Fullscreen HTML5Med Res (??MB)HTML4

5 channels of home theatre sound in headphones Is it nonsense or does it really work? TM Just recently there have been stories in the daily press about Dolby Headphone, a new system for reproducing home theatre sound in headphones. Is such a system possible or is it another marketing gimmick? By LEO SIMPSON I recently had the chance to talk to the developers of the Dolby Headphone system and experience a demonstration. To be honest, I did not know what to expect. On the one hand, how can it be possible to provide or simulate five separate channels of audio in stereo-phonic headphones? On the other hand, it is called “Dolby Headphone” and with a brand like 34  Silicon Chip that, it must be a genuine innovation. So I went along with an open mind (sort of). The demonstration was in a typical home theatre set-up: large screen for the video side of things and five speakers for the surround sound: left and right front, centre front and two rear speakers. There was no sub-woofer though, as far as I was aware, although that would normally be tucked away out of sight. I sat on the lounge in front of the video monitor and was handed a pair of normal stereo headphones to put on but I was told that the sound would come from the five speakers. The demo consisted of a spoken commentary, along the lines of “this is my voice coming from the left front speaker... This is my voice coming from the right front speaker... This is my voice coming from the left rear Dolby Digital speaker layout for cinema surround sound systems (above) and for the home theatre 5.1 system (below). speaker... and so on”. The demo starts with the sound clearly coming from the speakers arrayed around the room but part-way during the narrative I was told that I could now take the headphones off. As I did I realised that at some point in the narrative, the sound had stopped coming from the speakers and was now coming solely from the headphones. And yes, there was no doubt about it; there really were five discrete channels of audio, each strongly located where they were supposed to be. After the short demo and listening to some specially recorded material, I was convinced. It was no longer a question of whether Dolby Headphone works but “How is it done?” In effect, the Dolby Headphone system creates up to five virtual loudspeakers in a virtual room. Not only that, but the system can model the sound of surround sound playback in up to three different listening “rooms”: • DH1 is a small, well-damped room, ie, with carpet, curtains and soft furnishings, suitable for both movies and music-only recordings. It is the socalled Dolby Headphone “Reference Room” and is provided on all Dolby Headphone equipped products. • DH2 is a more acoustically live room particularly suited to music listening. • DH3 is a larger room, more like a concert hall or movie theatre. DH2 and DH3 are optional and may not be offered on some Dolby Headphone products. How it was done Normally, when you listen to a stereo program via headphones, the localisation of sound is quite unrealistic. Left channel sounds appear intimately in your left ear, right channels sounds in your right ear and sounds diffused over the stereo stage appear to come from right inside your head or for many listeners, over the top of May 2000  35 must be performed for all five channels simultaneously; with all the necessary acoustic delays for the direct sounds and the multiple reflections. While a number of companies have attempted to simulate surround sound via headphones, none have really caught on in the marketplace. Part of the problem has been that the simulations have not be able to cope with the huge number of signal For each speaker placed in a room a unique combination of direct and reflected sounds reaches the listener. Dolby Headphone simulates the acoustic effects for a complete surround experience over stereo headphones. your head. There is no “front” or “rear” localisation and if you have listened to stereo headphones over the years and understand the normal processes of audio recording, it is difficult to imagine how front and rear localisation could be provided, let alone left front, right front, centre front and so on. Think about how our ears and brain let us strongly localise sound. The process of localisation depends on the brain perceiving the difference in time of a arrival for a sound to reach our ears. But not only do we hear and perceive the sounds arriving via the shortest path to our ears, we also perceive all the reflections off walls and other objects to gain a sense of space, height and so on. Furthermore, our ears also provide a different frequency response to sounds coming from the front than they do from the rear. So much so, that even if we are blindfolded, we usually have no trouble knowing from where a sound originates. For example, if you were blind-folded or in a completely dark room, you would instantly be able to locate the source of most sounds, such an object falling to the floor, knocking on the wall and so on, even if the room was quite unfamiliar. This wonderful system of sound localisation, whereby our brain and ears work together, has been evolved over millions of years. It has enabled us to escape being eaten by predators because we could tell which direction they were coming from – and incidentally, allowed us to successfully hunt 36  Silicon Chip and survive. But the whole process of sound localisation by our ears had to be thoroughly understood before five channels of audio could be simulated electronically. Acoustic delays If you are going to simulate a sound arriving from the left front speaker at the left and right ears on a person’s head, you must provide acoustic delays which not only produce the direct path difference but also the delays for multiple reflections for any sound from the left front speaker off the walls, ceiling and floor. If you think in terms of computing power, the encoding and recording system becomes exceedingly complex, just to precisely locate the left front speaker via a set of headphones. But consider that the same process reflections involved for a period of perhaps half a second – the sort of reverberation time that can be experienced in a large listening room. Naturally, all of the simulation and filtering processes referred to above are done using DSP (digital signal processing). And that is where the Australian company Lake Technology Ltd, the developer of the Dolby Headphone system, comes into the picture. Lake Technology are experts in “convolution”, a mathematical operation used in the mixing of signals with applications in the processing of audio signals, radar signals and even in radio astronomy. Using their experience in convolution and DSP, Lake Technology developed algorithms to simulate the surround sound experience in head- phones using an FIR (finite impulse response) filter with low latency (meaning it’s very fast) and with 278,244 taps of convolution (meaning it can simulate vast numbers of room reflections). As part of their research, they went to the trouble of setting up a typical listening room with a home theatre setup of five loudspeakers and then recorded all sorts of signals as heard by a typical listener when seated in the “sweet-spot”. Real, live dummies! But they did not use a dummy head for the recordings; they used a real person and they fitted microphones in the ear canals of that person (must have been uncomfortable). Using their Huron acoustic virtual reality simulation platform, they then went on to produce a simulation of the recordings and subsequently, the algorithms. Such was the success of Lake Technology that the system is now licensed to Dolby Laboratories for full commercialisation. Already a number of semiconductor manufacturers, such as Motorola, Analog Devices and Sanyo, have produced chipsets for Dolby Headphone and the first commercial product, the Hitachi Prius computer, incorporating a DVD player and Dolby Headphone, has been released in Japan. Not only is it envisaged that Dolby Headphone will be incorporated into products such as DVD players and surround sound amplifiers but also into portable CD and MP3 players. As the release of the Hitachi Prius system proves, possibly the biggest market will be in computers and computer games, allowing users to enjoy full surround sound without the need for an array of tiny speakers. Standard headphones One of the beauties of the Dolby Headphone system is that any pair of stereo headphones can be used although naturally, the better the quality of the headphones, the better will be results. So while it can produce very good results for movie sound tracks with just average headphones, it will be even better with good quality phones and should be tops for music discs recorded with surround sound en- coding. Lake Inflight Theatre Nor are the benefits of surround sound confined to home users; there is a very big potential market in the airlines. Recognising this, Lake Technology and Dolby Laboratories Inc have acted to provide the system for in-flight movies. In this system, the sound portion of the program is not encoded as Dolby Digital with six channels but as two channels with the full surround sound simulation. This means that no decoders are required on the plane and all existing equipment can be used whether it is based on VCRs or DVD players. This system is already available on Qantas and Singapore Airlines and is available to all airlines. In fact, the LIFT program provides testing and accreditation for the entire inflight entertainment installation, including all the headsets. Further information on Dolby Headphone and related products is available from the following websites: www.dolby.com/headphone/ www.lake.com.au SC May 2000  37