Posted by Tony Churnside on , last updated
I'm currently working for R&D in the North Lab which I've blogged a bit about here. I'm working in Production Magic, Graham Thomas' section, on the future of surround sound, and I thought that it might be interesting to write a little about the project. In the media sector audio is often seen as the underdog to video. This is even true in some parts of the BBC, where we produce much more audio than video content (we don't make silent TV!). R&D has strong audio team, led by Andrew Mason, who's talked a bit about Ambisonics in a video here.
The majority of the audio that the BBC creates is stereo. The two exceptions to this are Radio 5 Live, which, for now, is broadcast in mono, and BBC HD, which has a mixture of stereo and 5.1 surround.
5.1 surround is one of the current multichannel surround standards. As a result of extensive testing in the 1990s the ITU recommends a set of speaker positions for a 5-channel surround, with 3 at the front and 2 at the back, at specific angles.
There are a number of disadvantages to this way of recording surround sound. One of the major issues is compatibility with formats with a different number of channels. The sound engineer has check compatibility with mono, stereo and 5.1. In the future the engineer may have to also check with 7.1, 22.2 and whatever other discrete channel surround system that may come next. That would require a lot of time and a room with enough speakers to cover every possible set up.
Another issue faced by an organisation like the BBC is how we archive our material. Theoretically, if we archived the stereo, 5.1 and 7.1 mixes of a piece of audio it would take 8 times the amount of space than just the stereo recording. These ITU standards were borne out from a lot of research into which angles gave the best sound, and are essential when setting up a studio or listening room. However, I would be surprised if many of our audience had their own ITU 5.1 set up, and the talks I've had with friends in the computer games industry suggest most of their customers who listen in 5.1 don't follow ITU's recommendation, preferring to use a square, perhaps because that set up fits best around their furniture. While games users may not be representative of the BBC's audiences, we shouldn't assume our 5.1 listeners are using an ITU recommended set up.
A possible alternative to these discrete channel formats is a system called Ambisonics. This system was developed in the 1970s and has had a cult following since but has yet to break into the mainstream, being of interest mainly to academics and select audio engineers. The fundamental idea behind Ambisonics is to attempt to represent a sound-field at a single point in space.
Without going into too much detail it is an extension of the Blumlein Pair, but capturing 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 sound-field.
Since the 1970s development of the system has lead to High Order Ambisonics which provides higher resolution in the localisation of sources within the sound-field, at the cost of needing more channels to represent the same recording.
So how might this technology help solve some of the problems described above? A major potential advantage of Ambisonics is its lack of dependency on speaker position. Unlike 5.1, the audio channels being carried in an Ambisonic signal do not map directly onto speakers. The number of speakers and the way they've been set-up by the listener is not as important and the same signal can be decoded to any speaker array. This flexibility would allow one common set of signals to be sent to everyone, and they would be able to decode it to suit their listening environment, regardless of the way they've chosen to set up their sound system. This also has obvious advantages from an archival point of view, and unlike stereo, 5.1 and 7.1, keeping the Ambisonics recordings could potentially help future-proof the archive. In my next post I'll talk about what we've done so far, and what we might do with Ambisonics in the future.