Switchable-Code Digital PWM Modulator for Low Audio THDN
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Solution Overview
Problem
Class-D amplifiers experience total harmonic distortion and noise (THDN) degradation due to inter-symbol interference (ISI) and quantization noise, particularly when edge separation is small, and existing solutions like dynamic digital gain switching introduce pop-click noise or increase hardware costs.
Innovation Solution
A digital pulse-width modulation (PWM) modulator with a state machine-controlled multi-bit quantizer that dynamically switches between two modes of quantization codes, adjusting edge separation to reduce ISI and THDN, while maintaining low idle channel noise and avoiding additional hardware costs.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If dynamic digital gain switching is performed outside the feedback loop to address THDN degradation caused by ISI, then THDN is improved, but pop-click noise is introduced
Solution Approach 1:
The patent implements a feedback mechanism where the modulator output is fed back to the quantizer input. The quantizer operates within this feedback loop, and its code set can be dynamically switched based on feedback signals. This ensures that any switching actions are compensated by the feedback mechanism, preventing pop-click noise while maintaining THDN improvement.
Solution Approach 2:
The patent employs dynamic switching of the quantizer's code set based on signal conditions. The quantizer can switch between different code sets (e.g., first code set for normal operation, second code set for ISI mitigation) depending on the instantaneous signal level and characteristics. This dynamic adaptation allows the system to optimize THDN without introducing abrupt gain changes that cause pop-click noise.
2Manufacturing precision
If edge separation is increased to reduce ISI, then THDN is improved, but quantization noise increases
Solution Approach 1:
The patent changes the quantizer's operational parameters by switching between different code sets. The first code set is optimized for low quantization noise with smaller edge separations, while the second code set provides larger edge separations to reduce ISI. The system dynamically selects between these parameter sets based on signal conditions, thereby optimizing the trade-off between quantization noise and ISI-induced THDN.
Solution Approach 2:
The patent applies different code set characteristics to different signal conditions. Instead of using a single uniform code set, the system selects specific code sets with appropriate edge separation characteristics matching the current signal level and dynamics. This local optimization ensures that quantization noise is minimized when possible, while ISI is reduced when signal conditions require it.
3Manufacturing precision
If multiple code sets are implemented to address ISI and quantization noise, then THDN is improved, but device complexity increases
Solution Approach 1:
The patent implements a universal quantizer structure that can operate with multiple different code sets. Rather than implementing separate quantizers for different functions, a single quantizer is designed to support multiple code sets, making it multi-functional. This universal design reduces overall device complexity compared to having separate dedicated quantizers for different operating conditions.
Solution Approach 2:
The patent uses dynamic code set selection rather than maintaining multiple static quantizer configurations. A control mechanism dynamically switches between code sets based on signal conditions, eliminating the need for complex hardware to support multiple simultaneous quantizer paths. This dynamic approach simplifies the overall device structure while maintaining the ability to address both ISI and quantization noise.
Data Source
AI summary
A digital PWM modulator modulates a digital input signal to drive a PWM signal to a PWM DAC susceptible to introducing inter-symbol interference (ISI) in small PWM edge separation presence causing audio THDN degradation. A multi-bit quantizer switches from a first to second mode when the input signal rises above a threshold. The quantizer quantizes the input signal into a quantized output signal, each sample of which has a code selected from respective first and second quantization code sets. The second set, relative to the first set, causes the digital PWM signal to have increased edge separation to reduce the ISI at high input levels. The first set includes small magnitude codes relative to the second set to reduce quantization noise at low input levels. The threshold is sufficiently low to cause the quantized output signal to be dominated by small codes when operating in the first mode.