Digital Vocoder Trill Sound Modulation Enhancement
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Solution Overview
Problem
Digital land mobile radios, particularly those using narrowband vocoders, distort speech sounds with high modulation rates, such as the alveolar trill, leading to intelligibility issues in languages like Spanish and Italian, due to low frame energy analysis rates and aliasing artifacts.
Innovation Solution
The implementation of pre- and post-processing techniques, including frame shifting, energy parameter modification, time expansion/compression, and modulation enhancement filtering, to enhance the modulation index of trill sounds without modifying the vocoder, by detecting and aligning with modulation nulls and adjusting the vocoder parameters.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If narrowband vocoders are used for digital radio transmission, then bandwidth efficiency is improved, but speech fidelity for high modulation rate sounds deteriorates
Solution Approach 1:
The system performs preliminary detection of trill sounds before vocoding and applies pre-processing techniques (frame shifting, time expansion) to prepare the signal for better vocoder handling, preventing distortion before it occurs
Solution Approach 2:
The system dynamically changes vocoder parameters (frame shift amount, time expansion factor) based on detected speech characteristics, particularly for trill sounds, to optimize the balance between bandwidth efficiency and speech fidelity
2Manufacturing precision
If vocoder frame rate is increased to improve trill sound encoding, then speech fidelity is improved, but data rate increases
Solution Approach 1:
The system performs preliminary time expansion on trill sounds before vocoding, which effectively lowers the modulation rate and allows accurate encoding at the standard vocoder frame rate without increasing data rate
Solution Approach 2:
The system changes the time expansion parameter dynamically based on detected trill characteristics, adjusting the effective sampling rate for trill sounds while maintaining the standard vocoder operating parameters for other speech
3Manufacturing precision
If frame shifting is applied to align with modulation nulls, then trill sound intelligibility is improved, but processing complexity increases
Solution Approach 1:
The system applies frame shifting selectively only to detected trill sounds rather than all speech, and uses localized processing around modulation nulls, reducing overall processing complexity while maintaining intelligibility improvement where needed
Data Source
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AI summary
A method and apparatus for enhancing modulation of certain speech sounds, such as trill sounds, are provided for radios which utilize digital vocoders. A digitized speech stream is sampled and the sampling is adjusted to determine, detect and enhance trill nulls in the digitized voice stream by one or more of: frame shifting the digitized speech input stream prior to vocoding, time expanding a digitized speech steam prior to vocoding, time compressing a digitized speech output stream after vocoding, and/or modulation enhancement and filtering of the a digitized speech output stream after vocoding.