Dolby noise reduction system
Encyclopedia
Dolby NR is the name given to a series of noise reduction systems developed by Dolby Laboratories
for use in analog magnetic tape
recording. The first was Dolby A, a professional broadband
noise reduction for recording studios in 1966, 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, and is common on 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. All the Dolby variants work by companding
, or compressing the dynamic range
of the sound during recording and expanding it during playback.
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).
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" (+3 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. 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 +3 VU on the VU meter(s). 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".
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.
. 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 cassette players. 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 prerecorded 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.
started to transmit programs with Dolby NR, and soon some 17 stations broadcasted with noise reduction, but by 1974 it was already on the decline.
techniques. Dolby C first appeared on higher end cassette decks in the 1980s.
In the motion picture industry, as far as it concerns distribution prints of movies, the Dolby A and SR markings refers 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.
SR prints are fairly well backward compatible with old Dolby A equipment. The Dolby SRD marking refers to Dolby Digital
and its variants.
was being replaced by the Compact Disc
as the dominant mass market music format. Dolby Labs claimed that most members of the general public couldn't differentiate between the sound of a CD and a Dolby S encoded cassette. Subsequently, Dolby S appeared only on high-end audio equipment.
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 played back on older Dolby B equipment with some benefit being realised. 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.
ry partly equivalent to that used in noise reduction, HX provides additional dynamic range
for high-frequency signals. HX or "Headroom eXtension" is a method for further increasing the dynamic range
of a cassette tape by dynamically modulating
the ultrasonic bias signal
, used by all analogue tape decks, to increase the headroom for high-frequency audio signals. This system was modified by Bang & Olufsen
for consumer equipment and marketed by Dolby as Dolby HX Pro. HX and HX Pro are used only in recording.
Because magnetic tape is magnetic, it is inherently non-linear in nature, due to the hysteresis
of the magnetic particles. 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. With strong signals of fixed frequency and high amplitude, the amount of bias needed is reduced. Due to group and phase delay
the audio signal itself creates a variable amount of self-bias. If the added bias remains constant, these high frequency signals become overbiased. This overbias creates distortion as the tape becomes saturated. Dolby HX Pro automatically reduces the bias signal in the presence of strong high frequency signals. This optimises the amount of self bias, reducing distortion caused from saturation of the magnetic tape. By adjusting the bias with respect to group and phase delay the overall distortion of high frequency signals is also greatly reduced. This kind of bias adjustment increases the high frequency dynamic range available. The net effect for the listener is a crisper sounding high frequency reproduction.
(in the form of compact disc
s, MP3
s, MiniDisc
s, and to a lesser extent DAT
) in the home for entertainment and professional studios for recording. In other words, Dolby NR is not becoming obsolete for analog recording, but analog recording itself is becoming obsolete as digital recording replaces it.
Dolby Laboratories
Dolby Laboratories, Inc. , often shortened to Dolby Labs, is an American company specializing in audio noise reduction and audio encoding/compression.-History:...
for use in analog magnetic tape
Magnetic tape
Magnetic tape is a medium for magnetic recording, made of a thin magnetizable coating on a long, narrow strip of plastic. It was developed in Germany, based on magnetic wire recording. Devices that record and play back audio and video using magnetic tape are tape recorders and video tape recorders...
recording. The first was Dolby A, a professional broadband
Broadband
The term broadband refers to a telecommunications signal or device of greater bandwidth, in some sense, than another standard or usual signal or device . Different criteria for "broad" have been applied in different contexts and at different times...
noise reduction for recording studios in 1966, 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, and is common on 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. All the Dolby variants work by companding
Companding
In telecommunication, signal processing, and thermodynamics, companding is a method of mitigating the detrimental effects of a channel with limited dynamic range...
, or compressing the dynamic range
Dynamic range
Dynamic range, abbreviated DR or DNR, is the ratio between the largest and smallest possible values of a changeable quantity, such as in sound and light. It is measured as a ratio, or as a base-10 or base-2 logarithmic value.-Dynamic range and human perception:The human senses of sight and...
of the sound during recording and expanding it during playback.
How Dolby noise reduction works
Dolby noise reduction is a form of dynamic preemphasisPreemphasis
In processing electronic audio signals, pre-emphasis refers to a system process designed to increase the magnitude of some frequencies with respect to the magnitude of other frequencies in order to improve the overall signal-to-noise ratio by minimizing the adverse effects of such phenomena as...
employed during recording, plus a form of dynamic deemphasis
Deemphasis
In telecommunication, de-emphasis is the complement of pre-emphasis, in the antinoise system called emphasis. Emphasis is a system process designed to decrease, , the magnitude of some frequencies with respect to the magnitude of other frequencies in order to improve the overall signal-to-noise...
used during playback, that work in tandem to improve the signal-to-noise ratio
Signal-to-noise ratio
Signal-to-noise ratio is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. It is defined as the ratio of signal power to the noise power. A ratio higher than 1:1 indicates more signal than noise...
. 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
White noise
White noise is a random signal with a flat power spectral density. In other words, the signal contains equal power within a fixed bandwidth at any center frequency...
, is most noticeable (usually above 1 kHz).
The Dolby preemphasis boosts the recorded level of the quieter audio
Sound
Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...
signal at these higher frequencies during recording, effectively compressing the dynamic range
Dynamic range
Dynamic range, abbreviated DR or DNR, is the ratio between the largest and smallest possible values of a changeable quantity, such as in sound and light. It is measured as a ratio, or as a base-10 or base-2 logarithmic value.-Dynamic range and human perception:The human senses of sight and...
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
Amplitude
Amplitude is the magnitude of change in the oscillating variable with each oscillation within an oscillating system. For example, sound waves in air are oscillations in atmospheric pressure and their amplitudes are proportional to the change in pressure during one oscillation...
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
Decibel
The decibel is a logarithmic unit that indicates the ratio of a physical quantity relative to a specified or implied reference level. A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power quantities...
(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" (+3 VU
VU meter
A VU meter is often included in audio equipment to display a signal level in Volume Units; the device is sometimes also called volume indicator ....
), 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. 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
Tape bias
Tape bias is the term for two phenomena, DC bias and AC bias, that improve the fidelity of analogue magnetic tape sound recordings. DC bias is the addition of a direct current to the audio signal that is being recorded. AC bias is the addition of an inaudible high-frequency signal to the audio...
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 +3 VU on the VU meter(s). 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
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 recordingMultitrack recording
Multitrack recording is a method of sound recording that allows for the separate recording of multiple sound sources to create a cohesive whole...
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.
Dolby B
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 cassettesCompact Cassette
The Compact Cassette, often referred to as audio cassette, cassette tape, cassette, or simply tape, is a magnetic tape sound recording format. It was designed originally for dictation, but improvements in fidelity led the Compact Cassette to supplant the Stereo 8-track cartridge and reel-to-reel...
. 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 cassette players. 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 prerecorded 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.
Dolby FM
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. In 1971 WFMTWFMT
WFMT is an FM radio station in Chicago, Illinois, featuring a format of fine arts, classical music programming, and shows exploring such genres as folk and jazz). The station is managed by Window To The World Communications, Inc., owner of WTTW, one of Chicago's two Public Broadcasting Service ...
started to transmit programs with Dolby NR, and soon some 17 stations broadcasted with noise reduction, but by 1974 it was already on the decline.
Dolby C
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. It utilises anti-saturation and spectral skewingSpectral skewing
Spectral skewing is the term used to describe the phenomenon of the changes in relative intensities of mass spectral peaks due to the changes in concentration of the analyte in the ion source as the mass spectrum is scanned; this situation occurs routinely as chromatographic components elute into a...
techniques. Dolby C first appeared on higher end cassette decks in the 1980s.
Dolby SR
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 refers to Dolby Surround
Dolby Surround
Dolby Surround was the earliest consumer version of Dolby's multichannel analog film sound decoding format Dolby Stereo introduced to the public in 1982 during the time home video recording formats were introducing Stereo and HiFi capability...
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.
SR prints are fairly well backward compatible with old Dolby A equipment. The Dolby SRD marking refers to Dolby Digital
Dolby Digital
Dolby Digital is the name for audio compression technologies developed by Dolby Laboratories. It was originally called Dolby Stereo Digital until 1994. Except for Dolby TrueHD, the audio compression is lossy. The first use of Dolby Digital was to provide digital sound in cinemas from 35mm film prints...
and its variants.
Dolby S
Dolby S was presented in 1989. It is found on some Hi-Fi and semi-professional recording equipment. It was intended that Dolby S would become standard on commercial prerecorded musicassettes in much the same way that Dolby B had in the 1970s, but this never happened, as Dolby S came to market at a time when the Compact CassetteCompact Cassette
The Compact Cassette, often referred to as audio cassette, cassette tape, cassette, or simply tape, is a magnetic tape sound recording format. It was designed originally for dictation, but improvements in fidelity led the Compact Cassette to supplant the Stereo 8-track cartridge and reel-to-reel...
was being replaced by the Compact Disc
Compact Disc
The Compact Disc is an optical disc used to store digital data. It was originally developed to store and playback sound recordings exclusively, but later expanded to encompass data storage , write-once audio and data storage , rewritable media , Video Compact Discs , Super Video Compact Discs ,...
as the dominant mass market music format. Dolby Labs claimed that most members of the general public couldn't differentiate between the sound of a CD and a Dolby S encoded cassette. Subsequently, Dolby S appeared only on high-end audio equipment.
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 played back on older Dolby B equipment with some benefit being realised. 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.
Dolby HX/HX-Pro
Developed in 1982, Dolby HX is not a noise reduction system, though it can indirectly decrease noise by allowing signals to be recorded at higher levels than they otherwise could. Using electronic circuitElectronic circuit
An electronic circuit is composed of individual electronic components, such as resistors, transistors, capacitors, inductors and diodes, connected by conductive wires or traces through which electric current can flow...
ry partly equivalent to that used in noise reduction, HX provides additional dynamic range
Dynamic range
Dynamic range, abbreviated DR or DNR, is the ratio between the largest and smallest possible values of a changeable quantity, such as in sound and light. It is measured as a ratio, or as a base-10 or base-2 logarithmic value.-Dynamic range and human perception:The human senses of sight and...
for high-frequency signals. HX or "Headroom eXtension" is a method for further increasing the dynamic range
Dynamic range
Dynamic range, abbreviated DR or DNR, is the ratio between the largest and smallest possible values of a changeable quantity, such as in sound and light. It is measured as a ratio, or as a base-10 or base-2 logarithmic value.-Dynamic range and human perception:The human senses of sight and...
of a cassette tape by dynamically modulating
Modulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...
the ultrasonic bias signal
Tape bias
Tape bias is the term for two phenomena, DC bias and AC bias, that improve the fidelity of analogue magnetic tape sound recordings. DC bias is the addition of a direct current to the audio signal that is being recorded. AC bias is the addition of an inaudible high-frequency signal to the audio...
, used by all analogue tape decks, to increase the headroom for high-frequency audio signals. This system was modified by Bang & Olufsen
Bang & Olufsen
Bang & Olufsen is a Danish company that designs and manufactures audio products, television sets and telephones. It was founded in 1925 by Peter Bang and Svend Olufsen, whose first significant product was a radio that worked with alternating current, when most radios were run from batteries...
for consumer equipment and marketed by Dolby as Dolby HX Pro. HX and HX Pro are used only in recording.
Because magnetic tape is magnetic, it is inherently non-linear in nature, due to the hysteresis
Hysteresis
Hysteresis is the dependence of a system not just on its current environment but also on its past. This dependence arises because the system can be in more than one internal state. To predict its future evolution, either its internal state or its history must be known. If a given input alternately...
of the magnetic particles. If an analogue signal
Analog signal
An analog or analogue signal is any continuous signal for which the time varying feature of the signal is a representation of some other time varying quantity, i.e., analogous to another time varying signal. It differs from a digital signal in terms of small fluctuations in the signal which are...
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. With strong signals of fixed frequency and high amplitude, the amount of bias needed is reduced. Due to group and phase delay
Group delay and phase delay
Group delay is a measure of the time delay of the amplitude envelopes of the various sinusoidal components of a signal through a device under test, and is a function of frequency for each component...
the audio signal itself creates a variable amount of self-bias. If the added bias remains constant, these high frequency signals become overbiased. This overbias creates distortion as the tape becomes saturated. Dolby HX Pro automatically reduces the bias signal in the presence of strong high frequency signals. This optimises the amount of self bias, reducing distortion caused from saturation of the magnetic tape. By adjusting the bias with respect to group and phase delay the overall distortion of high frequency signals is also greatly reduced. This kind of bias adjustment increases the high frequency dynamic range available. The net effect for the listener is a crisper sounding high frequency reproduction.
Technological obsolescence
Dolby's analogue noise reduction systems, though still used in some professional applications, as well as in the large installed base of consumer tape decks, are becoming increasingly obsolete due to the widespread adoption of digital audioDigital audio
Digital audio is sound reproduction using pulse-code modulation and digital signals. Digital audio systems include analog-to-digital conversion , digital-to-analog conversion , digital storage, processing and transmission components...
(in the form of compact disc
Compact Disc
The Compact Disc is an optical disc used to store digital data. It was originally developed to store and playback sound recordings exclusively, but later expanded to encompass data storage , write-once audio and data storage , rewritable media , Video Compact Discs , Super Video Compact Discs ,...
s, MP3
MP3
MPEG-1 or MPEG-2 Audio Layer III, more commonly referred to as MP3, is a patented digital audio encoding format using a form of lossy data compression...
s, MiniDisc
MiniDisc
The disc is permanently housed in a cartridge with a sliding door, similar to the casing of a 3.5" floppy disk. This shutter is opened automatically by a mechanism upon insertion. The audio discs can either be recordable or premastered. Recordable MiniDiscs use a magneto-optical system to record...
s, and to a lesser extent DAT
Digital Audio Tape
Digital Audio Tape is a signal recording and playback medium developed by Sony and introduced in 1987. In appearance it is similar to a compact audio cassette, using 4 mm magnetic tape enclosed in a protective shell, but is roughly half the size at 73 mm × 54 mm × 10.5 mm. As...
) in the home for entertainment and professional studios for recording. In other words, Dolby NR is not becoming obsolete for analog recording, but analog recording itself is becoming obsolete as digital recording replaces it.
See also
- Dolby SR (Spectral Recording)
- Dynamic Noise Reduction (DNR), a playback-only noise reduction system often confused with Dolby
- dbx (noise reduction)Dbx (noise reduction)dbx is a family of noise reduction systems developed by the company of the same name. The most common implementations are dbx Type I and dbx Type II for analog tape recording and, less commonly, vinyl LPs. A separate implementation, known as dbx-TV, is part of the MTS system used to provide stereo...
, a rival system
External links
- More information available at Dolby Laboratories, Inc.