The Journal of Korean Institute of Communications and Information Sciences (한국통신학회논문지)

The Korean Institute of Commucations and Information Sciences (한국통신학회)
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An Enhanced MELP Vocoder in Noise Environments

MELP 보코더의 잡음성능 개선

  • 전용억 (대덕대학 정보통신계열) ;
  • 전병민 (충북대학교 컴퓨터공학과)
  • Published : 2003.01.01
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Abstract

For improving the performance of noise suppression in tactical communication environments, an enhanced MELP vocoder is suggested, in which an acoustic noise suppressor is integrated into the front end of the MELP algorithm, and an FEC code into the channel side of the MELP algorithm. The acoustic noise suppressor is the modified IS-127 EVRC noise suppressor which is adapted for the MELP vocoder. As for FEC, the turbo code, which consists of rate-113 encoding and BCJR-MAP decoding algorithm, is utilized. In acoustic noise environments, the lower the SNR becomes, the more the effects of noise suppression is increased. Moreover, The suggested system has greater noise suppression effects in stationary noise than in non-stationary noise, and shows its superiority by 0.24 in MOS test to the original MELP vocoder. When the interleave size is one MELP frame, BER 10-6 is accomplished at channel bit SNR 4.2 ㏈. The iteration of decoding at 3 times is suboptimal in its complexity vs. performance. Synthetic quality is realized as more than MOS 2.5 at channel bit SNR 2 ㏈ in subjective voice quality test, when the interleave size is one MELP frame and the iteration of decoding is more than 3 times.

전술통신 환경에서 MELP 보코더의 잡음에 대한 성능을 개선시키기 위해 전단에는 음향잡음억제기를, 그리고 채널과 접면에는 FEC 코드를 설치한 개선된 MELP 보코더를 제안한다. 여기서 음향잡음억제기는 IS-127 EVRC의 잡음억제 알고리즘을 MELP 보코더에 정합시켜 구현한 것이고, 채널오류 정정기는 recursive convolution 방식의 rate 1/3 인코딩 및 BCJR-MAP 디코딩 알고리즘을 근간으로 구현된 turbo 코드를 사용한다. 성능실험 결과 제안 시스템은 음향잡음 환경에서 SNR이 낮아질수록 잡음 억제 효과가 증가된다. 또한 비정체성 잡음 보다 정체성 잡음에서 더 나은 성능을 나타내며, 원래 MELP 보코더 보다 MOS가 0.24 정도 개선된다. 채널잡음 환경에서는 채널오류 정정기의 인터리버가 MELP 프레임 한 개 크기일 때 채널 비트 SNR 4.2 ㏈에서 BER 10-6이 달성된다. 디코딩 반복횟수는 3회 정도가 복잡도 대비 성능 측면에서 최적으로 판단된다. 또한 인터리버가 MELP 프레임 한 개 크기이고 디코딩 반복횟수가 3회 이상인 경우 채널 비트 SNR 2 ㏈에서 MOS 2.5 이상의 합성음질이 실현된다.

Keywords

References

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