The Dolby Headphone Story

Dolby^® Headphone technology works by creating an acoustic illusion: it makes you think you are hearing a five-speaker home theater system, when in fact you are listening over ordinary stereo headphones.

The illusion is based on a fundamental understanding of how we are able to judge which direction a sound is coming from, how far away it is, and whether it is in motion. We can even locate and track many sounds at the same time.

We can do all this because the sound from a given source arrives at each ear with slightly different characteristics. Our brain needs only those slight differences to derive all we need to know about the sound. Our vision works in much the same way, with the brain deriving a three-dimensional image from the slight differences in the views provided by our two eyes.

As understanding of the mechanisms behind the human hearing system increased, audio researchers in the late 1960s and early 1970s began to speculate that electronic signal processing might someday be able to trick the brain into perceiving sounds as originating some distance away, from a particular direction, when in fact only headphones were being used.

It turned out to be much more difficult to achieve a convincing impression of distance and positioning than those early researchers anticipated.

Early Attempts

Studies of human hearing revealed that our head and shoulders imprint a unique signature on a sound arriving from a particular direction. Sounds coming from the left, for instance, arrive at the right ear slightly later than at the left ear, and at a lower level. There is also a filtering effect caused by acoustic shadowing of the head.

Researchers found that these subtle time, level, and frequency response differences, called head-related transfer functions or HRTFs, are all the brain needs to determine the origin and motion of a sound. As it became possible to measure HRTFs with a high degree of accuracy using microphones and sophisticated instruments, researchers began to envision how signal processing systems might be able to duplicate HRTFs electronically.

When the HRTFs of a large number of subjects were measured, however, it was found that they varied widely due to differences in the distance between each person's ears, and in the shapes of their ears and head. As a result, when a particular HRTF was replicated on a computer with signal processing software, the sound appeared to come from the correct direction only for the listener with that particular HRTF. The orientation varied widely with other listeners.

This discovery dashed the hopes of researchers for a practical, headphone-based 3D audio system that would work equally well for everyone and be incorporated in widely available consumer products. But then, in the early 1990s, Lake Technology Limited in Australia made a series of discoveries in signal processing and psychoacoustics that appeared to offer hope of a solution.

Dolby Headphone Technology is born

In 1996, Lake undertook a determined effort to expand the applications of its unique, and by now patented, signal processing systems. A strong impetus was the digital multichannel surround-sound technology, originally developed for movie theaters, that Dolby Laboratories was then introducing to home theater via Laserdisc and the embryonic DVD format.

In 1997, Lake Personal Surround, the technology underlying what is now known as Dolby Headphone, was born. Lake approached Dolby Laboratories as a potential conduit for making their technology available to Dolby licensees. Impressed with what Lake had accomplished, Dolby Laboratories agreed to assist Lake in further refining the process and making it available to consumer electronics and PC manufacturers under the umbrella of the well-established Dolby licensing program.

The result is Dolby Headphone technology, a cost-effective signal-processing system that provides a convincing simulation of a five-speaker surround system on any pair of stereo headphones.

The Breakthroughs

Dolby Headphone technology provides several advantages over earlier attempts to provide surround sound via headphones.

First of all, Dolby Headphone technology simulates the sound of speakers that are clearly and unambiguously located outside the listener's head. With conventional headphone listening, audio images are perceived as inside the head. This is because there are none of the cues that acoustics impart to the sound of loudspeakers in a room. Dolby Headphone technology, however, simulates the complex acoustics of a listening room where sounds reflect off various surfaces, effectively convincing listeners that the sound is originating from multiple speakers around them. Moreover, it can simulate a variety of rooms with different acoustics.

Secondly, Dolby Headphone technology is universal to all listeners. Due to a significant breakthrough in signal-processing technology, the system does not require custom HRTF settings to accommodate the differences between individuals, making it simple to implement and operate.

A third advantage is relative simplicity, allowing Dolby Headphone technology to be included in consumer products with no loss of quality. It can be implemented via a wide range of digital signal-processing (DSP) chips, opening the potential for implementation wherever headphones are used.

A fourth advantage of Dolby Headphone technology is that even though it provides all the benefits of a highly detailed finite-impulse response (FIR) filter, it does so with virtually no latency. This ensures video programs (stereo or multichannel) can maintain sync between sound and picture.

Finally, no special headphones are required. The process works well with everything from inexpensive airline headsets to high-end electrostatics. Although higher quality headphones give a higher quality experience. Lake studied headphone performance intensively and developed testing and optional compensation methods for optimal performance on all types of equipment.

Applications

The Dolby Headphone process can be embedded into DSP chips or implemented in software for use home theater systems, digital TV, personal computers, and portable equipment where headphones are used exclusively. Special application-specific integrated circuit (ASIC) design that offers all the performance at less cost than a typical DSP solution. It enables the enjoyment of high-quality, cinema-style surround sound anytime, anywhere, without disturbing family or neighbors.

Dolby Headphone technology was designed from the outset to encompass all possible audio formats. Conventional stereo, Dolby Surround, and 5.1-channel digital content can all be enhanced for headphone reproduction by the process. The result is far more natural and less fatiguing sound over any conventional stereo headset on all programs, from 5.1-channel DVDs to stereo MP3s.