Dolby Noise-reduction System - Dolby HX/HX-Pro

Dolby HX/HX-Pro

Developed in 1982, Dolby HX is not a noise reduction system, though it can indirectly decrease noise by allowing signals to be recorded at higher levels than they otherwise could. Using electronic circuitry partly equivalent to that used in noise reduction, HX provides additional dynamic range for high-frequency signals. HX or "Headroom eXtension" is a method for further increasing the dynamic range of a cassette tape by dynamically modulating the ultrasonic bias signal, used by all analogue tape decks, to increase the headroom for high-frequency audio signals. This system was modified by Bang & Olufsen for consumer equipment and marketed by Dolby as Dolby HX Pro. HX and HX Pro are used only in recording.

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. With strong signals of fixed frequency and high amplitude, the amount of bias needed is reduced. Due to group and phase delay the audio signal itself creates a variable amount of self-bias. If the added bias remains constant, these high frequency signals become overbiased. This overbias creates distortion as the tape becomes saturated. Dolby HX Pro automatically reduces the bias signal in the presence of strong high frequency signals. This optimises the amount of self bias, reducing distortion caused from saturation of the magnetic tape. By adjusting the bias with respect to group and phase delay the overall distortion of high frequency signals is also greatly reduced. This kind of bias adjustment increases the high frequency dynamic range available. The net effect for the listener is a crisper sounding high frequency reproduction.