The speech signal has variability due to language, speakers, and communication channels. In this work, variability due to language is referred to as variability due to different phones in the language. It is also referred to as (inter-phone or) phone variability. Variability due to speakers is referred to as speaker variability, and variability due to different communication channels is referred to as channel variability. The remaining variability in the signal is referred to as residual variability.