Dolby A
Dolby A was the company's first noise reduction system, presented in 1966. It was intended for use in professional recording studios, where it became commonplace, gaining widespread acceptance at the same time that multitrack recording became standard. The input signal is split into four frequency bands with 12 dB/oct slopes: lowpass @ 80 Hz; bandpass 80 Hz - 3 kHz; highpass @ 3 kHz; highpass @ 9 kHz. The compansion circuit has a threshold of -40dB, with a ratio of 2:1 for a compression / expansion of 10dB, except for the 9 kHz highpass which has only 5dB of gain change. This provides about 10 dB of broadband noise reduction, which increases to a possible 15dB at 15 kHz according to articles written by Ray Dolby in JAES (Oct 1967) and Audio (June / July 1968).
Dolby A also saw some use as the method of noise reduction in optical sound for motion pictures.
Read more about this topic: Dolby Noise-reduction System