Quantization noise in digital speech

The amount of quantization noise generated in a digital- to-analog converter is dependent on the number of bits or quantization levels used to digitize the analog signal in the analog-to-digital converter. The minimum number of quantization levels and the minimum sample rate are derived for a digital voice channel. The minimum calculated parameters for a digital voice channel with 100 percent sentence intelligibility are 16 quantization levels and a sample rate of 5000 samples per second. Lowpass filters at the input of the analog-to-digital converter and the output of the digital-to- analog converter must have a 3 db cutoff frequency of 2000 Hz and a minimum rolloff of 36 db per octave. Laboratory results show that the calculated sample rate is slightly optimistic. A sample rate of 6000 samples per second and lowpass filters with a 3 db cutoff of 2400 Hz are required for 100 percent sentence intelligibility. Consonant sounds are the first speech components to be degraded by quantization noise. A compression amplifier can be used to increase the weighting of the consonant sound amplitudes in the analog-to-digital converter. An expansion network must be installed at the output of the digital-to- analog converter to restore the original weighting of the consonant sounds. This technique results in 100 percent sentence intelligibility for a sample rate of 5000 samples per second, eight quantization levels, and lowpass filters with a 3 db cutoff of 2000 Hz.