PCM voice encoding and decoding systems research
|Keywords||Switched-Capacitor Filter Voltage reference Voice codec Anti-aliasing filter Smoothing filter|
This thesis mainly focuses on a PCM Codec-System. A PCM Codec-System is used for digitizing and reconstructing the human voice. When the voice is digitized, it can be transmitted long distance without degradation. This system has a wide field of application, including digital microwave communication, satellite communication and so on.To digitize intelligible voice requires a signal-to-distortion ratio of about 26dB over a dynamic range of about 40dB. This may be accomplished with a linear 13-bit ADC and DAC, but will far exceed the required signal-to-distortion ratio at larger amplitudes. The excess performance is at the expense of data per sample. Data reduction is implemented by compressing the 13-bit linear scheme to pseudo-logarithmic 8-bit schemes. The companding scheme follows a "piecewise-linear" curve similar toμLaw function and is formatted as sign bit, three chord bits, four step bits.On the system level, a 8kHz sampling rate is adopted in the transmission section. This sampling process requires a low-pass filter to limit the high frequency energy above 3.4 kHz from distorting the in-band signal. An anti-aliasing filter is added to prevent the spectrum aliasing introduced by the low-pass filter. The telephone line is also subject to 50Hz power line coupling, which must be attenuated from the signal by a high-pass filter before the ADC. This high-pass filter is designed to have 14 dB degradation at 50Hz. The receive section includes DAC, a low-pass filter with compensation and a smoothing filter. The digital-to-analog conversion process reconstructs a staircase version of the desired signal. A smoothing filter is needed to get the desired signal.On the circuit-design level, this thesis put emphasis on the following parts: sc-filter designed as the high-pass filter; RC active filter designed as the anti-aliasing filter and smoothing filter; the analog circuitry involved in DAC (1.575v voltage reference, RDAC, CDAC); the changeable-gain amplifier and so on.In the last part, simulation is made with the help of Cadence spectre simulator. From the simulation results, it is known that the desired performance of the design is achieved.