Correct Option
Dolby B and Dolby C refer to specific noise reduction circuits developed by Dolby Laboratories. These systems were primarily integrated into analog audio recording equipment, such as cassette tape recorders, to improve sound quality. Their function is to minimize unwanted background noise, particularly tape hiss, which is inherent in analog magnetic tape recording. This is achieved through a two-stage process: during recording, certain high-frequency, low-level audio signals are boosted (encoded); during playback, these boosted signals are attenuated (decoded), thereby reducing the noise that was introduced during the recording process. Dolby C provided a more advanced and greater degree of noise reduction compared to Dolby B.
Incorrect Options
Options (A) and (B) are incorrect because Frequency Modulation (FM) and Amplitude Modulation (AM) are techniques used for transmitting information by varying the frequency or amplitude of a carrier wave, respectively. They are modulation techniques for radio transmission and are distinct from audio noise reduction circuits.
Option (D) is incorrect as the terms Dolby B or Dolby C describe an audio processing technology for noise reduction, not the type of electrical power supply (AC or DC) that an electronic device utilizes.