Dolby noise-reduction system
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A Dolby noise-reduction system, or Dolby NR, is one of a series of noise reduction systems developed by Dolby Laboratories for use in analog audio tape recording. The first was Dolby A, a professional broadband noise reduction for recording studios in 1965, but the best-known is Dolby B (introduced 1968), a sliding band system for the consumer market, which helped make high fidelity practical on cassette tapes which used a relatively noisy tape size and speed. It is common on high fidelity stereo tape players and recorders to the present day. Of the noise reduction systems, Dolby A and Dolby SR were developed for professional use. Dolby B, C, and S were designed for the consumer market. Aside from Dolby HX, all the Dolby variants work by companding, or compressing the dynamic range of the sound during recording and expanding it during playback.
When recording a signal on magnetic tape, there is a low level of "noise" in the background which sounds like hissing. One solution is "low-noise" tape which records more signal, and less "noise". Other solutions are to run tape at a higher speed or use a wider tape. Cassette tapes were originally designed to trade off fidelity for convenience of recording voice by using a very narrow tape run at a very slow speed of 1 7/8 inches per second (ips) in a simple plastic shell when 15 or 7.5 ips was for high fidelity and 3 3/4 ips was lower fidelity. When Advent designed one of the first cassette tape decks for high fidelity, noise reduction was one way to fix problems with the format, along with chrome and high-bias tapes with extended high frequency response. Within 10 to 20 years, the original reel-to-reel high fidelity tape recorders which did not need noise reduction were eventually replaced by cassette decks for home use. The dominant noise reduction scheme "Dolby B" was widely accepted because if an inexpensive cassette player lacked the switch, they would just sound brighter which often offset the dull sounds of cheap players.
The signal to noise ratio is simply how large the music signal is compared to the low level of the "noise" with no signal. When the music is loud, the low hiss is not noticeable, but when the music is soft or in silence, most of what can be heard is the noise. If the recording level is adjusted so that the music is always loud, then it could in theory be turned down later, and the noise volume would also be turned down. The idea is for electronics to automatically increase the recording volume when it is soft, but reduce the volume on playback. Some schemes like Dolby B concentrate only on the high frequencies so that the "hiss" sound of noise will be masked when volume is turned down for playback.
Dolby noise reduction is a form of dynamic preemphasis employed during recording, plus a form of dynamic deemphasis used during playback, that work in tandem to improve the signal-to-noise ratio. While Dolby A operates across the whole spectrum, the other systems specifically emphasize the audible frequency range where background tape hiss, an artifact of the recording process that is similar to white noise, is most noticeable (usually above 1 kHz, or two octaves above Middle C).
The Dolby preemphasis boosts the recorded level of the quieter audio signal at these higher frequencies during recording, effectively compressing the dynamic range of that portion of the signal, so that quieter sounds above 1 kHz receive a proportionally greater boost. As the tape is recorded, the relative amplitude of the signal above 1 kHz is used to determine how much pre-emphasis to apply - a low-level signal is boosted by 10 dB (Dolby B) or 20 dB (Dolby C). As the signal rises in amplitude, less and less pre-emphasis is applied until at the "Dolby level" (0 VU), no signal modification is performed.
The sound is thus recorded at a higher overall level on the tape relative to the tape's overall noise level, requiring the tape formulation to preserve this specially recorded signal without distortion. On playback, the opposite process is applied (deemphasis), based on the relative signal component above 1 kHz. Thus as this portion of the signal decreases in amplitude, the higher frequencies are progressively more sharply attenuated, which also filters out the constant background noise on the tape when and where it would be most noticeable.
The two (pre and de-emphasis) processes are intended to cancel each other out as far as the actual recorded program is concerned. Only de-emphasis is applied to the incoming signal and noise during playback. After playback de-emphasis is complete, apparent noise in the output signal is reduced, and this process should not produce any effect noticeable to the listener (other than reduced noise of course). Playback without noise reduction produces a noticeably brighter sound, however.
The calibration of the recording and playback circuitry is therefore critical for faithful reproduction of the original program content, and this is easily offset by poor quality tape, dirty recording/playback heads, or using inappropriate bias levels/frequency for the tape formulation, as well as tape speed, when recording or duplicating. This can manifest itself as muffled-sounding playback, or "breathing" of the noise level as the signal varies.
On some high end consumer equipment, Dolby calibration control is included: for recording, a reference tone at Dolby level may be recorded for accurate playback level calibration on another transport; at playback, the same recorded tone should produce the identical output, as indicated by a Dolby logo marking at 0 VU on the VU meter(s). (In consumer equipment Dolby Level is defined as 200 nWb/m; calibration tapes were available to assist correct level setting.) For accurate off-the-tape monitoring during recording on 3-head decks, both processes must be employed at once, and circuitry provided to accomplish this is marketed under the rubric "Double Dolby".
Dolby A was the company's first noise reduction system, presented in 1965. 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 frequency bands by four filters with 12 dB per octave slopes, with cutoff frequencies (3 dB down points) as follows: high–pass at 80 Hz; band–pass from 80 Hz to 3 kHz; a low–pass from 3 kHz; and another low–pass at 9 kHz. (The stacking of contributions from the two low-pass bands allows greater noise reduction in the upper frequencies.) The compander circuit has a threshold of -40 dB, with a ratio of 2:1 for a compression/expansion of 10 dB. This provides about 10 dB of noise reduction increasing to a possible 15 dB at 15 kHz, according to articles written by Ray Dolby in JAES (October 1967) and Audio (June/July 1968).
As with the "B" system, correct matching of the compression and expansion processes is important. The calibration of the expansion (decoding) unit for magnetic tape uses a flux level of 185 nWb/m, which is the level used on industry calibration tapes such as those from Ampex; this is set to 0 VU on the tape recorder playback and to Dolby Level on the noise reduction unit. In the record (compression or encoding) mode a characteristic tone (Dolby Tone) generated inside the noise reduction unit is set to 0 VU on the tape recorder and to 185 nWb/m on the tape.
Dolby A also saw some use as the method of noise reduction in optical sound for motion pictures.
Dolby B was developed after Dolby A and presented in 1968, as a single sliding band system providing about 9 dB noise reduction (A-weighted), primarily for cassettes. It was much simpler than Dolby A and therefore much less expensive to implement in consumer products. Dolby B recordings are acceptable when played back on equipment that does not possess a Dolby B decoder, such as most inexpensive portable and car cassette players. Without the de-emphasis of the decoder, the sound will be perceived as brighter as high frequencies are emphasized, which can be used to offset "dull" high frequency response in inexpensive equipment. However, Dolby B provides less effective noise reduction than Dolby A, generally by a factor of more than 3 dB.
From the mid-1970s, Dolby B became standard on commercially pre-recorded music cassettes in spite of the fact that some low-end equipment lacked decoding circuitry, although it allows for acceptable playback on such equipment. Most pre-recorded cassettes use this variant. VHS video recorders used Dolby B on linear stereo audio tracks.
Up to 1980 tape recorders supporting Dolby B were often just labelled Dolby NR.
In the early 1970s, some expected Dolby NR to become normal in FM radio broadcasts and some tuners and amplifiers were manufactured with decoding circuitry; there were also some tape recorders with a Dolby B "pass-through" mode. In 1971 WFMT started to transmit programs with Dolby NR, and soon some 17 stations broadcast with noise reduction, but by 1974 it was already on the decline. Dolby FM was based on Dolby B, but used a modified 25 µs pre-emphasis time constant and a frequency selective companding arrangement to reduce noise.
A similar system named High Com FM was tested in Germany between July 1979 and December 1981 by IRT. It was based on the Telefunken High Com broadband compander system, but never introduced commercially in FM broadcasting.
Dolby C was introduced in 1980. It provides about 15 dB noise reduction (A-weighted). It is constructed by combining the effect of two Dolby B systems together with an expansion to lower frequencies. The resulting recordings sound much worse when played back on equipment that does not have Dolby C noise reduction. Some of this harshness can be mitigated by using Dolby B on playback. Dolby C first appeared on higher end cassette decks in the 1980s. The first commercially available cassette deck with Dolby C was NAD 6150C, which came into the market in ca. 1981. It was also used on professional video equipment for the audio tracks of the Betacam and Umatic SP videocassette formats. In Japan the first cassette deck with Dolby C was the AD-FF5 from Aiwa.
The Dolby SR (Spectral Recording) system, introduced in 1986, was the company's second professional noise reduction system. It is a much more aggressive noise reduction approach than Dolby A. It attempts to maximise the recorded signal at all times using a complex series of filters that change according to the input signal. As a result, Dolby SR is much more expensive to implement than Dolby B or C, but Dolby SR is capable of providing up to 25 dB noise reduction in the high frequency range. It is only found on professional recording equipment.
In the motion picture industry, as far as it concerns distribution prints of movies, the Dolby A and SR markings refer to Dolby Surround which is not just a method of noise reduction, but more importantly encodes two additional audio channels on the standard optical soundtrack, giving left, center, right, and surround.
Dolby S was introduced in 1989. It was intended that Dolby S would become standard on commercial pre-recorded music cassettes in much the same way that Dolby B had in the 1970s, but it came to market when the Compact Cassette was being replaced by the Compact Disc as the dominant mass market music format. Dolby Labs claimed that most members of the general public could not differentiate between the sound of a CD and a Dolby S encoded cassette. Dolby S mostly appeared on high-end audio equipment and was never widely used.
Dolby S is much more resistant to playback problems caused by noise from the tape transport mechanism than Dolby C. Likewise, Dolby S was also claimed to have playback compatibility with Dolby B in that a Dolby S recording could be played back on older Dolby B equipment with some benefit being realized. It is basically a cut down version of Dolby SR and uses many of the same noise reduction techniques. Dolby S is capable of 10 dB of noise reduction at low frequencies and up to 24 dB of noise reduction at high frequencies.
B&O immediately licensed HX-Pro to Dolby Laboratories, stipulating a priority period of several years for use in consumer products, to protect their own Beocord 9000 cassette tape deck.
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.
If the audio signal contains strong high frequency content, in particular from percussion instruments such as a high-hat, this adds to the constant bias causing magnetic saturation on the tape. Dolby HX Pro automatically reduces the bias signal in the presence of strong high frequency signals, making it possible to record at a higher signal level, leading to its name: HX = Headroom eXtension.
HX-Pro only applies during recording; the improved signal to noise ratio is available no matter which tape deck the tape is played back on, and therefore HX-Pro is not a noise-reduction system in the same way as Dolby A, B, C & S, although it does help to improve noise reduction encode/decode tracking accuracy by reducing tape performance non-linearity.
Some record companies issued HX-Pro pre-recorded cassette tapes during the late 1980s and early 1990s.
The widespread proliferation of digital audio in professional and consumer applications (e.g. compact discs, music download) has made analogue audio production less prevalent, but Dolby's analogue noise reduction systems are still widely used in niche analogue production environments.
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