Dolby HX, commercially known as HX Pro, is a signal processing system used in analog tape recorders, both reel-to-reel and cassette, to increase high-frequency recording capacity. The acronym HX stands for Headroom eXtension. Unlike Dolby noise reduction systems, this feature acts only during recording and requires no decoding upon playback. Its operation is based on the dynamic and automatic adjustment of the bias current. Since very high-pitched audio signals naturally act as a type of bias, the system reduces the bias generated by the machine at those moments to prevent magnetic tape saturation. This allows for recording at higher volume levels while preserving treble clarity without distortion.