Direct Mapping of Single-Bit Audio for Constant Edge Rate
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Solution Overview
Problem
Single-bit audio streams with data-dependent edge rates cause undesirable playback artifacts, such as inter-symbol interference and noise, in current-steering digital analog converters, and lack effective volume control, affecting audio quality in consumer devices like mobile phones.
Innovation Solution
The conversion of a single-bit audio stream to a modified stream with a constant edge rate using direct mapping, which combines a pre-filter bank and a multi-bit symbol mapper to select symbols, maintaining the modulation index and allowing for volume control through digital and analog stages, thereby reducing noise and improving audio quality.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If a return-to-zero (RTZ) format is used to obtain a constant edge rate audio stream, then the edge rate becomes constant, but the analog output is attenuated and noise is introduced
Solution Approach 1:
The patent introduces a level adjuster as an intermediary component between the RTZ converter and the DAC. This level adjuster compensates for the signal attenuation caused by the RTZ format by adjusting the signal level before it reaches the DAC, thereby maintaining constant edge rate while preventing noise introduction and signal loss
Solution Approach 2:
The patent changes the modulation index parameter from its original value to a reduced value that is compatible with the RTZ format. This parameter adjustment allows the system to maintain constant edge rate while avoiding the harmful effects of signal attenuation and noise, as the modified modulation index works optimally with the RTZ conversion process
2Manufacturing precision
If gaps are inserted between symbols in RTZ block to obtain transitions, then constant edge rate is achieved, but the analog output is attenuated and modulation index changes
Solution Approach 1:
The patent deliberately changes the modulation index parameter to a reduced value that is specifically designed to work with the RTZ format and symbol gap insertion. This parameter modification ensures that while gaps are inserted for constant edge rate, the modulation index remains stable and appropriate for the modified signal structure
Solution Approach 2:
The level adjuster acts as an intermediary that compensates for the effects of gap insertion. It maintains the appropriate signal level after gaps are introduced, ensuring that the modulation index remains stable and the analog output is not attenuated despite the structural changes to the audio stream
3Ease 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 replaces the conventional digital modulator-based volume control system with an analog volume control system that operates on the analog output signal. This substitution eliminates the audio quality degradation caused by digital modulation while maintaining full volume control capability, as the analog control path does not introduce the same artifacts and distortions
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.