Monday 24 January 2011, 12:25
Simon Tuff, a senior engineer at the BBC, takes up the technological challenges being met in producing surround sound for online audio.
Alan Ogilvie (in the first surround sound blog post) talked about some of the challenges we face when delivering surround sound online as well as some of the changes that make this increasingly feasible. In this second part I'll look at what broadcasters like the BBC are doing to create surround sound content for you to enjoy as well as some of the challenges this has created!
The BBC has, in fact, been experimenting with multi-channel sound since at least the days of four-channel quadrophonic in the 1970s (and possibly earlier... but I haven't found the records yet). One of the reasons these experiments didn't flourish was the lack of a good method for broadcasting more than two channels of audio (i.e. stereo) to the home in a way that a significant number of our audience could listen to it. So although tests were conducted and some of the results were impressive, nobody had quite worked out why you should need more than two loud speakers for most programmes or how to get it to the home. Without attempting to blog a history of surround sound, it's worth noting that what helped to answer these questions for the broadcasters was the experience of surround sound in cinema which became wide spread in the 1990s and by the 21st century this combined with DVD technology (in home cinemas) was beginning to have a significant influence on television as the audio accompaniment to HDTV.
At the programme making end of the process we had film production as a starting point and at home we had DVD-type home cinema systems with Dolby or DTS 5.1 surround sound. Then in the middle of the signal chain companies, like Dolby, provided 2 main innovations to help us. Firstly they enabled us to move 6 or even 8 channels of audio over infrastructures that had been built for stereo (mostly the wires within our buildings and studios) and secondly they allowed us to deliver surround sound to these home cinema systems, via digital broadcasting, providing an alternative to DVD as the source of both high quality pictures and sound.
The subject of surround sound distribution is another complicated topic worthy of its own blog post, but first: why and how surround sound? Why is it worth the effort and how does broadcasting differ from film?
Answering these questions has also been a journey of learning and innovation, which we haven't finished by any means but by adapting some ideas from the film and recording industries, plus plenty of homegrown innovation, we have developed reasonably cost-effective techniques that work for HDTV and, we hope, radio too.
We know from research on human perception that hearing is very much a three dimensional experience and that this is especially true for the sound we detect from behind. Anthropologists suggest that this rearward hearing sensitivity was developed to help protect us from threats we couldn't see (e.g. attacks from behind) and thus it is one area of the senses that surround sound can use to enhance the media experience by adding the two rear speakers of the 6 speakers required for 5.1 surround sound. With these we can provide a better sense of space with background noise and reflections of the sound sources in front, coming from the rear, but also by proving a sense of motion as objects leave what we can see on screen and pass to either side or move across behind us.
Next, the addition of a centre speaker to the stereo pair (one left, one right). One of the differences between cinema surround and that for TV is the way we place people speaking (or dialogue) in the in the middle of the sound stage in front of the viewer. With a very large screen and a much larger audience, placing all the dialogue in a centre speaker (often hidden behind that screen) works well for movies in the cinema. However, we have found that for home viewing on smaller screens, with people sat closer, the effect is more realistic if we spread the sound (to some extent) across the front 3 speakers but having this centre speaker still gives TV viewers clearer dialogue and better, more stable, sound images for more of those sat on the sofa.
Another aspect of our audio world that surround sound technology aims to better reproduce is the dynamic range of real life i.e. the difference between the quietest and loudest sounds. This can be very dramatic in cinemas, although broadcasters have had to tame this technology to some extent for TV viewing (if we aren't going to annoy the neighbours etc.). We have several tools to help here but perhaps the most well know is the LFE channel (Low Frequency Effects). This is the 0.1 of 5.1. The idea is that the rumbles and bangs that make movies so exciting can be created best by a loudspeaker specifically designed for this purpose. This is partly because the main components of these sounds are at low frequencies as well as great level (or loudness). In this case the low frequencies are typically below 100Hz where humans and at this frequency can't work out which direction they come from... so, if done properly, you only need one speaker and this doesn't damage the sound image. It is this sort of effect that shakes your chest and perhaps the furniture.
Having a technology that can deliver 'Hollywood levels of excitement' is a good place to start and a lot of good surround sound comes with great films - but television and radio programmes cover a broader range and provide a more intimate experience. This is true of other types of drama certainly but the ability to create a sense of space, of really being there, allows other programme types to benefit. Sport is transformed by the sound of the crowd and the atmosphere of a stadium. Sky's premier league football and the BBC's coverage of Wimbledon are both transformed by the roar of the crowd all around you when a goal is scored or the hushed murmurer of expectation on centre court awaiting a match wining serve. Live music is also greatly enhanced by what surround sound brings. Not only can the acoustics of great performance spaces, from concert venues to cathedrals, be better captured but the thrill of being in a festival audience or standing next to the stage can also be magicked up.
The BBC is still learning how to make the best of 5.1 for TV as well as doing experiments to explore ways of bringing surround sound to radio audiences and considering what the next generation of audio technology, perhaps to accompany 3D pictures, might be!
Simon Tuff is Principle Technologist at BBC FM&T
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Friday 7 January 2011, 18:37
Monday 24 January 2011, 16:25