http://liquidsdr.org/doc/quantization/

The audio module in liquid provides several objects and functions for compressing, digitizing, and manipulating audio signals. This is particularly useful for encoding audio data for wireless communications. Continuously variable slope delta (CVSD) source encoding is used for data compression of audio signals.

http://liquidsdr.org/doc/cvsd/

This tutorial explains how to write and simulate a phase-locked loop in the C programming language.

http://liquidsdr.org/blog/pll-howto/

A super simple phase-locked loop in 50 lines of C.

http://liquidsdr.org/blog/pll-simple-howto/