VMS to HTK Converter

VMS (Voice Messaging System) is a compressed audio format designed for telephony and voice mail applications, originally used in Germany. Files with the .vms extension encode spoken audio using Continuously Variable Slope Delta modulation (CVSD), a method suited to low-bandwidth voice transmission over telephone networks. The format operates at 8 kHz, matching the standard digital telephony sampling frequency, and produces self-describing files that embed encoding parameters within a short header. This header distinguishes VMS from raw CVSD streams, letting playback tools process recordings without external configuration. The SoX audio toolkit provides native read and write support, making it straightforward to convert VMS recordings into WAV or other modern formats. A practical advantage is the format's small file size — CVSD compression keeps voice mail messages compact enough for systems with limited disk capacity, which was critical in early telephony infrastructure. The encoding degrades gracefully under noisy channel conditions, preserving speech intelligibility even when errors occur. Although VMS has been superseded by modern codecs in current voice messaging platforms, it remains relevant for recovering legacy voice mail archives.