80-403: 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 system for recording studios that was first demonstrated in 1965, but the best-known is Dolby B (introduced in 1968), a sliding band system for
160-490: A 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 approximately +3 VU on the VU meter(s). In consumer equipment, Dolby Level is defined as 200 nWb/m , and calibration tapes were available to assist with
240-405: 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
320-400: 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 of 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,
400-663: A de-emphasis process applied at playback. Systems include the professional systems Dolby A and Dolby SR by Dolby Laboratories , dbx Professional and dbx Type I by dbx , Donald Aldous' EMT NoiseBX, Burwen Noise Eliminator [ it ] , Telefunken 's telcom c4 [ de ] and MXR Innovations' MXR as well as the consumer systems Dolby NR , Dolby B , Dolby C and Dolby S , dbx Type II , Telefunken's High Com and Nakamichi 's High-Com II , Toshiba 's (Aurex AD-4) adres [ ja ] , JVC 's ANRS [ ja ] and Super ANRS , Fisher / Sanyo 's Super D , SNRS , and
480-400: A de-emphasis process applied during playback. Modern digital sound recordings no longer need to worry about tape hiss so analog-style noise reduction systems are not necessary. However, an interesting twist is that dither systems actually add noise to a signal to improve its quality. Dual-ended compander noise reduction systems have a pre-emphasis process applied during recording and then
560-521: A decoder. The Telefunken High Com integrated circuit U401BR could be utilized to work as a mostly Dolby B –compatible compander as well. In various late-generation High Com tape decks the Dolby-B emulating D NR Expander functionality worked not only for playback, but, as an undocumented feature, also during recording. dbx was a competing analog noise reduction system developed by David E. Blackmer , founder of Dbx, Inc. It used
640-418: A device's mechanism or signal processing algorithms . In electronic systems , a major type of noise is hiss created by random electron motion due to thermal agitation. These agitated electrons rapidly add and subtract from the output signal and thus create detectable noise . In the case of photographic film and magnetic tape , noise (both visible and audible) is introduced due to the grain structure of
720-529: A greater or lesser degree. The local signal-and-noise orthogonalization algorithm can be used to avoid changes to the signals. Noise reduction techniques in Digital Signal Processing (DSP) are essential for improving the quality of signals in various applications, including audio processing, telecommunications, and biomedical engineering. Noise, which is unwanted random variation in signals, can degrade signal clarity and accuracy. DSP offers
800-415: 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 hi-hat cymbals ), this adds to the constant bias causing magnetic saturation on the tape. Dynamic, or adaptive, biasing automatically reduces the bias signal in
880-585: A high-level stage and a low-level stage) staggered action arrangement of series-connected compressors and expanders, with an extension to lower frequencies than with Dolby B. As in Dolby ;B, a "sliding band" technique (operating frequency varies with signal level) helps to suppress undesirable breathing , which is often a problem with other noise reduction techniques. As a result of the extra signal processing, Dolby C-type recordings will sound distorted when played back on equipment that does not have
SECTION 10
#1732772859469960-418: A maximum volume they can record, so already-loud sounds will become distorted. The idea is to increase the volume of the recording only when the original material is not already loud, and then reduce the volume by the same amount on playback so that the signal returns to the original volume levels. When the volume is reduced on playback, the noise level is reduced by the same amount. This basic concept, increasing
1040-450: A priority period of several years for use in consumer products, to protect their own Beocord 9000 cassette tape deck. By the middle of the 1980s the Bang & Olufsen system, marketed through Dolby Laboratories, became an industry standard under the name of Dolby HX Pro. HX-Pro only applies during the recording process. The improved signal-to-noise ratio is available no matter which tape deck
1120-500: A range of algorithms to reduce noise while preserving the integrity of the original signal. Spectral subtraction is one of the simplest and most widely used noise reduction techniques, especially in speech processing. It works by estimating the power spectrum of the noise during silent periods and subtracting this noise spectrum from the noisy signal. This technique assumes that noise is additive and relatively stationary. While effective, spectral subtraction can introduce "musical noise,"
1200-456: A root-mean-squared (RMS) encode/decode algorithm with the noise-prone high frequencies boosted, and the entire signal fed through a 2:1 compander. dbx operated across the entire audible bandwidth and unlike Dolby B was unusable without a decoder. However, it could achieve up to 30 dB of noise reduction. Since analog video recordings use frequency modulation for the luminance part (composite video signal in direct color systems), which keeps
1280-408: A tape speed slightly in excess of 30 inches per second (76.8 cm/sec). The AC biased Magnetophon machines reduced the harmonic distortion to well under 3 percent; extended the dynamic range to 65 dB and the frequency response was now from 40 Hz to 15 kHz at the same tape speed. These AC biased magnetophons provided a fidelity of recording that outperformed any other recording system of
1360-419: A type of artificial noise, if the noise spectrum estimate is inaccurate. Applications: Primarily used in audio signal processing, including mobile telephony and hearing aids. Advantages: Simple to implement and computationally efficient. Limitations: Tends to perform poorly in the presence of non-stationary noise, and can introduce artifacts. Adaptive filters are highly effective in situations where noise
1440-419: A variety of sources. Further use of these images will often require that the noise be reduced either for aesthetic purposes, or for practical purposes such as computer vision . Tape bias Tape bias is the term for two techniques, AC bias and DC bias, that improve the fidelity of analogue tape recorders . DC bias is the addition of direct current to the audio signal that is being recorded. AC bias
1520-472: A working machine that could record sound. The earliest magnetic recording systems simply applied the unadulterated ( baseband ) input signal to a recording head, resulting in recordings with poor low-frequency response and high distortion. Within short order, the addition of a suitable direct current to the signal, a DC bias, was found to reduce distortion by operating the tape substantially within its linear-response region. The principal disadvantage of DC bias
1600-497: Is a statistical approach to noise reduction that minimizes the mean square error between the desired signal and the actual output. This technique relies on knowledge of both the signal and noise power spectra, and it can provide optimal noise reduction if these spectra are accurately estimated. Applications: Frequently applied in image processing, audio restoration, and radar. Advantages: Provides optimal noise reduction for stationary noise. Limitations: Requires accurate estimates of
1680-575: Is an advanced noise reduction technique that uses redundancy in the signal by averaging similar patches across the signal or image. While computationally more demanding, NLM is highly effective in removing noise from images and audio signals without blurring. Applications: Applied primarily in image denoising, especially in medical imaging and photography. Advantages: Preserves details and edges in images. Limitations: Computationally intensive, often requiring hardware acceleration or approximations for real-time applications. Boosting signals in seismic data
SECTION 20
#17327728594691760-494: 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. Magnetic tape is inherently non-linear in nature due to hysteresis of the magnetic material. If an analog signal were recorded directly onto magnetic tape, its reproduction would be extremely distorted due to this non-linearity . To overcome this,
1840-530: Is concentrated about it. Yet another approach is the automatic noise limiter and noise blanker commonly found on HAM radio transceivers, CB radio transceivers, etc. Both of the aforementioned filters can be used separately, or in conjunction with each other at the same time, depending on the transceiver itself. Most digital audio workstations (DAWs) and audio editing software have one or more noise reduction functions. Images taken with digital cameras or conventional film cameras will pick up noise from
1920-537: Is considered compatible with Dolby B. JVC eventually abandoned the ANRS standard in favor of official Dolby B support; some JVC decks exist whose noise-reduction toggles have a combined "ANRS / Dolby B" setting. 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
2000-492: Is effective from approximately 1 kHz upwards; the noise reduction that is provided is 3 dB at 600 Hz, 6 dB at 1.2 kHz, 8 dB at 2.4 kHz, and 10 dB at 5 kHz. The width of the noise reduction band is variable, as it is designed to be responsive to both the amplitude and the frequency distribution of the signal. It is thus possible to obtain significant amounts of noise reduction down to quite low frequencies without causing audible modulation of
2080-400: Is especially crucial for seismic imaging , inversion, and interpretation, thereby greatly improving the success rate in oil & gas exploration. The useful signal that is smeared in the ambient random noise is often neglected and thus may cause fake discontinuity of seismic events and artifacts in the final migrated image. Enhancing the useful signal while preserving edge properties of
2160-592: Is frequently confused with the far more common Dolby noise-reduction system . Unlike Dolby and dbx Type I and Type II noise reduction systems, DNL and DNR are playback-only signal processing systems that do not require the source material to first be encoded. They can be used to remove background noise from any audio signal, including magnetic tape recordings and FM radio broadcasts, reducing noise by as much as 10 dB. They can also be used in conjunction with other noise reduction systems, provided that they are used prior to applying DNR to prevent DNR from causing
2240-417: Is the addition of an inaudible high-frequency signal (generally from 40 to 150 kHz ) to the audio signal. Most contemporary tape recorders use AC bias. When recording, magnetic tape has a nonlinear response as determined by its coercivity . Without bias, this response results in poor performance, especially at low signal levels. A recording signal that generates a magnetic field strength less than
2320-564: Is the process of removing noise from a signal . Noise reduction techniques exist for audio and images. Noise reduction algorithms may distort the signal to some degree. Noise rejection is the ability of a circuit to isolate an undesired signal component from the desired signal component, as with common-mode rejection ratio . All signal processing devices, both analog and digital , have traits that make them susceptible to noise. Noise can be random with an even frequency distribution ( white noise ), or frequency-dependent noise introduced by
2400-424: Is to define a dynamic threshold for filtering noise, that is derived from the local signal, again with respect to a local time-frequency region. Everything below the threshold will be filtered, everything above the threshold, like partials of a voice or wanted noise , will be untouched. The region is typically defined by the location of the signal's instantaneous frequency, as most of the signal energy to be preserved
2480-653: Is unpredictable or non-stationary. In adaptive filtering, the filter's parameters are continuously adjusted to minimize the difference between the desired signal and the actual output. The Least Mean Squares (LMS) and Recursive Least Squares (RLS) algorithms are commonly used for adaptive noise cancellation. Applications: Used in active noise-canceling headphones, biomedical devices (e.g., EEG and ECG processing), and communications. Advantages: Can adapt to changing noise environments in real-time. Limitations: Higher computational requirements, which may be challenging for real-time applications on low-power devices. Wiener filtering
Dolby noise-reduction system - Misplaced Pages Continue
2560-408: The 1980s. It was called RMS (from Rauschminderungssystem , English: "Noise reduction system"). The Dolby C-type noise reduction system was developed in 1980. It provides about 15 dB noise reduction ( A-weighted ) in the 2 kHz to 8 kHz region where the ear is highly sensitive and most tape hiss is concentrated. Its noise reduction effect results from the dual-level (consisting of
2640-455: 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. SR prints are fairly well backward compatible with old Dolby A equipment. The Dolby SR-D marking refers to both analog Dolby SR and digital Dolby Digital soundtracks on one print. Dolby S
2720-561: The Hungarian/East-German Ex-Ko system. In some compander systems, the compression is applied during professional media production and only the expansion is applied by the listener; for example, systems like dbx disc , High-Com II , CX 20 and UC used for vinyl recordings and Dolby FM , High Com FM and FMX used in FM radio broadcasting. The first widely used audio noise reduction technique
2800-438: 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
2880-461: 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, although Dolby has as of 2016 ceased licensing the technology for new cassette decks. Of the noise reduction systems, Dolby A and Dolby SR were developed for professional use. Dolby B , C , and S were designed for
2960-485: The consumer market. Aside from Dolby HX , all the Dolby variants work by companding : 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 to this issue is to use low-noise tape, which records more signal, and less noise. Other solutions are to run
3040-495: The effective headroom of the cassette tape system. As a result, recordings are cleaner and crisper with a much improved high-frequency response that the cassette medium heretofore lacked. With a good quality tape, the Dolby C response could be flat to 20 kHz at the 0 dB recording level, a previously unattainable result. An A-weighted signal-to-noise ratio of 72 dB (re 3% THD at 400 Hz) with no unwanted "breathing" effects, even on difficult-to-record passages,
3120-463: The first), cassette hardware supporting Dolby B and cassettes encoded with it would be labeled simply "Dolby System," "Dolby NR", or wordlessly with the Dolby symbol. This continued in some record labels and hardware manufacturers even after Dolby C had been introduced, during the period when the new standard was relatively little-known. JVC 's ANRS [ ja ] system, used in place of Dolby B on earlier JVC cassette decks,
3200-450: The incoming off-tape signal and noise. After playback de-emphasis is complete, the apparent noise in the output signal is reduced, and this process should not produce any other effect noticeable to the listener other than reduced background noise. However, playback without noise reduction produces a noticeably brighter sound. The correct calibration of the recording and playback circuitry is critical in order to ensure faithful reproduction of
3280-410: The low level of tape noise with no signal. When the music is loud, the low background hiss level is not noticeable, but when the music is soft or in silence, most or all of what can be heard is the noise. If the recording level is adjusted so that the music is always loud, then the low-level noise would not be audible. One cannot simply increase the volume of the recording to achieve this end; tapes have
Dolby noise-reduction system - Misplaced Pages Continue
3360-468: The medium. In photographic film, the size of the grains in the film determines the film's sensitivity, more sensitive film having larger-sized grains. In magnetic tape, the larger the grains of the magnetic particles (usually ferric oxide or magnetite ), the more prone the medium is to noise. To compensate for this, larger areas of film or magnetic tape may be used to lower the noise to an acceptable level. Noise reduction algorithms tend to alter signals to
3440-452: The noise by the signal ("breathing"). From the mid-1970s, Dolby B became standard on commercially pre-recorded music cassettes even though 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. Prior to the introduction of later consumer variants (Dolby C being
3520-473: The noise reduction unit is set to 0 VU on the tape recorder and to 185 nWb/m on the tape. The Dolby A-type system also saw some use as the method of noise reduction in optical sound for motion pictures. Dolby B-type noise reduction was developed after Dolby A, and it was introduced in 1968. It consisted of a single sliding band system providing about 9 dB of noise reduction ( A-weighted ), primarily for use with cassette tapes . It
3600-458: The original patent, and Bell simply kept silent about their rediscovery of AC bias. Teiji Igarashi, Makoto Ishikawa, and Kenzo Nagai of Japan published a paper on AC biasing in 1938 and received a Japanese patent in 1940. Marvin Camras (USA) also rediscovered high-frequency (AC) bias independently in 1941 and received a patent in 1944. The reduction in distortion and noise provided by AC bias
3680-431: The original program content. The calibration can easily be upset by poor-quality tape, dirty or misaligned 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 volume level of the signal varies. On some high-end consumer equipment,
3760-409: The original signal volume. For instance, in Dolby B, a low-level signal will be boosted by 10 dB , while signals at the "Dolby Level", +3 VU , receive no signal modification at all. Between the two limits, a varying level of pre-emphasis is applied. On playback, the opposite process is applied (de-emphasis), based on the relative signal component above 1 kHz. Thus, as this portion of
3840-645: The other noise reduction system to mistrack. One of DNR's first widespread applications was in the GM Delco car stereo systems in US GM cars introduced in 1984. It was also used in factory car stereos in Jeep vehicles in the 1980s, such as the Cherokee XJ . Today, DNR, DNL, and similar systems are most commonly encountered as a noise reduction system in microphone systems. A second class of algorithms work in
3920-523: The playback of phonograph records to address scratches, pops, and surface non-linearities. Single-ended dynamic range expanders like the Phase Linear Autocorrelator Noise Reduction and Dynamic Range Recovery System (Models 1000 and 4000) can reduce various noise from old recordings. Dual-ended systems (such as Dolby noise-reduction system or dbx ) have a pre-emphasis process applied during recording and then
4000-500: The presence of strong high-frequency signals, making it possible to record at a higher signal level. The original Dolby HX, where HX stands for Headroom eXtension , was invented in 1979 by Kenneth Gundry of Dolby Laboratories, and was rejected by the industry for its inherent flaws. Bang & Olufsen continued work in the same direction, which resulted in a 1981 patent (EP 0046410) by Jørgen Selmer Jensen. Bang & Olufsen immediately licensed HX-Pro to Dolby Laboratories, stipulating
4080-399: The recording medium at the time of recording. Single-ended hiss reduction systems (such as DNL or DNR ) work to reduce noise as it occurs, including both before and after the recording process as well as for live broadcast applications. Single-ended surface noise reduction (such as CEDAR and the earlier SAE 5000A, Burwen TNE 7000, and Packburn 101/323/323A/323AA and 325 ) is applied to
SECTION 50
#17327728594694160-641: The required Dolby C decoding circuitry. Some of this harshness can be mitigated by using Dolby B on playback, which serves to reduce the strength of the high frequencies. With Dolby C-type processing, noise reduction begins two octaves lower in frequency in an attempt to maintain a psychoacoustically-uniform noise floor. In the region above 8 kHz, where the ear is less sensitive to noise, special spectral-skewing and anti-saturation networks come into play. These circuits prevent cross modulation of low frequencies with high frequencies, suppress tape saturation when large signal transients are present, and increase
4240-451: 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: low-pass at 80 Hz; band-pass from 80 Hz to 3 kHz; a high-pass from 3 kHz; and another high-pass at 9 kHz. (The stacking of contributions from the two high-pass bands allows greater noise reduction in
4320-642: The seismic profiles by attenuating random noise can help reduce interpretation difficulties and misleading risks for oil and gas detection. Tape hiss is a performance-limiting issue in analog tape recording . This is related to the particle size and texture used in the magnetic emulsion that is sprayed on the recording media, and also to the relative tape velocity across the tape heads . Four types of noise reduction exist: single-ended pre-recording, single-ended hiss reduction, single-ended surface noise reduction, and codec or dual-ended systems. Single-ended pre-recording systems (such as Dolby HX Pro ), work to affect
4400-438: The signal and noise statistics, which may not always be feasible in real-world applications. Kalman filtering is a recursive algorithm that estimates the state of a dynamic system from a series of noisy measurements. While typically used for tracking and prediction, it is also applicable to noise reduction, especially for signals that can be modeled as time-varying. Kalman filtering is particularly effective in applications where
4480-404: The signal decreases in amplitude, the higher frequencies are progressively increasingly attenuated, which also reduces in level the constant background noise on the tape when and where it would be most noticeable. The two processes (pre- and de-emphasis) are intended to cancel each other out as far as the actual recorded program material is concerned. During playback, only de-emphasis is applied to
4560-666: The signal into different frequency components using a wavelet transform and then removes the noise by thresholding the wavelet coefficients. This method is effective for signals with sharp transients, like biomedical signals, because wavelet transforms can provide both time and frequency information. Applications: Commonly used in image processing, ECG and EEG signal denoising, and audio processing. Advantages: Preserves sharp signal features and offers flexibility in handling non-stationary noise. Limitations: The choice of wavelet basis and thresholding parameters significantly impacts performance, requiring careful tuning. Non-local means (NLM)
4640-427: The signal is dynamic and the noise characteristics vary over time. Applications: Used in speech enhancement, radar, and control systems. Advantages: Provides excellent performance for time-varying signals with non-stationary noise. Limitations: Requires a mathematical model of the system dynamics, which may be complex to design for certain applications. Wavelet -based denoising (or wavelet thresholding) decomposes
4720-455: The signal-to-noise ratio on tape up to 10 dB depending on the initial signal volume. When it was played back, the decoder reversed the process, in effect reducing the noise level by up to 10 dB. The Dolby B system (developed in conjunction with Henry Kloss ) was a single-band system designed for consumer products. The Dolby B system, while not as effective as Dolby A, had the advantage of remaining listenable on playback systems without
4800-478: 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
4880-429: The subject in the 8 September 1888 issue of The Electrical World as "Some possible forms of phonograph" . By 1898, Valdemar Poulsen had demonstrated a magnetic recorder and proposed magnetic tape. Fritz Pfleumer was granted a German patent for a non-magnetic "Sound recording carrier" with a magnetic coating, on 1 January 1928, Years earlier, Joseph O'Neil had created a similar recording medium, yet had not made
SECTION 60
#17327728594694960-459: The tape at a higher speed or use a wider tape. Cassette tapes were originally designed to trade off fidelity for the convenience of recording voice by using a very narrow tape running at a very slow speed of 1 + 7 ⁄ 8 in/s (4.8 cm/s) housed in a simple plastic shell when 15 in/s (38 cm/s) or 7 + 1 ⁄ 2 in/s (19 cm/s) tape speeds were for high fidelity, and 3 + 3 ⁄ 4 in/s (9.5 cm/s)
5040-438: The tape at saturation level, audio-style noise reduction is unnecessary. Dynamic noise limiter ( DNL ) is an audio noise reduction system originally introduced by Philips in 1971 for use on cassette decks . Its circuitry is also based on a single chip . It was further developed into dynamic noise reduction ( DNR ) by National Semiconductor to reduce noise levels on long-distance telephony . First sold in 1981, DNR
5120-753: 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, and S, although it does help to improve noise reduction encode/decode tracking accuracy by reducing tape 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 , music streaming) has made analog audio production less prevalent and therefore changed Dolby's focus on Dolby Vision , but Dolby's analog noise reduction systems are still widely used in niche analog production environments. Audio noise reduction Noise reduction
5200-511: The tape's coercivity cannot magnetise the tape and produces little playback signal. Bias increases the signal quality of most audio recordings significantly by pushing the signal into more linear zones of the tape's magnetic transfer function . Magnetic recording was proposed as early as 1878 by Oberlin Smith , who on 4 October 1878 filed, with the U.S. patent office, a caveat regarding the magnetic recording of sound and who published his ideas on
5280-467: The task of correct level setting. For accurate off-the-tape monitoring during recording on 3-head tape decks, both processes must be employed at once, and circuitry provided to accomplish this is marketed under the "Double Dolby" label. Dolby A-type noise reduction was the Dolby 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
5360-415: The time-frequency domain using some linear or nonlinear filters that have local characteristics and are often called time-frequency filters . Noise can therefore be also removed by use of spectral editing tools, which work in this time-frequency domain, allowing local modifications without affecting nearby signal energy. This can be done manually much like in a paint program drawing pictures. Another way
5440-406: The total amount of distortion of the original signal can be reduced and focused only on the problematic frequencies. The differences in the various Dolby products are largely evident in the precise set of frequencies that they use and the amount of modification of the original signal volume that is applied to each of the frequency bands. Within each band, the amount of pre-emphasis applied depends on
5520-522: 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 and published by the Audio Engineering Society (October 1967) and Audio (June/July 1968). As with the Dolby B-type system, correct matching of
5600-424: The volume to overwhelm inherent noise, is known as pre-emphasis, and is found in a number of products. On top of this basic concept, Dolby noise reduction systems add another improvement. This takes into account the fact that tape noise is largely heard at frequencies above 1,000 Hz. It is the lower-frequency sounds that are often loud, like drum beats, so by only applying the companding to certain frequencies,
5680-511: Was accidentally rediscovered in 1940 by Walter Weber while working at the Reichs-Rundfunk-Gesellschaft (RRG) when a DC-biased Magnetophon that he had been working on developed an 'unwanted' oscillation in its record circuitry. The last production DC biased Magnetophon machines had harmonic distortion in excess of 10 percent; a dynamic range of 40 dB and a frequency response of just 50 Hz to 6 kHz at
5760-410: Was developed by Ray Dolby in 1966. Intended for professional use, Dolby Type A was an encode/decode system in which the amplitude of frequencies in four bands was increased during recording (encoding), then decreased proportionately during playback (decoding). In particular, when recording quiet parts of an audio signal, the frequencies above 1 kHz would be boosted. This had the effect of increasing
5840-550: Was evaluated in Germany between July 1979 and December 1981 by IRT , and field-trialed up to 1984. It was based on the Telefunken High Com broadband compander system, but never introduced commercially in FM broadcasting. Another competing system was FMX , which was based on CX . A fully Dolby B-compatible compander was developed and used on many tape recorders in the former German Democratic Republic in
5920-469: 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
6000-640: 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 many inexpensive portable and car cassette players. Without the de-emphasis effect 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 an amount of more than 3 dB. The Dolby B system
6080-408: Was of lower fidelity. As a result of their narrow tracks and slow speed, cassettes make tape hiss a very severe problem. Dolby noise reduction is a form of dynamic pre-emphasis employed during recording, plus a form of dynamic de-emphasis used during playback, which work in tandem to improve the signal-to-noise ratio . The signal-to-noise ratio is simply how large the music signal is compared to
6160-502: Was possible. Dolby C first appeared on higher-end cassette decks in the 1980s. The first commercially available cassette deck with Dolby C was the NAD 6150C, which came onto the market around 1981. Dolby C 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
6240-544: Was replaced by AC bias but was later re-adopted by some very low-cost cassette recorders. The original patent for AC bias was filed by Wendell L. Carlson and Glenn L. Carpenter in 1921, eventually resulting in a patent in 1927. The value of AC bias was somewhat masked by the fact that wire recording gained little benefit from the technique and Carlson and Carpenter's achievement was largely ignored. The first rediscovery seems to have been by Dean Wooldridge at Bell Telephone Laboratories , around 1937, but their lawyers found
6320-422: Was that it left the tape with a net magnetization, which generated significant noise on replay because of the grain of the tape particles. However: the earlier wire recorders were largely immune to the problem due to their high running speed and relatively large wire size. Some early DC-bias systems used a permanent magnet that was placed near the record head. It had to be swung out of the way for replay. DC bias
6400-486: Was the AD-FF5 from Aiwa . Cassette decks with Dolby C also included Dolby B for backward compatibility, and were usually labeled as having "Dolby B-C NR". 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 maximize the recorded signal at all times using
#468531