Old-school surround – the return of binaural1 May 2014
Since the earliest days of sound reproduction, inventors have strived to accurately replicate how humans hear in everyday life, with sensations of height and depth as well as width and length. Cinema has again led the way with Dolby Atmos and Auro by Barco, but in broadcast, developers are looking towards older technologies to create spatial audio.
Just when some technologies appear to have been consigned to the technical history books, they suddenly make a comeback to fulfil a need in modern broadcast and media production. Two prime examples from the sound world are binaural and Ambisonics, both of which have formed the basis of the BBC’s research into new formats for immersive sound in television, radio and general distribution to new mobile devices.
The appetite for so-called 3D sound is certainly there, as shown by the success of Dolby’s Atmos object-based spatial cinema system. A competitor is Auro 11.1, developed by Auro Technologies and marketed for the theatrical market by Barco. Dolby also sees potential for surround sound in the mobile device market, initially with its Dolby Digital Plus 5.1 system, but now is taking the concept further after demos of Atmos for handhelds during Mobile World Congress in February.
DTS is another company looking to headphones and handheld devices. Its Headphone:X technology delivers virtual 11.1 surround to standard headphones, based on a customisable 3D audio renderer for localisation and spatialisation. DTS is aiming Headphone:X both at music/video and the games market, with a feature to allow gamers to save audio preferences on multiple devices for a consistent experience using many platforms.
Headphones formed part of the earliest experiments into what became known as stereo sound, often allowing more of a emulation of the full spatial information people naturally hear. Stereo was publicly demonstrated in 1881 during the International Exposition of Electricity, when Clarence Adler placed 10 telephone transmitters around the stage of the Paris Opera. These were connected to corresponding pairs of telephone receivers at the Palace of Industry, where the public could hear the performances picked up through a spaced pair of microphones. It is said this basic equipment allowed listeners to identify the position of different sound sources.
Binaural techniques were refined during the 20th century by placing microphones in a dummy head (pictured is a modern Bruel & Kjær model) so that listeners wearing headphones would hear exactly what the recording setup “heard” in terms of the positioning of sounds.
The technique fell out of favour as a listener had to sit very still for true reproduction of the original audio picture, and the general preference moved towards people listening on loudspeakers for greater convenience and freedom of movement. But binaural never really went away, and in recent years BBC R&D at its MediaCityUK facility in Salford has used it as a basis for experiments into immersive audio.
In parallel with this, researchers have also been using Ambisonics for 3D sound. Ambisonics was devised during the early 1970s by recording engineer and mathematician Michael Gerzon, with recordings made through a specially designed SoundField microphone to produce an accurate audio picture comprising absolute sound pressure level and the three pressure gradients: left/right, front/back and up/down.
But BBC R&D decided that binaural was more effective for the height element in 3D sound, and produced a number of test recordings, including a radio drama production of The Wizard of Oz (which gave the sensation of objects being thrown about by the tornado that takes Dorothy to the fantasy land) and a recording of Elbow in concert.
During a demonstration of this work in 2011 a BBC R&D spokesman said much of the work had focused on “how to personalise characteristics [for] accurate localisation” of the sound to suit each individual listener. To make this possible, a head-tracking device is now part of the process, and special apps that can be attached to devices such as Bluetooth receivers – so the wearer is able to receive all the spatial audio information, even if they are moving about while wearing the headphones – are under development.
To make this work, the dummy head has been replaced by software to process six-channel surround-sound recordings, preserving head-related transfer functions (HRTFs) – the variations in sound frequency caused by the ear, head and shoulders.
Recent BBC productions made using 3D sound include the Nine Lessons and Carols concert and radio drama Private Peaceful, although these were based on standard surround signals converted into binaural.
The reappearance of this old technology has not been well-received in some sections of the technical press. As well as observing that the “sexing up” of binaural as 3D sound would not make up for existing “imperfections” in the format, one commentator pointed out that sounds could be potentially disconcerting and even dangerous for someone listening while out walking or riding a bicycle.
Despite such negative reactions, the BBC is confident 3D sound/binaural will have a new lease of life. “There are lots of potential applications for binaural sound and when done really well it’s very convincing,” comments Chris Pike, senior scientist with BBC R&D. To appeal to a younger audience, and playing on recent trends, engineers have been working to integrate sound recorded on smartphones and tablets into broadcasts of concerts and other live events.