Interactive video-on-demand (VOD) television services first appeared in the 1990s. Up until then, it was not thought possible that a television programme could be squeezed into the limited telecommunication bandwidth of a copper telephone cable to provide a VOD service of acceptable quality, as the required bandwidth of a digital television signal was around 200Mbps, which was 2,000 times greater than the bandwidth of a speech signal over a copper telephone wire. VOD services were only made possible as a result of two major technological developments: discrete cosine transform (DCT) video compression and asymmetric digital subscriber line (ADSL) data transmission. DCT is a lossy compression technique that was first proposed by Nasir Ahmed in 1972, and was later adapted into a motion-compensated DCT algorithm for video coding standards such as the H.26x formats from 1988 onwards and the MPEG formats from 1991 onwards. Motion-compensated DCT video compression significantly reduced the amount of bandwidth required for a television signal, while at the same time ADSL increased the bandwidth of data that could be sent over a copper telephone wire. ADSL increased the bandwidth of a telephone line from around 100kbps to 2Mbps, while DCT compression reduced the required bandwidth of a television signal from around 200Mbps down to 2Mpps. The combination of DCT and ADSL technologies made it possible to practically implement VOD services at around 2Mbps bandwidth in the 1990s.