Electronic noise reduction techniques were also developed to increase the signal-to-noise ratio and dynamic range of analog sound recordings. Dolby noise reduction includes a suite of standards (designated A, B, C, S and SR) for both professional and consumer recording. The Dolby systems use frequency dependent compression/expansion (companding) during the recording/playback, respectively. Initially, Dolby was offered via a stand-alone box that would go between a recorder and amplifier. Later audio devices often included Dolby. DBX is another noise reduction system that uses a more aggressive companding technique to improve both dynamic range and noise level. However, DBX recordings do not sound acceptable when played on non-DBX equipment.