A critical development that enabled practical streaming media is the discrete cosine transform (DCT). The DCT algorithm formed the basis for the first practical video coding format, H.261, in 1988. It was initially used for online video conferencing. It was followed by more popular DCT-based video coding standards, most notably MPEG video formats from 1991 onwards. The DCT algorithm was also adapted into the modified discrete cosine transform (MDCT), which is fundamental to the MP3 audio format introduced in 1994, and Advanced Audio Coding (AAC) format introduced in 1999.