| • Science | • People | • Locations | • Timeline |
Several types of Dolby NR were developed, including A, B, C, S, and SR. Most widely used, by consumers, is the B type, which allows for acceptable playback on devices without noise reduction. Most pre-recorded cassettes available on the market use this variant. In the mid-1970s, some expected Dolby NR to become normal in FM radio broadcasts and some tuners and amplifiers were manufactured with decoding circuitry.
The noise reduction systems available are designed specifically with either professional or consumer users in mind– Dolby A and Dolby SR were developed for professionals, as the equipment requires precise alignment in order to reproduce properly, while Dolby B , C , and S were designed for the consumer market, requiring less intervention by the user, but providing poorer noise reduction
Dolby has another noise reduction system called HX Pro , which works by modifying the ultrasonic bias signal, used by all analogue tape decks, to increase the headroom for high-frequency audio signals. HX stands for "headroom extension".
Dolby's analogue noise reduction systems, though still used in some professional applications, have been made obsolete by the widespread adoption of digital audio (in the form of compact discs, MP3s, MiniDiscs, and to a lesser extent DAT) in the home for entertainment and recording.
Dolby B (and C which is similar) is a form of dynamic preemphasis. The background hiss of a tape white noise is unnoticeable if it is masked by a stronger audio signal, especially at higher frequencies. This is called psychoacoustic masking. When the tape is recorded, the amplitudeAmplitude is a nonnegative scalar measure of a wave's magnitude of oscillation. In the following diagram, the distance y is the amplitude of the wave. Sometimes that distance is called the "peak amplitude", distinguishing it from another concept of amplit of the signal in the higher frequencyFrequency is the measurement of the number of times that a repeated event occurs per unit time. To calculate the frequency, one fixes a time interval, counts the number of occurrences of the event within that interval, and then divides this count by the l registers is used to determine how much pre-emphasis to apply - a lower level signal is boosted by about 10 dBThe decibel is a "dimensionless unit" (like percent) that is a measure of ratios on a logarithmic scale. Usually, it is ten times the base-10 logarithm of the ratio. It's not an SI unit, although the International Committee for Weights and Measures (BIPM) (Dolby B) or 20dB (Dolby C). As the signal rises in amplitude, less and less pre-emphasis is applied until at the "Dolby level" (+3 VU), no signal modification is performed. On playback, the opposite process is applied (deemphasis), based on the signal level. Thus as the signal level drops, the higher frequencies are progressively more strongly filtered, which also filters the constant background noise level. The two processes cancel out as far as the signal is concerned, so it is reproduced faithfully, but only one process (the de-emphasis) is applied to the noise, which is thereby reduced.
The calibration of the recording and playback circuitry is important for faithful cancellation of the complementary processes, and is easily upset by poor quality tapes, dirty playback heads or using incorrect bias levels. This usually manifests itself as muffled-sounding playback, or "breathing" of the noise level as the signal varies.