The HC-55564 is a half duplex modulator/demodulator CMOS
intergrated circuit used to convert voice signals into serial NRZ
digital data and to reconvert that data into voice. The conversion
is by delta-modulation, using the Continuously Variable
Slope (CVSD) method of modulation/demodulation.
While the signals are compatible with other CVSD circuits, the internal
design is unique. The analog loop filters have been replaced by
very low power digital filters which require no external timing components.
This approach allows inclusion of many desirable features
which would be difficult to implement using other approaches.
The fundamental advantages of delta-modulation, along with its
simplicity and serial data format, provide an efficient (low data
rate/low memory requirements) method for voice digitization.
The HC-55564 is usable from 9kbits/s to above 64kbps. See the
Harris Military databook for a MIL-STD-883C compliant CVSD.
Application Note 607.