Dolby C-type (Dolby C) noise reduction system, was developed in 1980 by the Dolby B-type, its noise processing using a low-level bypass of the dual-path system, consisting of high-level and low-level two systems in series. This system also does not sound frequency range of all noise suppression, only part of the frequency band noise suppression, in the recording and playback process of more than 1kHz high-frequency noise reduction effect of about 20dB, and can improve the maximum output level of 8dB (15kHz). Because the Dolby C noise reduction system can be a cassette tape noise nearly completely eliminated, but also reduces the high-frequency loss and distortion, so it is a business-class two-band noise reduction system, is a high-level home cassette tape recorder indispensable to the setup.
Dolby S-type (Dolby S) noise reduction system, was adopted in 1989 professional SR (Spectral Recording) technology and A-type noise reduction system combined with Dolby B, C-type noise reduction system and other related technologies derived from its noise reduction performance in the low-frequency range of 10dB, high-frequency range of up to 24dB, and to reduce distortion, enhance the role of linear It also reduces distortion, improves linearity, realizes a wider dynamic range, and can record at higher levels, and also reduces the phenomenon of tracking error caused by transmission band error, which makes the listening experience better. The Dolby S noise reduction system is used in advanced home cassette recorders such as the Quasi-Pro. Software and hardware with Dolby noise cancellation are marked with a double-write D.