Digital Speech Processing

CVSD at 16kbps

The Continuously Variable Slope Delta-modulation (CVSD) algorithm attempts to reconstruct the exact waveform at the receiver that was input to the transmitter; thus it is classified as a waveform coder. The digital output from the transmitter is 1 bit per input sample upsampled to 16Kbps. The transmitted bit stream is used to indicate the slope of the input waveform. The slope-limiting detection logic looks at the 3 most recent bits transmitted. If these bits are all 1's or all 0's, the step size is doubled. For all other combinations, the step size is cut in half. The ratio between maximum and minimum step size is 16. The sign of the slope is positive if the current bit is a 1 and negative if the current bit is 0.

Performance Measures - An evaluation of the intelligibility of various implementations of the CVSD algorithm operating at 16kbps was performed by ARCON in March of 2001: CVSD Intelligibility.
In addition, results of the 16kbps CVSD algorithm were compared to 2.4kbps MELP and other DoD Voice Processors in a poster session presented at the IEEE International Conference on Acoustics, Speech, and Signal Processing in Munich Germany, 1997 (ICASSP '97) entitled: A Comparison of the New 2400 BPS MELP Federal Standard with Other Standard Coders

Sound Samples - The accompanying table entries are links to sound samples of the 16kbps CVSD algorithm. These are 8KHz sampled, 16 bit linear PCM files in WAV format.

Algorithmic Delay - The 16kbps CVSD algorithm, implemented with minimal buffering, has a speech throughput delay of 125.0 µsecs.