The term "track" refers to multi-track music recordings (isolating the singer, bassist, and pianist on single recordings). Despite the fact that 8-tracks are not separated by distinct instruments as they are in music, the notion was inspired by the music business. Each 8-track cassette has four "programs." These are channels of information that can be accessed by turning a dial on the side of the tape player. Track is also used to describe the channel on an audio recording device where each channel represents a separate instrument or voice part. Modern recording devices usually have three or more tracks, allowing several parts to be recorded at once. 8-tracks were popular in the 1970s and 1980s. An 8-track cartridge contained eight minutes of stereo music recording time. The first generation of 8-tracks could only be played back in special players. However, newer models allow them to be played on regular stereo systems too. In modern usage, the term track refers to one of several layers of magnetic material on which audio signals can be stored. The term comes from the fact that these layers were originally made out of brass or copper, similar to a phonograph record. Today, these layers are mostly made of polycarbonate but some manufacturers still use metal sheets. In a recording studio, there are typically between three and nine tracks available for use.