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Dolby Noise Reduction - Reducing noise without harming the music

When the cassette tape was first introduced in the early 1960s it did not pretend to be a high-quality format. It was actually developed as a convenient format for voice recording, especially for secretarial work. In 1970, the arrival of noise reduction for cassettes changed the course of the medium. Dolby B-type both reduced tape hiss significantly and spurred many basic improvements in cassette transports, electronics and tape formulations.

Soon the cassette was sounding as good as the LP records of the day and it went on to become an extremely popular and widespread medium for recorded music — one which, even today, continues to provide an unmatched combination of affordable costs, portability, playback versatility and recording capability.

As listeners' expectations for high-quality playback grew, various improvements on the original Dolby B-type system arose to the challenge. Dolby C-type and S-type noise reduction systems apply the same principles to even more dramatically reduce tape hiss and enhancements like Dolby HX Pro headroom extension improve the quality still further.

Dolby B-type Noise Reduction

Dolby B-type noise reduction is the original Dolby system designed for consumer tape recorders. It is included in all but the least expensive cassette machines today and is used in the preparation of the vast majority of pre-recorded cassettes. It is also incorporated in many stereo VHS videocassette recorded to improve their (mono) linear audio tracks and, in a modified form, in Dolby Surround decoders. Dolby B-type provides 10 dB of noise reduction at the higher frequencies where tape hiss predominates.

Dolby C-type Noise Reduction

Dolby C-type noise reduction was developed to improve the cassette medium still further by providing twice the tape hiss reduction (20 dB) of Dolby B-type. It is offered along with Dolby B-type in a variety of mid-range and premium cassette decks and players. Dolby C-type is also used in many professional VCRs.

Dolby S-type Noise Reduction

Introduced in 1990, Dolby S-type is based upon the principles of the professional Dolby SR (Spectral Recording) process. It not only provides still more tape hiss reduction (24 dB), but also reduces low-frequency noise by 10 dB. Thus, it permits recording high-level signals at the frequency extremes more accurately and cleanly. In listening tests it compares very favourably with CD recordings, with most listeners judging the quality of Dolby S-type encoded tapes as comparable with or in some cases superior to the playback quality of the CD. Dolby S-type is still steadily gaining momentum and is being incorporated first in state-of-the-art decks for the most discerning listeners.

Dolby HX Pro

Not a noise reduction system, Dolby HX Pro makes it possible to record loud musical passages with fewer high-frequency losses and less distortion. It is available in better cassette decks and is also widely used by the recording industry to improve the quality of pre-recorded cassettes through its process of reducing the effects of tape saturation. As no encoding of the signal takes place, no playback decoding circuitry is required to realize the benefits of Dolby HX Pro.

Dolby Headphone Stereo is a new, powerful, digital signal processing system that was developed in 2002 to recreate the sound from each audio channel as if it were being played back from a speaker placed in an acoustically-designed room. In effect, Dolby Headphone Stereo creates virtual loudspeakers in a virtual room. The result is much more dynamic, listening experience: a more natural way of listening to music. Dolby Headphone has the technology to offer three impressive 'Listening rooms' which are controlled from the main unit or the in-line remote control they offer. These are:

The Studio: replicates the music played in a small acoustically-damped room, giving perfect sound

Live: recreates a more open live sound as if you were listening to your favourite band on tour

Dome: gives the impression of hearing music in a Large concert hall. Without having to be there

Dolby Headphone FAQs:

1. What is Dolby Headphone?

Dolby Headphone is a signal-processing system that enables ordinary stereo headphones to portray the sound of a five-speaker surround playback system. It does this by virtualising the sound of up to five speakers properly set in a good listening room. It can also be used to simulate a two-speaker stereo system

2. Do I need to use any particular type of headphone?

No, Dolby Headphone works well with all types of headphones, from the inexpensive headsets the airlines provide to high-end electrostatics. However, as with speakers, the better the headphones the better the overall sound

3. Is the ".1" channel on 5.1-channel material reproduced by Dolby Headphone?

Yes. During playback of 5.1 material, the low-frequency effects ".1" channel is mixed into the left and right channels prior to Dolby Headphone processing

4. Does Dolby Headphone require any special adjustments to my system?

No. Dolby Headphone works well for all listeners with normal hearing without the need for any individual adjustments or fine-tuning. This is a significant advantage over previous attempts at virtualising the sound of speakers in a room over headphones where the effect could be quite different from person to person. You will, however, need a system capable of processing audio with Dolby Headphone

5. Can Dolby Headphone be used with DVD-Audio?

Yes. Dolby Headphone can be used with any multi-channel format after it has been decoded back to PCM, including multi-channel music programmes encoded with MLP Lossless on DVD-Audio discs. This means that music lovers will be able to enjoy one of the most exciting features of DVD-Audio on the road as well as at home. For this reason, Dolby Headphone could well give added impetus to broader acceptance of this new super-high-fidelity format

6. What is Dolby Headphone Stereo?

Two-channel stereo products such as CD, MiniDisc, and MP3 and AAC players will feature a two-channel-only version of Dolby Headphone, indicated by the Dolby Headphone Stereo logo. On stereo material, just like multi-channel material, Dolby Headphone gives you a much more natural and less fatiguing listening experience, equivalent to a good two-speaker playback system in a room with good acoustics

7. Does Dolby Headphone create "surround" from stereo programmes?

No, Dolby Headphone does not synthesise quasi-surround from conventional stereo programmes. Think of Dolby Headphone simply as the virtual equivalent of a five-speaker system for multi-channel programmes and as a two-speaker system for stereo material. Just as with your actual speakers, what you hear depends upon how the material was recorded and what kind of surround decoding your playback system incorporates (if any)

8. Does Dolby Headphone work with processors that do create a surround effect?

Yes. Just like a multi-channel speaker system, Dolby Headphone can be used in conjunction with processors that are intended to give a surround effect on stereo material. For example, Dolby Surround Pro Logic II, the latest Dolby multi-channel matrix decoder, provides a wider, more enveloping sound field on many conventional stereo recordings, which Dolby Headphone (in its five-channel mode) will reproduce to great effect

9. I've heard that some airlines are showing movies with soundtracks that are pre-encoded with Dolby Headphone. Will I be able to get these movies for playback at home?

Some international airlines offer the benefits of Dolby Headphone to their passengers by showing films with soundtracks that have been specially pre-encoded with the Dolby Headphone process. This eliminates the need to add playback processors to their in-flight entertainment systems. However, there are no plans at this time to offer films with Dolby Headphone pre-encoded tracks to consumers

About Dolby Noise Reduction

While they differ in the details of their operation and the degree of noise reduction they provide, all three Dolby noise reduction systems answer the same question: how can noise be reduced without harming the music?

Unlike simple noise filters, Dolby Noise Reduction systems make no attempt to remove noise once it has been mixed in with the music. Rather, it prevents noise from being added to the music as it is recorded in the first place.

This two-step process first encodes the music when it is recorded and then decodes it when the tape is played back. This is why the Dolby noise reduction system in a cassette recorder should be switched on both when you record a cassette and when you play it back.

In recording, the Dolby NR circuit makes the quiet parts of the music (which are most susceptible to noise), louder than normal. When the encoded tape is played back, the Dolby NR circuit is switched around to in turn lower the previously boosted parts of the music.

This automatically lowers any noise added to the music by the recording process and it restores the music to its original form so that nothing is changed or lost but the noise.

As simple as it sounds in theory, highly sophisticated technology is used in all Dolby NR systems to ensure their unique combination of effective noise reduction and freedom from side effects.

Created: 10th January 2007
Modified: 2nd April 2007

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