This feature works along with noise reduction technology, and is yet another attempt to compensate for the loss of high frequencies. Dolby HX Pro uses a different technology to reduce the effects of tape saturation, which then makes it possible to record loud musical passages with fewer high-frequency losses and less distortion. It is available in higher-end cassette decks and is also widely used by the recording industry to improve the quality of prerecorded cassettes. This technology is found in almost all modern cassette decks. You'll hear exactly what your recorded tape sounds like.
Every recording has a perfect setting where the levels are ideal for recording.
If too much bias is used, high-frequency response will not be audible, causing the recording to sound dull. If too little is used, high frequencies will be emphasized, causing the recording to sound harsh and maybe even distorted. Cheap cassette decks come with an automatic bias adjustment, which means when you pop in a tape, the bias is set after sensing what kind it is. More expensive decks allow you to adjust the bias, and may have an automatic feature as well. There are three basic types of cassette tapes available.