MELP Vocoder Transcoding for HF Radio Bit-Rate Reduction
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Solution Overview
Problem
Current 2.4 Kbps vocoders using the LPC10e standard have limited success in providing reliable secure communications over narrow-band HF channels, requiring high signal-to-noise ratios and suffering from degraded speech quality and intelligibility due to background noise and low-end frequency response issues, especially in tactical ManPack Radios.
Innovation Solution
A vocoder and method that transcodes Mixed Excitation Linear Prediction (MELP) encoded data to operate at different speech frame rates, reducing or increasing bit-rate by a factor of four, utilizing time interpolation and quantization to maintain or improve speech quality and intelligibility, especially in harsh conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If 2.4 Kbps LPC10e vocoder is used, then communication can be established on narrow-band HF channels, but speech quality and intelligibility are degraded due to background noise and low-end frequency response issues
Solution Approach 1:
The patent changes the vocoder parameters by switching from LPC10e to MELP algorithm and adjusting the operating bit-rate from 2.4Kbps to 600 bps. This parameter change enables the system to maintain communication reliability on narrow-band HF channels while improving speech quality and intelligibility by reducing sensitivity to background noise and low-end frequency roll-off through the MELP model's integrated noise pre-processor
Solution Approach 2:
The patent employs a lower bit-rate (600 bps) that can be considered a 'simpler' or 'cheaper' version of the standard 2.4Kbps LPC10e system. This lower-rate system achieves comparable or better performance in harsh conditions by using more efficient coding techniques, effectively replacing the older standard with a more resilient alternative
2Productivity
If 2.4 Kbps LPC10e vocoder is used, then communication can operate at standard bit rate, but high signal-to-noise ratio is required which limits availability in low-power conditions
Solution Approach 1:
The patent changes the operating parameters by reducing the bit-rate from 2.4Kbps to 600 bps and switching to MELP algorithm. This parameter change allows the system to achieve comparable or better communication availability in low-power conditions by reducing the signal-to-noise ratio requirement, enabling tactical radios to maintain connectivity with lower transmit power over HF channels
3Duration of action of stationary object
If bit-rate is reduced to 600 bps, then secure voice availability increases and connectivity duration extends, but transcoding between different frame rates becomes complex
Solution Approach 1:
The patent performs preliminary actions by pre-calculating and storing conversion tables and parameters for transcoding between different MELP frame rates (600 bps and 2400 bps). This preliminary preparation simplifies the actual transcoding process during operation, allowing the system to efficiently convert between frame rates without complex real-time calculations, thus enabling extended connectivity at low bit-rates while managing system complexity
Data Source
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AI summary
A vocoder and method transcodes Mixed Excitation Linear Prediction (MELP) encoded data for use at different speech frame rates. Input data is converte (100) into MELP parameters such as used by a first MELP vocoder. These parameters are buffered (102) and a time interpolation (104) is performed on the parameters with quantization to predict spaced points. An encoding function (106) is performed on the interpolated data as a block to produce a reduction in bit-rate as used by a second MELP vocoder at a different speech frame rate than the first MELP vocoder.