In this thesis the performances of adaptive differential pulse code modulation (ADPCM) and adaptive delta modulation (ADM) systems are compared in dynamic range, noise tolerance and peak signal-to-noise ratio (SNR) at different bit rates. The ADPCM system selected for the study has an adaptive quantizer with a one word memory. For ADM first order constant factor delta modulation (CFDM) has been selected. Also, continuously variable slope delta modulation(CVSD) has been chosen as a reference. Extensive computer simulations have been done using real speech for the comparative study.
According to the simulation results, dynamic ranges of ADPCM and CFDM are about the same (55-58 dB) for a 60 dB compander, but that of CVSD is much narrower (20-25 dB) than the other two. Those three coders studied have significant SNR gains over non-adaptive coders. The performance of ADPCM system is always better than that of CFDM system regardless of bit rate, but in low bit rate (16 Kbits/s) CVSD system in a little (about 1 dB) better than ADPCM system.
In a noisy channel, however, CVSD outperforms the other two coding methods. Comparing CFDM and ADPCM, the performance of the former is slightly better than that of the latter in a noisy channel.
In addition, an analytical study has been done with R-C filtered band limited Gaussian signal as an input. In this analysis overload average step size and granular average step size have been used to modify the firmulas for granular and slope overload noise obtained by Van de Weg and Protonotarios, respectively. The theoretical and computer simulation results agree closely.
본 논문에서는 실제의 음성을 가지고 ADM system중에서는 CVSD 와 CFDM 그리고 ADPCM system 에서는 Jayant 의 한개의 기억소자를 갖는 양자기를 택해서 computer simulation 에 의하여 bit rate 에따른 peak SNR 과 dynamic range 그리고 잡음이 있는 channel 에 대하여 비교 연구 하였다.
ADPCM 은 bit rate 에따라 직선적으로 SNR 이 증가하여 high bit rate 에서는 가장 좋았다. Low bit rate (16 Kbits/s) 에서는 CVSD 가 가장 좋은 것으로 나타났다.
Dynamic range 에 있어서는 CFDM 과 ADPCM 은 주어진 stepsize 의 dynamic range 와 거의 같았으나 CVSD 는 dynamic range 의 개선이 없었다. Error rate 가 증가함에 따라 CFDM 과 ADPCM 둘다 성능이 급속히 떨어지나 CVSD 는 완만하게 떨어졌다. 양자화 잡음에 대한 해석적 연구는 평균 stepsize 의 식들을 추정하여 Gaussian 신호에 의한 simulation 결과와 비교하였다. 또한 overload 와 granular 각각의 평균 stepsize 의 식을 추정하여 지금까지 얻어진 LDM 식을 변형하여 대입하였다. 좀 더 많은 연구가 필요할 것 같다.
