A circuit employed by many modern analog tape recorders to improve recording quality. Developed by Dolby Laboratories, HX Pro varies the bias signal (see WFTD archive Bias) in record mode to reduce the tendency toward self-erasure. In magnetic tape recording, loud high frequencies in the signal can sometimes look like bias to the tape and will tend to erase the signal as it is being recorded. The effect is called high frequency compression and is a fault of magnetic recorders in general and cassette recorders in particular. The HX Pro system senses the level of high frequencies and reduces the level of bias accordingly. Because it is made by Dolby Labs and bears the Dolby trademark many users think it is a form of noise reduction since that is what Dolby has been most known for. It is not noise reduction, and does not require a “decode” process like typical noise reduction schemes (Dolby included) and is only applied during recording. The benefit of HX Pro is increased headroom during recording, especially with high frequencies. HX Pro recorded tapes play back just fine on any tape machine.