Single-Bit Audio Mapping for Constant Edge Rate Playback
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Solution Overview
Problem
Single-bit audio streams with data-dependent edge rates produce undesirable playback artifacts due to inter-symbol interference in current-steering DACs, leading to poor THD+N and limited control over audio volume, which affects the quality of reproduced audio in consumer devices.
Innovation Solution
Direct mapping of a single-bit audio stream to a modified stream with a constant edge rate using a pre-filter bank and multi-bit symbol mapper, combined with two-stage volume control in digital and analog circuitry, to maintain modulation index and achieve precise volume adjustments.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If a return-to-zero (RTZ) format with gap insertion is used to obtain constant edge rate, then the edge rate becomes constant, but the analog output is attenuated and noise is introduced
Solution Approach 1:
The patent introduces a levelizer as an intermediary component between the PDM input and the constant edge rate output. The levelizer accumulates input samples and generates output symbols based on accumulated level comparisons, thereby mediating the transition from variable edge rate to constant edge rate without the harmful gap insertion effects of RTZ formats
Solution Approach 2:
The levelizer performs preliminary accumulation and level comparison before generating the constant edge rate output. By pre-processing the input samples through accumulation and level threshold comparison, the system establishes the constant edge rate property in advance, avoiding the need for post-processing gap insertion that causes noise and attenuation
2Ease of operation
If conventional modulators are used for volume control on digital signals, then volume control is achieved, but the quality of reproduced audio is affected
Solution Approach 1:
The patent implements volume control by changing the threshold parameter of the levelizer rather than using conventional modulators. By adjusting the threshold level at which output symbols are generated, the system achieves volume control while maintaining audio quality, as the constant edge rate property is preserved throughout the process
3Device complexity
If single-bit audio streams with data-dependent edge rate are used, then the system is simpler, but inter-symbol interference occurs resulting in poor THD+N
Solution Approach 1:
The patent segments the single-bit audio stream processing into distinct functional stages: input sampling, accumulation in the levelizer, level comparison, and constant edge rate output generation. This segmentation allows the system to maintain relative simplicity while achieving constant edge rate property that eliminates inter-symbol interference and improves THD+N performance
Data Source
AI summary
A single-bit audio stream can be converted to a modified single-bit audio stream with a constant edge rate while maintaining a modulation index of the original audio stream using direct mapping. With direct mapping, a pre-filter bank may be combined with a multi-bit symbol mapper to select symbols for the modified audio stream with a constant edge rate per symbol and the same modulation index as the original audio stream. The output of the pre-filter bank may be an audio stream with no consecutive full-scale symbols. Using the output of the pre-filter bank, a multi-bit symbol mapper may use the symbol selector to output a symbol with a constant edge rate per symbol and the same modulation index as the original signal. The symbols may be converted to an analog signal for reproduction of audio content using a transducer.