Posted by Chris Pike on , last updated
We’re really excited that this Saturday a special binaural version of “Knock Knock”, episode 4 in the latest series of Doctor Who, will be available on the BBC iPlayer. It gives an immersive spatial sound experience for headphone listeners. This version was remixed using the BBC R&D binaural production tools at BBC Wales in Cardiff. This post is for those wishing to understand a little more about the technology behind it and our vision for developing it in the future.
Here at BBC R&D we have been investigating spatial audio technology for several years. We believe it can offer BBC audiences a more immersive and engaging experience. We are developing the technology for spatial audio over both loudspeakers and headphones, but for headphones the opportunity to improve things seems particularly strong right now. Our listening habits have changed dramatically in recent years and now many more people are listening to BBC programmes with headphones. The traditional stereo format is designed to create a frontal sound stage on a pair of loudspeakers, but when listened to on headphones it gives a very different image, often inside the listener’s head. This is where binaural techniques come into play. Sounds are processed so that when listened to on headphones they seem to come from a point in space outside of the head. So we can create a virtual acoustic scene around the listener, immersing them in the world of the programme.
What is binaural sound?
So how does binaural sound actually work? Rather than the simple left-right amplitude panning of stereo, we accurately simulate the acoustic transmission from a point in space to the two ears of the listener. The sounds we hear are affected by the shape of our head and ears, and the effect varies with the position of the sound source. Our auditory systems can recognise these patterns to localise sounds, particularly frequency-dependent time and level differences between the ears. By modelling these effects we have built production tools that allow sound engineers to freely position sounds in a 3D scene. This is not a new concept, it was heavily developed by researchers in the 1990s when virtual reality technology was a hot topic (of course there has been renewed interest in this lately). We developed these tools at BBC R&D so that we could study and improve the quality of the signal processing techniques and also to work out the best way to fit the technology into the tools and workflows of our production teams, which is an ongoing process.
Now onto The Doctor. It’s so great that we can show you the potential of binaural sound with one of the BBC’s biggest and most loved programmes, which has a long history of innovative sound. We’ve come a long way with the project to get to this point and have already used the tools on a number of programme productions, some of which are linked to from our project page. In the last year we have established facilities in Cardiff for spatial audio production, working with the brilliant Cathy Robinson, who did the sound for the Fright Night binaural audio drama Ring in 2015. Having an expert on the ground in Cardiff we were able to show the Doctor Who team the potential of binaural sound and were thrilled when they decided to try it out for Knock Knock in this series. I don’t want to ruin the suspense, but this episode is great for using spatial audio. I think it adds a lot to the spooky atmosphere and really draws you into the scene with Bill and her friends...
Hear (and see) a preview of Knock Knock with binaural sound
Cathy and I supported dubbing mixer Darran Clement in using the tools to remix the episode, working from the surround sound loudspeaker mix which will go out on the HD TV broadcast. Darren did a fantastic job of enhancing the spatial impression for the binaural mix, giving a more natural sense of the space of the environments and movement of the characters (and creatures). In this mix we used an approach often called 'bed plus objects', where much of the sound is mixed to a virtual 3D loudspeaker array (the bed) and then certain key sound effects are processed as separate objects within the scene. This has the benefit of speeding up the mixing process, partly due to compatibility with existing production tools such as reverb and dynamics effects. Whilst the tools for spatial positioning are working well, there’s still a long way to go before all of the tools in a sound engineers toolbox support spatial audio.
So what next? Our hope is that we can offer BBC audiences many more programmes with spatial sound. We are working on improving the tools and workflows, making them more efficient and enhancing the creative possibilities. This is a rapidly advancing area, particularly with the current interest in virtual reality, and we are partnering with academia and industry to develop the technology we use. We are also building the knowledge amongst our production teams in how best to use this technology.
This work also fits into the wider BBC R&D strategy on object-based media. We are working towards programmes that are responsive to each user, allowing personalisation for individual preferences or accessibility needs and adaptation to the environment and context that you are in. With spatial audio, rather than mix and present a special headphone version as we’ve done this time, we are working towards creating 3D sound scenes that are automatically adapted to the playback system of the listener. Hopefully in the future when you plug your headphones in on the iPlayer you’ll automatically get a 3D headphone version, from the same mix as the surround sound loudspeaker version. For those interested, there is a lot more information about object-based media and the other audience benefits we believe it can have here on the BBC R&D website.
The enhanced binaural version of 'Knock Knock' is available on the BBC iPlayer now.
This post is part of the Immersive and Interactive Content section