ADPCM distortion of digital data transmitted at 9.6 kbit/s
This work is concerned with the effects of Adaptive Differential Pulse Code Modulation (ADPCM) on digital data transmitted at 9.6 kbit/s. To facilitate the study, a system model was developed. The model simulates the complete communication system, including true bandpass simulation of the modulation and demodulation processes. The model was used to observe the behaviour of the ADPCM link and the distortion it produces when digital data are transmitted at 9.6 kbit/s. The observations were employed to devise two ADPCM distortion cancellation schemes. Scheme-1 employs a look-up-table. The cancellation is here determined by the last two detected symbols and the sampling phase. Scheme-2 operates without a memory. The cancellation is achieved by signal level compensation. The performances of these schemes, operating with different detectors, are compared with the performance of the system when a non-linear equaliser is applied to the digital data in the absence of ADPCM distortion. Tests were conducted using two channels: CH-1 which introduces mild distortion into the transmitted digital signal, and CH-2 which introduces significant distortion. The detectors used in the tests are a linear equaliser, a non-linear equaliser and a near-maximum-likelihood detector. Scheme-2, which is the simpler of the two schemes, when used with the near-maximum-likelihood detector has a significantly better performance than the other systems tested.