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How it works

 

 

Introduction to SVS and the Smyth Realiser

Recorded "surround sound" is typically delivered through five, six, seven or more speakers.  The sounds of the real world come to us from an infinity of locations. We readily sense direction in all axes of three-dimensional space, and yet: the human auditory system is two-channel. One route into that system is headphones.

The weakness of headphones has been their inability to create a spacious and completely accurate sonic image in three dimensions.  Some "virtual surround" processors have made incremental progress in this regard.  But there is no reason in principle why headphones cannot provide a sonic experience fully as spacious, precisely localised and vivid as that created by multiple speakers in a real room.

Hearing in three dimensions

The fact that we can hear direction left and right makes intuitive sense since our ears are on the left and right sides of our heads.  How do we hear front and back, up and down?  Sounds coming from various directions are altered as they encounter the shape and dimensions of the head and upper torso, and the shape of the outer ear (pinna). Our brains are highly sensitive to these modifications but we do not hear them as tonal alterations; rather we experience them, quite accurately, as localisation up, down, front, back, or in between. This acoustic alteration is called the Head Related Transfer Function or HRTF.
 

Binaural

Historically, one type of recording has recognised that two audio channels can recreate a three-dimensional experience. Binaural recordings are made with a single pair of closely-spaced microphones and are intended for headphone listening. Sometimes the microphones are embedded in a dummy head or head/torso to create an HRTF, in which case the sense of three-dimensionality is enhanced. The reproduced sound space can be convincing, though with no reference to the original environment, its accuracy cannot be attested. In any case, these are specialised recordings rarely seen in the commercial catalogue. Instead, for sound beyond left
and right, we have...

 

   
 

Surround sound

Recordings intended to capture sounds both front and rear, and sometimes above, are made with multiple microphones, are stored on multiple channels, and are intended to be played back on multiple speakers arrayed about the listener.

   
  Virtual surround


 

 

For headphones, the multiple channels of surround sound can be combined, adding the directional signatures that would be imposed on each channel by the positions of the loudspeakers.  Various attempts have been made to "virtualise" the experience of multiple loudspeakers, with varying degrees of success. Surround virtualisers are sold or licensed by a variety of well-known and lesser-known companies. These apply HRTF alterations to the incoming channels, using generalised or average data for pinna, head, and torso shape and dimensions.

A monophonic signal in headphones sounds as though it's in the middle of the head; a stereo signal localises sounds in a line from left to right through the middle of the

   
  head. Neither localises anything outside the head, as the real world and loudspeakers do. With a multichannel source, surround virtualisers do indeed open up the headphone experience and expand it somewhat outward. Rearward localisation is achieved, making use of the surround channels; and frontward localisation is achieved to a limited extent, with the center front channel especially difficult to virtualise forward. None of these products would ever be confused with the much more precise localisation available from five or more loudspeakers about the listener.    

The Realiser

The Realiser provides a completely different experience in which a multichannel (or stereo) recording sounds indistinguishably the same through headphones as it does through a loudspeaker array in a real room. In basic principle the Realiser is similar to the other systems in that it applies HRTFs to multichannel sound to drive the headphones. But along with other refinements, the Realiser employs three critical components not seen in the other products:  personalisation, head tracking, and the capture of the properties of any real listening space and sound system.

Personalisation

As mentioned above, surround virtualisers have used a generalised or average HRTF for all listeners. The Realiser provides a method of measuring and storing the individual HRTF of each listener's unique ears, head, and torso. This measurement is easy, quick, and need be done only once. The difference between hearing one's own HRTF and the generalised one can be tremendous -- often larger than the difference between a typical virtualiser and plain stereo.
 
 

Head tracking

 

 

 

 

 

In the real world, the direction of a sound remains stationary as we move our heads -- in fact, we deliberately move our heads to discern the stationary direction more finely. With headphones, the sounds move with our heads. This is highly unnatural and a prime cue that we are listening to headphones, not the real world. For sound with picture, the fact that sounds move with the head while the screen remains fixed destroys the credibility of the experience and greatly weakens the association of picture and sound.

The Realiser system includes a small device atop the headphone headband and another small device at the front of the room, which together monitor the position of the listener's head every five milliseconds. As the head moves, and no matter

   
how rapidly it moves, the Realiser recalculates all the HRTF factors, seamlessly and in real time, maintaining accurate localisation with respect to the screen and room. The importance of this to the perceived reality of the experience cannot be overstated. The sense of wearing headphones disappears.
The measurements

The Realiser package includes a pair of tiny microphones, inserted into earplugs, which are placed in the listener's ears for measurement. (Additional plugs are provided in each of three sizes, and they are washable.)

The listener sits at the listening position, within the array of speakers, typically 5.1 or 7.1, but any configuration including height channels can be accommodated. A brief set of test signals is played through the speakers, introduced by very simple spoken instructions. Then the listener puts on the headphones, and a second brief set of measurements is taken. The whole procedure takes less than five minutes.

In the measurement with the speakers, the Realiser captures not only the personal HRTF of the listener, but completely characterises the room, the speakers, and the electronics driving the speakers.

In the measurement with the headphones, the system gathers data to correct for the interaction of the headphones and the ears, and the response of the headphone itself.

The composite data is stored in memory, and can also be written to a memory card for archiving and portability.
The comparison

The supplied headphones contain an accelerometer which allows the loudspeakers to play when the headphones are not worn, and which silences the speakers and switches on the headphones when they are placed on the head. This allows an immediate comparison between the sound system in the room and the headphone emulation. This is the most demanding possible test and will reveal the smallest discrepancy. The first-time listener's typical reaction is that the speakers are always playing. (No, it's the headphones.)
Headphone quality
 
As noted above, the headphone measurement corrects for headphone response, suggesting that any brand and model of headphone can be used. Indeed, any model will be corrected as much as possible, but the inherent quality of the headphones is a limiting factor. Therefore, to ensure a fully satisfactory result, the Realiser comes with a pair of STAX SR-207 headphones.
 

Low frequencies

Headphones are capable of respectable low frequency response, but cannot provide the full-body physical impact that woofers and subwoofers afford. Therefore the Realiser provides a pair of outputs to drive tactile transducers such as a shaker motor coupled to the listener's chair. The result is quite satisfying, and fun. Flexible bass management controls are provided to derive the tactile outputs (which can also be used to drive subwoofers).



Other listeners and rooms

With ample internal memory supplemented by memory cards, there is no limit to the number of listeners and environments whose characteristics can be captured.

  Professional users

For professional users, the Realiser affords the ability for a mixing or mastering engineer to replicate a preferred monitoring environment in another location. The studio can be brought home, or taken on location. A variety of consumer playback systems and rooms can be called up for checking the suitability of a music mix for commercial release. A client can hear a mix in the environment where the engineer created it without travelling there. Within an organisation, a proven good monitoring environment can be replicated as many times as there are users who need the room, and the replicated studios can be used simultaneously at any number of workstations, greatly relieving demand on limited
     

facilities and saving the cost of building additional rooms. Or different monitoring environments suited to different tasks can be accessed from the same physical workstation. Because headphones are used, the workstations themselves need not be isolated.

The Realiser's eight inputs/outputs are more than sufficient for presently established multichannel delivery formats, but if more than eight channels of emulation are needed, two or more Realisers can be used together, with the same head-tracking input.

Home users

The home user's first experience with the Realiser is typically the exact emulation of his own speaker system. But any audio system in any room can be measured, stored, and emulated at will. Audiophiles can emulate each others' rooms, speakers, and electronics. An audio dealer could make an unaffordable system in an acoustically outstanding room available for emulation, or might have arrangements with a studio or cinema for the same purpose. Or the layout of an existing room can be improved. For example, a center channel virtual speaker can be located behind a solid screen rather than above or below it. Playback can of course occur anywhere. A home user, for example, can take the sound of his home theater into his bedroom for spectacular sound later at night and without disturbing others in the house.
 
Gamers

Today's computer and console games offer brilliantly realistic graphics that immerse the player in the action. Many games also generate surround sound, but many gamers either use very poor computer speakers, or modest headphones limited to stereo. With the Realiser, the sound of the game rises to the level of the graphics, hugely enriching the gaming experience.
iPods and other personal players

While it would be impractical to carry a Realiser around, the output of the Realiser can be recorded into a device such as an iPod. For the optimum effect, the listener can make a one-time measurement through the ear buds to correct for the bud/ear interaction and to improve the earbud response. Then the mobile listener can enjoy the full dimensionality, and much of the quality, of a good surround speaker system while mobile. Since there is no picture and the listener is constantly changing direction, head tracking is unnecessary.
 
Tools and adjustments

The Realiser is designed to be simple for basic setup and operation, including the personalisation measurement procedure. However, a large range of adjustments and tools is available for those who want more finely to polish or optimise their setups.

For example, extensive level, EQ, delay, crossover and bass management controls are provided for every channel. A choice of test signals is provided for the best result in various room acoustics. Measurements can be repeated and combined to lower measurement noise. The angles of the virtual speakers can be fine-adjusted to correct for slight misalignments of the listener's head during measurement; also, a pilot tone system provides audible feedback for precise head alignment just before measurement. And there are many more such facilities, all accessible by straightforward menus.

It must be emphasised that the above tools and adjustments can and often will be ignored, because a completely satisfactory experience can be achieved without them. They are provided for flexibility, convenience, system interface, and to provide the ultimate emulation accuracy for those inclined to go beyond the basic procedure.
 
The default emulation

What if the user has a poor listening environment and/or modest equipment which he does not wish to emulate, and no access to better rooms or equipment? The Realiser comes with a default virtual room and virtual loudspeakers. This non-personalised experience will be competitive or superior to other virtualisers, especially if head tracking is employed. But we strongly urge that all listeners personalise, which is, after all, a principal reason for acquiring the Smyth Realiser!
  In conclusion

The Smyth Realiser is an unprecedented audio product. It will redefine the way audio professionals work and the way people enjoy music, movies, and games. It will no doubt create opportunities not yet recognised. We invite you to read the comments and articles by journalists and audio practitioners who have used the Realiser, and to audition the Realiser yourself. We would be pleased to answer any questions you may have; kindly address them to: info@smyth-research.com
   
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