PWM Frame Rate Control to Prevent AM Interference in Audio Receivers
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Solution Overview
Problem
Conventional digital audio systems using class D output amplifiers face significant electromagnetic interference (EMI) due to clock rates involved in pulse-width-modulation (PWM) control signals, which degrade signal-to-noise ratio and reduce sensitivity, particularly when selecting PWM frame rates that interfere with amplitude-modulated (AM) radio frequencies.
Innovation Solution
A digital audio receiver is designed to adjust the PWM sampling frequency in response to changes in the AM tuning frequency using a sample rate converter (SRC) that upconverts the signal sampling frequency without additional filtering, minimizing audible noise and avoiding interference by precalculating PWM frame rates to avoid AM carrier and intermediate frequencies.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If PWM frame rate is increased to improve audio signal processing speed and reduce quantization noise, then audio quality is improved, but electromagnetic interference with AM radio frequencies increases
Solution Approach 1:
The patent dynamically adjusts the PWM frame rate based on the tuned AM frequency. When an AM station is tuned, the system modifies the PWM frame rate to avoid generating electromagnetic interference at frequencies that would interfere with the AM carrier and its harmonics, thus resolving the contradiction between high audio processing quality and AM radio interference
Solution Approach 2:
The system transitions from a fixed PWM frame rate to a dynamic frame rate that adapts in real-time based on the AM tuning frequency. This dynamic adjustment allows the audio processing to maintain high performance while adapting to external electromagnetic environment conditions to minimize interference
2Object-affected harmful factors
If PWM frame rate is adjusted to avoid AM interference frequencies, then AM radio reception quality is improved, but audio processing speed and signal fidelity may be reduced
Solution Approach 1:
The system changes the PWM frame rate parameter dynamically based on the AM frequency being received. By selecting appropriate frame rates that avoid interference with the AM carrier and its harmonics, the system maintains both clean AM reception and adequate audio processing performance
Solution Approach 2:
The system uses feedback from the AM tuning frequency detection to automatically adjust the PWM frame rate. This closed-loop control ensures that the PWM operations do not generate electromagnetic interference at critical frequencies while maintaining optimal audio processing speed
3Manufacturing precision
If additional filtering is added to remove PWM-related artifacts from audio signal, then audio quality is improved, but device complexity and manufacturing cost increase
Solution Approach 1:
Instead of adding complex filtering circuitry to remove PWM artifacts, the patent extracts the source of the problem by adjusting the PWM frame rate itself. By changing the operating parameters of the PWM generator to avoid generating interfering frequencies, the need for additional filtering hardware is eliminated
Solution Approach 2:
The patent replaces the mechanical/electrical solution of adding physical filtering components with a digital/control solution of adjusting the PWM frame rate parameter. This substitution reduces hardware complexity while achieving the same goal of improving audio signal quality
Data Source
AI summary
A digital audio system including a digital audio amplifier with reduced AM interference. The digital audio amplifier includes a pulse-width-modulation (PWM) processor in which a digital datastream is upsampled by an interpolation filter. A sample rate converter resamples the upsampled datastream to produce a datastream at a converted sampling frequency, or PWM frame rate. The converted datastream is then applied to pulse-width-modulation circuitry which generates a PWM signal at the PWM frame rate. The sample rate converter resamples according to a sample rate conversion ratio associated with the AM tuned frequency. For example, the sample rate conversion ratio can be selected so that the PWM frame rate and its lower harmonics avoid an AM tuned frequency, any intermediate frequency in the AM tuner, and also an image frequency in the tuner.