af acj1980 » fre 10. jan 2014 14:48
Jeg fandt lige denne her:
Dolby HX-Pro[9] was invented in 1980 and patented in 1981 (EP 0046410) by Jørgen Selmer Jensen[10] of Bang & Olufsen.
B&O immediately licensed HX-Pro to Dolby Laboratories, stipulating a priority period of several years for use in consumer products, to protect their own Beocord 9000[11] cassette tape deck.
Magnetic tape is inherently non-linear in nature due to hysteresis of the magnetic material. If an analogue signal were recorded directly onto magnetic tape, its reproduction would be extremely distorted due to this non-linearity.
To overcome this, a high frequency signal, known as bias, is mixed in with the recorded signal, which "pushes" the envelope of the signal into the linear region.
If the audio signal contains strong high frequency content, in particular from percussion instruments such as a high-hat, this adds to the constant bias causing magnetic saturation on the tape. Dolby HX Pro automatically reduces the bias signal in the presence of strong high frequency signals, making it possible to record at a higher signal level, leading to its name: HX = Headroom eXpansion.
HX-Pro only applies during recording; the improved signal to noise ratio is available no matter which tape deck the tape is played back on, and therefore HX-Pro is not a noise-reduction system, in the same way as Dolby A, B & C.
HX-Pro was widely used on pre-recorded cassette tapes, and is the main reason these sound better than cassette tapes recorded on consumer equipment
Kilde: Wikipedia.