De-emphasis Filtering for FM Transmitter Over-modulation Control

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Solution Overview

Problem

FM transmitters often experience over-modulation, leading to interference with adjacent FM bands due to excessive bandwidth occupation, which existing technologies fail to effectively control.

Innovation Solution

The method involves measuring the deviation of audio signal amplitude from a reference level, filtering the audio signal with frequency-dependent gain or attenuation factors, and using the filtered digital values to frequency modulate a carrier signal, thereby controlling over-modulation by adjusting the amplitude levels of frequency components within FM transmitters.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If the amplitude of the audio signal is increased to improve signal quality, then the signal quality is improved, but the bandwidth occupied by the FM signal exceeds the maximum allowable bandwidth causing over-modulation

Engineering Contradiction:

Improvesignal qualityVSAvoidover-modulation

Core Design Contradiction:

ReliabilityVSObject-affected harmful factors

Solution Approach 1:

The patent applies de-emphasis filtering to change the amplitude parameters of different frequency components of the audio signal. By applying frequency-dependent attenuation through digital filtering, the signal amplitude is reduced in a controlled manner across different frequency bands, preventing over-modulation while maintaining acceptable signal quality.

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The patent applies different filtering magnitudes to different frequency components of the audio signal. The de-emphasis filter applies varying attenuation levels across the frequency spectrum, with higher attenuation applied to higher frequency components, thereby locally adjusting the signal characteristics to prevent over-modulation in specific frequency ranges.

Inventive Principle:

Principle #3Local quality

2Object-affected harmful factors

If de-emphasis filtering is applied to control over-modulation, then over-modulation is controlled, but the audio signal quality may be degraded

Engineering Contradiction:

Improveover-modulation controlVSAvoidaudio signal quality

Core Design Contradiction:

Object-affected harmful factorsVSReliability

Solution Approach 1:

The patent implements adaptive de-emphasis filtering where the filtering characteristics are dynamically adjusted based on the input signal conditions. The system monitors the audio signal and adjusts the de-emphasis filter parameters in real-time, applying stronger filtering only when necessary to prevent over-modulation, thereby maintaining audio quality under normal conditions while preventing harmful over-modulation effects.

Inventive Principle:

Principle #15Dynamics

Data Source

PatentUS9350393B2De-emphasis filtering audio signals in response to composite control signal

Publication Date: 2016.05.24 TEXAS INSTRUMENTS INC

AI summary

Digital values representing an audio signal are formed in a FM transmitter. The deviation of the amplitude of the audio signal as represented by the digital values, from a reference amplitude level, is measured. The audio signal is filtered based on the deviation measured by processing the digital values, with the filtering designed to filter different frequency components of the audio signal with different respective magnitudes. Another set of digital values representing the filtered audio signal is generated, and used to frequency modulate a carrier signal to generate a FM signal. Over-modulation of the FM signal is thereby controlled.