Immersive audio systems explored6 June 2011
Broadcasters are exploring immersive audio systems to keep pace with the advances made in 3DTV and higher resolution displays. Stefan Meltzer a technical consultant who represents the Fraunhofer IIS at DVB level says there are different degrees of interest among European broadcasters about adding a third dimension to audio. “The big question is how to do it technically and also how many additional speakers will user’s accept,” he says. Pieter Schillebeeckx, head of R&D at Soundfield has met with broadcasters who have moved from SD to HD and 3D on the video side but “feel the audio has stagnated at 5.1 and they are asking what next?” he says. The 5.1 surround format has its limitations, the most obvious being reproduction of audio restricted to the horizontal plane. The 7.1 format adds two further channels in an attempt to improve the sense of envelopment in the horizontal plane, but again does not reproduce a sensation of height. “There are problems with discrete channel formats such as 5.1, including the effect of non-standard speaker layouts at the user’s end which are beyond the control of the broadcaster,” says BBC R&D’s Chris Dunn. The next generation of Dolby’s audio format Dolby Digital Plus (DD+), is capable of providing channel configurations in 7.1, 9.1, even to 22.2. The latest DVB transmission systems also include this capability supporting DD+ and HE-AAC/AAC. The BBC for example is using AAC and HE-AAC for Freeview HD. AAC/HE-AAC supports in its bit stream up to 48 channels. “It depends on how much spectrum you wish to take up with audio and how many speakers consumers want in their living room,” says Dolby Broadcast Marketing manager James Caselton. “You get to 22.2 and it becomes a question of designing wires into the fabric of a house.”
One alternative to discrete channel formats, currently being explored by BBC R&D, is Ambisonics. Invented in the 1970s, this approach attempts to represent a sound-field at a single point in space.
As well as reproducing sound in the horizontal plane, Ambisonic systems have the advantage over surround formats in that they are also capable of reproducing the sensation of height, in principle offering a fully-immersive sound experience.
Ambisonic recordings require omnidirectional mics to capture width, height and depth information, and these are available from Soundfield (pictured) and others. Ambisonic recordings can be coded and transmitted using as few as four channels (so-called B-format). Ambisonics does have problems though, including a tendency to restricted ‘sweet-spots’ in the listening area, and relatively expensive mics and recording equipment. BBC R&D’s work is focussing on the trade-offs between the technique’s advantages and limitations. Another option, being investigated at Fraunhofer IDMT, is IOSONO based on Wave Field Synthesis which was originally devised in the mid-1980s at Delft Technical University. The basic idea is to emulate sound sources and waves by using a ring of loudspeakers around the listening space. The level, position and distance of each sound source is recorded and processed separately from the acoustic characteristics of the room. Using this separation, it is possible to manipulate the source characteristics and the room information separately. Playing 5.1 surround over Wave Field Synthesis is said to drastically enlarge the optimal listening area but the technology is mainly applicable to live events, multimedia installations, or virtual-reality-applications. For example, IONOSO is installed at Hollywood’s Mann Chinese 6 Theatre and Disney World. “Fraunhofer IDMT and IOSONO are also looking at how sound reproduction in the home can be improved dependent on the reception device,” says Meltzer. “We want to avoid solutions where the audio is downmixed or upmixed because each time you are compromising quality. “Instead of a channel-based representation, Wave Field Synthesis has an object-based approach which should make it easier to scale the reproduction of audio up or down according to the number of speakers in a home. This also has advantages for the broadcaster, since it only needs to produce and transmit one signal. Such a format would also be a future-proofed archiving format.” www.soundfield.comwww.bbc.co.uk/rd/www.dolby.com