3D audio - a dimension worth adding


Will Strauss# TV-Bay Magazine
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3D audio - a dimension worth adding
Those of you that have just returned from NAB may have been surprised to witness the distinct lack of stereo 3D hype during the show. I certainly was.
After several conventions in a row when stereoscopy seemed to unnecessarily dominate the press announcements, the halls and some of the chatter, 2012 finally saw this latest incarnation of 3D return to what is probably its rightful place as just another storytelling tool.
However, I’m sure that’s not the last we’ll hear of it, not least because there is another form of 3D garnering some interest: three-dimensional audio.
Adding a third dimension to TV and film sound is likely to be the next big audio advance as boffins the world over try to make the noises that come out of our screens and sets appear more realistic, an advance that should aid the story telling process (unlike, I might add, some of the more spurious uses of stereoscopy).
But what will that advance be? Let’s look at some of the options.
As it stands, there are a number of innovations to be aware of (some of which have been around for more than a hundred years I should note). I’m going to look at just three.
The first is binaural sound. Also known as the ‘two-ear effect’, binaural sound is recorded on microphones that are positioned near to a person's eardrums.
When this acquired sound is played back on headphones it can give the listener the impression that he or she was present at the location where the recording was made.
How does it work? I’m glad you asked. For an explanation, let’s take a leaf out of Roland’s book. Well, actually let’s take a paragraph from its website.
Roland explains binaural sound like this: “The sounds that reach our ears are given unique signatures as they reflect and refract against body parts, including features of our ear lobes. In technical terms, this is called the head-related transfer function (HRTF). Eardrums in our ears pick up these sounds that have been altered by reflection and refraction, and this information is transmitted to the brain via complex organs in the ear. The brain is then able to recognize the location of the sound source as well as the space in which the sound was made based on subtle differences in the sound registered in our two ears, including differences in the intensity of the sound and the time the sound arrives. Therefore, by recording sounds near our eardrums sounds that have been colored with a signature peculiar to the human ear and playing these sounds back on headphones, the reproduced sounds give the illusion that you are actually listening in the physical location where the recording was made.”
Roland already makes a set of stereo condenser microphones that are built into earphones for recording binaural sound, the CS-10EM. And there are various iPhone games that utilize binaural sound. So, is it the next big advance in mainstream audio?
Possibly not as, unfortunately, if you play binaural sound back over speakers rather than headphones the effect is lost. There is a way around it though using what is called Transaural processing. This sees the left channel signal shifted to the opposite phase, given a very slight delay, and then mixed into the right channel and vice versa.
This process produces a binaural sweet spot. Unfortunately this sweet spot is only a couple of inches wide and the two speakers and the listener must form an exact equilateral triangle in order to achieve it. If the listener then moves his head, the spatial imaging will be disturbed and the effect will be lost. Which doesn’t really lend itself to either the home or the cinema.
Another way of adding a new dimension – and therefore more realism - to audio is Ambisonics. First developed in the 1970s, Ambisonics introduces ‘height’ to sound, providing a layer that 5.1 surround sound cannot.
BBC Research and Development describes it thus: “Ambisonics captures audio from three perpendicular figure of eight microphones all positioned at the same point in space. When combined with an omnidirectional microphone these four signals are know as B-format. This signal represents the three-dimensional soundfield.”
To play this back this you’d expect to need more speakers but that’s the beauty of Ambisonics. It actually uses four signals that are interpreted by the receiver to give us any number of loudspeaker signals.
The major benefit of Ambisonics is that, unlike surround sound (or Transaural processing), speaker positioning is not essential. So, from a broadcast perspective, one common set of signals could be sent to all viewers who would then be able to decode it to suit their listening environment, regardless of how their surround sound system is set-up.
Unfortunately, mass adoption of Ambisonics is some way off, not least because sound localization using four streams is not considered to be quite good enough yet and to improve it would currently require more channels, which kind of defeats the point. More work required.
For a market-ready 3D audio perhaps we should look to Dolby Atmos.
A cinema technology at present, this new format is being used on Pixar’s latest animated movie, Brave, which opens next month.
It is said to envelope the listener, giving the illusion that there is an infinite number of channels all around and above them.
According to the marketing blurb Dolby Atmos enables “adaptive rendering” to make sure that the playback experience is as close as possible to the creator's original vision regardless of where the speakers are.
And we’re not just talking about speakers on the ceiling and behind the screen. Dolby Atmos is said to be able to transmit as many as 128 channels and render anything from 5.1 surround to 64 discrete speaker feeds.
Quite how this would adapt to the home is another matter. But from a cinema perspective it looks capable of producing a genuinely three-dimensional sound that is significantly more realistic than what we can currently re-produce with 5.1 or 7.1 surround sound.
In my opinion, regardless of whether it’s Binaural, Ambisonics, Doly Atmos or something else entirely that leads the way, if 3D audio can genuinely make recorded sound appear more realistic and therefore aid the narrative of a TV programme of film, it’s certainly a technology advance worth pursuing.
Unlike some of the more overhyped 3D developments I could mention.

Tags: iss065 | doby atmos | Ambisonics | Transaural processing | binaural sound | roland CS-10EM | Will Strauss#
Contributing Author Will Strauss#

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