Quadraphonic (or quadrophonic and sometimes quadrasonic) sound – equivalent to what is now called 4.0 surround sound – uses four audio channels in which speakers are positioned at the four corners of a listening space. The system allows for the reproduction of sound signals that are (wholly or in part) independent of one another. Four channel quadraphonic surround sound can be used to recreate the highly realistic effect of a three-dimensional live concert hall experience in the home. It can also be used to enhance the listener experience beyond the directional limitations of ordinary two channel stereo sound. Quadraphonic audio was the earliest consumer product in surround sound. Since it was introduced to the public in the early 1970s many thousands of quadraphonic recordings have been made. Quadraphonic sound was a commercial failure when first introduced due to a variety of technical issues and format incompatibilities. Four channel audio formats can be more expensive to produce than standard two-channel stereo. Playback requires additional speakers and amplifier channels. It may also require specially designed decoding equipment. The introduction of home cinema products in the 1990s were first intended for movie sound, but also brought multi-channel music reproduction into popularity again. By this time new digitally based formats had been created. Many four channel recordings from the 1970s have been reissued in modern surround sound systems such as Super Audio CD, DTS, Dolby Digital, DVD-Audio and Blu-ray. Multichannel home audio reproduction has experienced a revival since 2000 and new four channel recordings have also been released to the public since this time. A quadraphonic system will reproduce left front, left rear, right front, and right rear audio signals in each of four separate speakers. Reproduction in the rear speakers should be of the same quality or almost the same quality as the front speakers. Ideally, it is preferred to use four identical speakers.