patent implements a feedback mechanism where the POTS signal is monitored for mains hum interference, and a compensation signal is generated based on the detected hum level and phase. This closed-loop approach dynamically adjusts the compensation to maintain effective hum cancellation while operating within the constraints of the Y-capacitor.
3Ease of operation
If the DC voltage output is made accessible for user connection, then convenience is improved, but the Y-capacitor must comply with current safety limits when touched by a person
Solution Approach 1:
The Y-capacitor serves as an intermediary safety element between the DC voltage output and the user. By limiting the capacitance to 2-3 nF, it provides electrical isolation that allows the DC output to be accessible while preventing dangerous current levels from reaching the user in case of contact.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution effectively suppresses the mains hum interference, ensuring that the hum is not audible to the user, thereby enhancing the quality of voice transmission in analog telephony systems.
Implementation Method 1
Through a winding capacitance between the primary and secondary side of the transformer, these voltage spikes may couple to the secondary side and there cause common-mode interference
Implementation Method 2
The compensation circuit is configured to generate a compensation signal which is opposite in phase with the mains hum signal
Implementation Method 3
By means of this Y-capacitor, high frequency signal contributions between the primary and secondary side can be short-circuited
Data Source
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AI summary
A device comprises an interface for transferring an analog telephony signal. Further, the device comprises a compensation circuit which detects a hum signal in the analog telephony signal, generates a compensation signal opposite in phase with the hum signal, and combines the compensation signal with the analog telephony signal.