Dolby Virtual Speaker Technology: Fundamental Principles

Transcription

1 Dolby Virtual Speaker Technology: Fundamental Principles Introduction More and more consumers are eager to recreate the movie theatre surround-sound experience in their homes. However, the multispeaker home theater systems necessary to do this are not always practical or affordable in many applications that would benefit from surround sound. Dolby Virtual Speaker technology is a practical new alternative to multispeaker systems. Using a new-generation virtualizing technique, it provides convincing multichannel surround sound with just two stereo speakers. It transforms such activities as watching TV and movies, or listening to music, into an accurate, natural, and realistic surround-sound experience, without the added expense and complexity of a traditional surround system. Dolby Virtual Speaker technology is a proprietary process that can be included in a wide variety of products (such as DVD players, TV sets, PCs, personal speaker systems, and home theater systems) to provide a true-to-life 5.1 surround-sound experience with just two speakers. Working in conjunction with Dolby Digital and Dolby Pro Logic II decoding, it enhances the playback of any program material, from 5.1-channel DVDs and satellite programming to stereo music CDs and MP3 files. A New Solution Typical home theater surround systems have five main speakers, plus an optional subwoofer. Depending on the program content and the type of surround decoding used, Left, Center, and Right channels are reproduced by speakers across the front of the listening area, while surround information is reproduced by two additional speakers at the sides or rear. Dolby Laboratories, Inc. 100 Potrero Avenue, San Francisco, CA Telephone Fax Wootton Bassett, Wiltshire SN4 8QJ, England Telephone (44) Fax (44) Dolby, Pro Logic, and the double-d symbol are registered trademarks of Dolby Laboratories Dolby Laboratories, Inc. All rights reserved. S02/14404/14469

2 Some smaller-scale surround systems, such as personal speaker systems for PCs, use four speakers Left and Right front speakers, plus Left and Right Surrounds and rely on the front speakers to create an effective phantom Center channel. However, doing without surround speakers and somehow convincing the listener that surround information is coming from the sides and/or rear of the listening area is much more difficult when using only two speakers. Dolby Virtual Speaker technology uses Dolby s proprietary technique to create a virtualized surround sound stage. It is so powerful and advanced that with just two speakers it convincingly creates the illusion of having precisely placed surround speakers in the room, creating a true-to-life 5.1 surround-sound experience. How Sound Reaches Our Ears To understand how Dolby Virtual Speaker technology achieves its advanced virtualization, it s important to first understand how sound travels from a sound source, such as a speaker, to our ears. As shown in Figure 1, some of the sound from the speaker reaches the listener directly, following a slighter shorter path to one ear than the other (A). Some of the sound also reflects off the walls of the room, following longer paths to the ears than the direct sound, thereby arriving slightly later (B). The reflected sounds are also lower in level than the direct sound, and differ in spectral balance. Figure 1 Direct and Reflected Sound Paths What we hear, therefore, consists of two independent composites of direct and reflected sound, one that reaches the left ear and one that reaches the right ear. By comparing and analyzing these two composites, or sonic signatures, the brain is able to determine where the sound originates, and the size and acoustic character of the environment. 2

3 It follows, then, that if these two sonic signatures could be reproduced with sufficient detail by just two speakers, it would seem to the listener that some of the sound is originating from sources elsewhere in the room, such as surround speakers placed to the sides towards the rear. This is exactly what Dolby Virtual Speaker technology achieves. Crosstalk Cancellation Virtualizing multiple sound sources over two speakers requires not only recreating the sonic signatures for each ear (discussed later), but also ensuring that neither ear can hear the signature intended for the other. In other words, the left ear must hear only the left speaker, and the right ear must hear only the right speaker. To accomplish this, Dolby Virtual Speaker technology uses a technique called crosstalk cancellation. Figure 2 shows how crosstalk cancellation prevents sound from the right speaker from reaching the left ear. A delayed, mirror image of the right speaker s sound is generated by the left speaker (1). When the mirror image reaches the left ear, it is perfectly time aligned, but of opposite phase, with the sound that arrives at the left ear from the right speaker (2), thereby canceling it out. As a result, the listener hears only the sound from the right speaker that arrives at the right ear (3). Figure 2 Crosstalk Cancellation 3

4 To experience the full effectiveness of this cancellation, the listener needs to be in a listening zone equidistant from each speaker as shown in Figure 3. As a result, Dolby Virtual Speaker technology is particularly appropriate for such activities as using a PC or casual TV viewing just the kind of applications where a full surround system might be unsuitable or impractical. Figure 3 Listening Zone Dolby Virtual Speaker uses crosstalk cancellation to independently control the acoustic image delivered to each ear, thereby making it possible to recreate the sonic signatures of a complete multichannel system for the listener. Sonic Signatures Now that we have the mechanism for delivering sonic signatures of virtual speakers, we need to recreate them in their totality for the listener to experience the sound as the director or artist intended. That this is no easy task is illustrated by the sonic signature, or impulse response, illustrated in Figure 4. It shows what happens during a fraction of a second to a single sound impulse generated by a speaker in a room. Figure 4 Typical Sonic Signature The first peak represents the sound reaching the ear via the direct path from the speaker, while the successive peaks are reflected sounds arriving at the ear via successively longer paths. For the ear to identify a precise location of a virtual speaker, the entire signature must be recreated, which requires a powerful tool. 4

5 Dolby Virtual Speaker technology has that tool: a new, breakthrough proprietary technique that can recreate multiple, complex sonic signatures in their entirety, in real time. Most virtual systems on the market today limit their processing to the direct sound, ignoring 99 percent of what is needed to deliver an accurate, natural, and realistic hometheater experience. As a result, these systems are unable to even approach the realism achieved by Dolby Virtual Speaker technology. Benefits Only two speakers needed. Dolby Virtual Speaker technology saves the cost of the three or more additional speakers required for conventional surround systems. The consumer doesn t have to find room for the extra speakers, or go to the trouble of hooking them up. It can also be used with stereo (two-channel) amplifiers, receivers, soundcards, TV sets, and so on, saving the consumer still more money. Precise localization. The apparent location of the virtual speakers is extremely realistic and stable. The listener hears the sound of two discrete surround sources, located at 105 degrees on the left and 105 degrees on the right, as recommended for surround speaker placement in a multispeaker home-theater system. The result with Dolby Virtual Speaker technology is convincingly realistic surround sound. Spatial cues. By delivering complete sonic signatures, Dolby Virtual Speaker gives the brain all of the information necessary to perceive the depth of the environment, a major innovation in virtualization. The listener not only hears the virtualized sound sources, but also hears these sources as though generated by speakers in a room, making the surround listening experience more natural and realistic. High sound quality. Dolby Virtual Speaker technology uses special filters to compensate for the spectral modifications inevitable in signal processing and virtualization techniques. As a result, the integrity and timbre of the program content is faithfully maintained, and the listener hears the program as intended by its producers, with a realistic and natural surround effect. Multichannel content heard as intended by its producers and sound mixers. With Dolby Virtual Speaker, the sound from each channel is placed just as it was when the soundtrack was mixed. A sound assigned to the left rear, for example, will be heard coming from the left rear. Stereo content in realistic surround. Dolby Virtual Surround technology works in conjunction with Dolby Pro Logic II to create a rich and enveloping surroundsound experience from stereo sources such as CDs, MP3 files, and FM broadcasts. 5

6 Long-term listening comfort. By recreating the full acoustic characteristics of multiple speakers in a real environment, Dolby Virtual Speaker technology ensures long-term listening comfort. This is a great improvement on other virtualizer systems that cause considerable listening fatigue over time because of their use of raw and oversimplified crosstalk-cancellation techniques. Selectable listening modes. Dolby Virtual Speaker technology provides the listener a choice of surround environments (described below). Listening modes Dolby Virtual Speaker offers two virtualization modes that differ in surround effect and apparent speaker localization. Each mode may be used with any type of program content. Reference mode (shown in Figure 5) creates convincingly realistic five-speaker surround sound from two speakers with the apparent width of the sound across the front image defined by the distance between the two speakers. Figure 5 Reference Mode 6

7 Wide mode (shown in Figure 6) provides a wider, more spacious front image when the two speakers are close together, with the same convincingly realistic five-speaker surround sound as the Reference mode. Figure 6 Wide Mode Conclusion Through its powerful and advanced proprietary techniques, Dolby Virtual Speaker technology transforms two-speaker listening into an exciting, richly enveloping true-tolife 5.1 surround-sound experience. Dolby Virtual Speaker delivers the most convincing and realistic surround-sound experience possible using only two speakers. 7