Dolby Headphone Technology
An Overview

Potentially, any multichannel sound source - Dolby Digital, Dolby Surround, or DTS Surround Sound - can be used in conjunction with Dolby Headphone to be converted into a special 2-channel Dolby Headphone soundtrack.

The resultant encoded two-channel soundtrack contains manipulated audio signals with sonic spatial cues by applying varying delays and phase-shifts between the different channels.

The Dolby headphone process combines this with other ambient information - like the characteristics of a Live Concert Hall - to trick the ears into believing that sound is coming from a real multichannel home theater loudspeaker system.

This means that sound no longer appears to be coming from inside your head, but rather from in front and around you in a similar manner to what you experience with a five-speaker system setup. There is one major difference though...

It is not the perfect headphone technology, but unlike a multi-channel speaker setup where the sound you hear is dependent on room variables and your seating position, with Dolby Headphone technology, you always have the best seat! What you hear is independent of variables such as room acoustics, listening position, etc.

To understand the benefits associated with this virtual surround headphone technology, one needs to first understand the major limitation associated with the use of an ordinary pair of stereo-headphones.

Normal headphones produce a rather unnatural in-the-head sound effect. Sound appears to be coming from within rather than from in front and around. Yet the mind knows very well that sound cannot be coming from inside the head and therefore no longer believes the sound the ears are hearing. Exposure to this unnatural sound effect for an extended period may eventually lead to what is referred to as 'listener fatigue'.

Diagram courtesy of Dolby Laboratories

This is a state of mental fatigue caused by the cognitive dissonance the mind experiences. In other words, this creates a feeling of uncomfortable tension in the mind as a result of the conflict between what the mind knows and what instead appears to be.

In contrast to this headphone listening experience, when we listen to sound originating from a set of speakers, we hear sound coming from the speakers' positions as well as sound reflected off any side or rear of the room.

The combined effect of direct and reflected sound is unique for each speaker in the system, and is dependent on the speaker and the listener's position, as well as the room acoustic characteristics.

This means that by the time sound from a specific speaker reaches the ear, it acquires a unique acoustical signature. These direct and reflected sounds provide spatial information to the brain about the speaker location, room dimensions, as well as other ambient information.

A correct speaker placement can provide enough spatial cues to the brain as a result of direct and reflected sounds, to enable the exact location of sounds in the horizontal and vertical planes. This creates what is referred to as a three-dimensional soundstage.

This three-dimensional soundstage provides a far richer sound experience than when hearing sound from just two distinct locations of the speakers, which is basically the case with the standard stereo headphone.
In the case of a 5.1 multichannel home theater speaker system, the soundstage is further enhanced by the addition of the extra channels. These enable the listener to precisely locate any sound in practically any location in the home theater room.

In other words, when we listen to a system of home theater loudspeakers, we hear the sound coming from various directions: front, from the left, from the right, from either side, or from somewhere in the middle of the room. This contrasts heavily with the in-the-head effect produced by an ordinary pair of stereo headphones.

Dolby Headphone technology makes use of powerful digital signal processing techniques. The aim is to manipulate the audio signal from a multichannel sound source - so that when it is reproduced by a pair of headphones, the resultant sound imaging appears as though one is listening to different virtual loudspeakers in a virtual room.

How it Works - Dolby Headphone Encoding?

As already indicated earlier on, the whole process makes use of powerful digital signal processing algorithms as defined by Dolby Headphone™.

It takes a multichannel audio source (up to 5-channels), and adds to it the necessary spatial and ambient information effects through simulated direct and reflected sounds, and outputs a special two-channel encoded audio signal that is reproduced through a pair of conventional stereo headphones.

The Dolby surround sound headphone process is based on the fact that sound emanating from different sources in a room would reach the ears at slightly different volumes and in different times. This applies to both direct and reflected sounds. The process combines all this information into two encoded channels. These deliver a unique mix of direct and reflected sounds from each of the five virtual speakers to each of the listener's ear - through the use of conventional stereo headphones.

Diagram courtesy of Dolby Laboratories

This means that by exploiting this differential effect between the various audio channels, Dolby Headphone technology is able to re-create a virtual acoustic environment of a 5-channel loudspeaker home theater environment over a two channel stereo headphone pair.

The leads to a more natural out-of-the-head sound effect that allows the listener to enjoy a more complete three-dimensional soundstage. The result is an enjoyable headphone listening experience that is much easier on the mind and hence less likely to result in 'listener fatigue'.

Dolby Headphone in Stereo Mode

Technically speaking, Dolby Headphone stereo mode is simply a subset of the five channel experience in that only the left and right channels are active. Through the same process described above, the Dolby stereo algorithm re-creates an encoded two-channel audio sound track to deliver a virtual soundstage as recreated by a two-speaker stereo step-up.

This yields a wide and detailed soundstage that appears in front of the Dolby Headphone listener - very much like performers and instruments spread across a stage in a live performance.  This works with virtually any 2-channel program source, such as audio CD, MP3-encoded file, and WMA-encoded file (Windows Media Audio).

We only discussed the 5-channels in a 5.1 home theater sound setup because 'sound imaging' is not much of an issue with low bass frequencies as these are direction independent.

In a Dolby-encoded Headphone sound track, the low frequency effect channel - also referred to as the [.1] - is simply superimposed over the Dolby Headphone sound track.

Different Simulated Acoustic Environments

Dolby DSP based technology for headphone listening caters for up to three possible simulated environments or 'listening room' options:

DH1: The 'Reference Room' Setting

This recreates the acoustic environment of a small, well-damped room that is ideal for a home theater.  It is provided on all Dolby Headphone technology equipped systems and is most appropriate for both movies and music-only recordings.

DH2: Live 'Wet' Room

This setting simulates a medium-sized live (or 'wet') room, which is most suited to music listening.

DH3: Large Room

This setting is designed to simulate the acoustic environment of a large, commercial movie theater.

It must be noted that not all headphone systems cater for these three different settings. By default, those systems with no setting adjustments provide only the 'reference room', or DH1 mode.