Posted by Tony Churnside on , last updated
In my last post I talked a bit about why BBC R&D is interested in Ambisonics. This week I am going to talk a bit about what I've done so far, and what we might do in the future.
As most academics will tell you, the first step when undertaking a new research project is a literature review. This is to find out what other people have done already (so you can avoid re-doing work) and to help you get an idea of what it is you need to find out. There is a fair amount of literature about Ambisonics and there has been a small but dedicated research community surrounding it since it was developed over 40 years ago by Michael Gerzon. I spent the first month or so of the project reading.
I've talked a bit about working in the North Lab, and Rowan's talked about setting up a new research facility. When I arrived in Manchester to start the Ambisonics and Periphony project (before the R&D North moved into the new interim lab facility) one thing that was missing was a listening room. Kingwood Warren had an excellent ITU standard listening room, but there was no such facility in our north base. New Broadcasting House, on Oxford Road was built in the 1970s and I think it would be fair to say parts of it are past their best. Taking a short cut back from the canteen I discovered a decommissioned radio studio in a dark corridor which turned out to have, perhaps not ITU standard, but good enough acoustics to set up a listening room. I set up the room with 14 speakers for the Ambisonic array; a cube (a square above and a square below) and six in a hexagon at listener head height. I also added two extra speakers in the horizontal plane so I could compare mono, stereo and 5.1 with Ambisonics.
Perhaps the most common way of making Ambisonic recordings is by using a Soundfield microphone. This is a microphone system that is capable of outputting a B-format signal, and there are a lot of high quality Soundfield recordings available online from sites such as SoundOfSpace.com and Ambisonia if you want to explore them.
One of the things we wanted to look at was how Ambisonics might be integrated into typical BBC production workflows. The perfect opportunity for this came up with the recording of The Last Night of the Proms, last September. The week before the beginning of the Proms season we slung a Soundfield microphone up above the conductor's position in the Royal Albert Hall. On the last night we came back with an external soundcard and a laptop to record the B-format signal from the Soundfield mic. The Proms is recorded for the BBC by a specialist outside broadcast company called SIS Live. I worked with SIS to get hold of all the close microphone audio tracks of the Proms (this filled a 300 gigabyte hard drive!). l converted these signals into B-format by mathematically placing them in a soundfield and mixed them with the original signal, that we recorded to recreate the sound of the event.
In addition to the recording of live events the BBC produces a lot of radio drama where the final programme is artificially created by mixing recordings of actors performing along with foley recordings and archive sound effects. To assess how Ambisonics might work in this type of production I made contact with a producer of Radio 4 Drama who was about to record The Wonderful Wizard of Oz. This drama was recorded in a radio drama studio (a fairly dead space) where Actors perform to a stereo microphone. We set up the Soundfield microphone in the studio and the actors performed around it. The sound effects were given to me afterwards as mono and stereo audio files. I produced this in a similar way to the Proms mix, but using a combination of B format ambience and mono effects mathematically placed in the sound-field. Another thing I experimented with here was using convolution of B format signals. Using B format impulse responses of reverberant spaces to add ambience to the dry B format recordings of the actors.
We also conducted a number of listening tests to assess subjectively some of the work we did. Chris Baume, who had been working at Kingswood Warren on the project, did some listening tests investigating how important periphony is to the listening experience, while I focused on comparing the audience's enjoyment of Ambisonics with their enjoyment of stereo and 5.1.
That pretty much brings us up to the present. I'm currently analysing the results of the listening tests and writing up the whole project into an AES paper that Chris and I will be presenting in May. While this project has shown what first order Ambisonics might do to for the BBC, it has also shown that there is more research to be done. To paraphrase Donald Rumsfeld, this project has turned quite a few unknown unknowns into known unknowns, and I look forward to turning them into known knowns in the future.
Thank you very much for all your comments on the previous Ambisonics research post. We've found them really useful in producing today's post so hopefully some of your questions have been answered. Please do leave comments below to continue the discussion.