MP3 Surround

The story of surround sound in the home has so far been one of heroic failure. Over the last 40 years, a succession of brilliant innovations have arrived with fanfares — then faded away to public indifference. The Quadraphonic system, launched in 1970, was implemented beautifully: to record four synchronous audio tracks onto a vinyl record is no mean achievement. But there was no agreement between manufacturers on standardising the system; technical difficulties and conflicting formats meant that very few people felt confident enough to buy into it, and so Quad died. But Quad was never going to accurately reproduce a 3D soundfield anyway, due to a basic flaw in the maths: four speakers placed in the corners of a room cannot produce the 'phantom images' that stereo or 5.1 can. At 90 degrees, the angles are too wide. It can give some good effects, but it's not proper surround sound.

Comparing the two formats was now simply a matter of switching between the DTS version playing from disc and the MP3 Surround version playing from the PC's hard drive. The Bilocation mix contains a lot of spatial movement, all of it 'real' rather than panned. MP3 Surround reproduced this quite well, with all the imaging pretty much as it is in the DTS version. There were differences, though, mostly with the 'above' effects. There was still a sense of 'above', but it was a bit vague, and not always in the right place. I assume this was due to the MP3 process removing too many 'unnecessary' frequencies. (I do realise this test is a bit unfair, as 5.1 isn't even supposed to have height!) One part of the track was recorded in a huge dome in India, where children clap and shout into a natural repeat echo. Although this still sounded good in MP3 Surround, it was here that the limitations showed up. The echoes that faded to silence were OK, but at one point there is a maxlevel (0dB) handclap, right next to the mics, that in the DTS version opens up the acoustic and explodes into ambience. The MP3 Surround version of this is disappointing and sounds very obviously 'compressed'. But really, that is my only criticism! Also, my material is hardly typical. If you'd like to try this experiment for yourself, both of the files are still available at www.bilocation.co.uk.

The MP3 format actually dates back to 1992, and has been in common use for over 10 years. For better or worse, it's fair to say that the MP3 format has transformed the music industry in that time. The format has proved to be extremely versatile, supporting all common sampling rates from 16 to 48 kHz and bit rates up to 320kbps. Because MP3 is so well established, it made sense to develop a new format for surround sound that would be completely compatible with existing stereo MP3, as well as other surround systems and playback equipment. This was made possible by the technique of 'Binaural Cue Coding' (BCC). In essence, this is not very different from the Middle + Sides (M+S) technique used historically for recording stereo, but it is a radical departure from previous surround codecs. BCC combines all the separate audio channels into one 'sum' channel, which is augmented with spatial 'side information' — in other words, instructions on how to separate them again. File sizes are drastically reduced, because the spatial side information for each channel is relatively small.

For many years, musicians and producers have been searching for a way to sell surround mixes over the Internet, but until now, file sizes have just been far too big to make this practical. MP3 Surround thus has an awful lot going for it. File sizes are only 10 percent bigger than stereo, sound quality is excellent, new hardware is not essential, the system is expandable and it is backwardscompatible with existing stereo MP3. Many audio manufacturers have already taken out licences, including Philips, Sony, Samsung, Yahoo and Yamaha. Funai launched the world's first MP3 Surround DVD player this year and more hardware products are on their way. Steinberg have worked closely with Fraunhofer for some years, and Cubase 4 and the upcoming Nuendo 4 offer full support for MP3 Surround. The codec has already been accepted by the video industry and is in use by several Internet radio stations, while gaming applications are on their way.

And the quality issue? The audiophile in me doesn't honestly really approve of MP3, but I want to sell surround recordings as Internet downloads for a mass market. Only MP3 Surround makes this a practical proposition, so I accept the slight loss in quality. And if anyone else shares my love of hifi surround sound, they can always buy the same tracks on disc: they'll cost a lot more, they'll have to be posted, but they'll be actual 'objects of desire', with nice packaging that you can store on a shelf, in a collection. I can envisage a market where many different formats and resolutions are offered, some physical, as well as differentsounding mixes of the same track. Why not? The 'long tail' economics of the Internet suggest that there may be more profit in lowvolume diversity than in a handful of blockbusters. If the medium is effectively free, as with MP3, than diversity is no problem. Stereo MP3 was not an overnight phenomenon: it actually took six years to take off, even though a vast body of stereo material already existed. So it will take time for a new format to grow, but let's get busy mixing in surround — it does seem to have a future at last. And really, for musicians and producers struggling to make a living in a chaotic market, this is a gift! 0

Transfer Your Own Head

Creating HRTFs is a complex and expensive business. Here at Salford University, a dummy head is placed in an anechoic chamber and test tones are recorded from strategically placed speakers.

A great deal of research is going into Head Related Transfer Functions at the moment, largely because of the mobile phone industry. The advent of stereo Bluetooth means that some people will soon be wearing stereo headphones most of the time! Personal gadgets are all rapidly merging into allpurpose 'devices' that combine phones, music and video players, Internet browsers, games consoles and so on. With HRTF technology comes the capacity for adding surround sound and 3D 'mobile environments'. The idea is that you can wear a stereo headset and take phone calls, listen to music, chill out in a 3D tropical rainforest — that kind of thing. Some systems will also incorporate noisecancelling to remove unwanted ambient sound.

All of this is possible even without the use of headphones. Spatial sound environments can actually be projected into the air, simply from a pair of microspeakers an inch apart, mounted in one end of a phone. The old but effective 'transaural' technique has been enhanced by incorporating HRTF data: a combination of phase cancellation and time delays is used to eliminate crosstalk between two speakers, and to simulate the effect of wearing headphones. Once each of our ears is receiving only the signal that is meant for it (and none of the other channel), we're into binaural territory. HRTF coding can then be used to make the sound appear to come from much further away, and even from bigger, virtual speakers.

Amazingly, spatial sound is even possible from only one loudspeaker! A 'dipole' transmits sound from both the front and the back surfaces of a driver, radiating in a figureofeight pattern (the two outputs will naturally be out of phase with each other). It's then possible to process the resulting output using HRTF algorithms, simulating the effect of several speakers placed around a room.

It seems likely that some form of HRTFbased transaural technology will eventually become the most convenient way of listening to surround sound at home. All those messy satellite speakers, stands and cables could be eliminated by only using one or two speakers inside a TV to simulate a 3D surround system. With HRTF processing the effect can be spectacular: a convincingly solid soundfield is projected out into the room, which now appears to be full of correctly positioned surround speakers. Such systems have been tried in the past without much success, but as the technology continues to get better and cheaper, we can expect to see affordable systems that actually work.

But if all these techniques depend on Head Related Transfer Functions, don't we have a potential problem? Our ears are all different. Ears come in many shapes and sizes, and, like fingerprints, none are exactly the same. One approach is to search for the universal HRTF that suits most people, and there are already many patented variations on this theme.

Alternatively, technology is available that can tailor a unique HRTF to each individual. Either by a series of simple listening tests, or by inspecting a photo of your ear, it is already possible to generate custom HRTFs, and we can expect to see some radical developments in this area in the near future.

It will eventually be possible to modify Ensonido with custom HRTFs, although this has not been implemented in the evaluation version. Fraunhofer do stress though, that Ensonido is not a pure HRTF processor, and that: "It combines the acoustic reception of a human head, room acoustics measurements and simulations and equalisation. The individual HRTF measurement is only one element in the design of Ensonido."

Making HRTFs

I visited the acoustics department of Salford University to see a typical setup for measuring Head Related Transfer Functions. It's not cheap or easy to do! An #8000 Bruel & Kjaer dummy head, fitted with measuring mics, sits in the centre of an anechoic room that is fully floating and acoustically isolated from the rest of the building. The foam wedges on the walls have to be as long as possible, to absorb the lower frequencies (they absorb at quarter wavelength). Even the floor is absorbent and has thick wire mesh suspended above it to walk on. This room is totally anechoic down to 100Hz and cost almost a million pounds to build!

Point One

The 'point one' of surround systems is the LFE channel, socalled because of its very limited bandwidth compared to the other channels. LFE actually means 'low frequency effects' and it is supposed to be used for occasional film sound effects, such as explosions — not kick drums. When I had originally made my Bilocation demo mix I didn't have an LFE track, as the Minnetonka software that I'd used for DTS coding doesn't require one. The MP3 Surround encoder expects an LFE track whether it's used or not, so I had to record a WAV of silence that matched the other five in length and sample rate.

MP3 Stereo eXtended

When the world gets used to MP3 Surround, ordinary stereo will seem far too flat and boring for us to listen to, right? I guess that's the reasoning behind MP3 SX. Simple to use, it can create new surround mixes from stereo: you just drag MP3 files onto the interface. Allegedly "the ambience of the original track is analysed and two rear channels are created to match". I still have lots of old mono records and have never felt the need to convert them to artificial stereo, but I tried to get enthusiastic about MP3 SX anyway. The word 'ambience' is very tantalising here, but misleading. I imagined that dry sounds would remain unchanged in the stereo mix, and that reverbs would become threedimensional: quite an exciting prospect. So I made some MP3s to test it with.

David Bowie's 'Heroes' is well known for its innovative vocal treatment. The vocal starts off quite dry, but when Bowie starts to belt it out after about three minutes, the room acoustics open up and he seems to be louder. It's a great effect in stereo, but converting the track to MP3 SX had the exact opposite effect to what I'd imagined. The whole track was generally 'smaller' and there was a very unpleasant effect like a very slow, pumping compression where the overall level would suddenly drop for no reason. Tony Visconti would not be happy.

I tried some other tracks. Medeski Martin & Wood are a Hammond organ trio with a trademark sound that is rough and sounds very 'roomy'. Here again, the ambience was reduced by MP3 SX. Only cymbals seemed to be expanded into the rear channels, and they seemed to be coming through a cheap '80s chorus pedal. Brian Eno's 'Shadow' from the On Land album was the most successful track I tried, but only because it features a sound like tropical insects that happens to be at the right frequency to get copied into the rear channels. Tracks by Joe Meek and Abba, featuring spring and plate reverb, were very disappointing: hardly anything came out of the rear channels except a bit of hihat.

I give it two out of 10... See me later.

MP3 Surround Links

Download the MP3 Surround software from Fraunhofer:

www.iis.fraunhofer.de

A wide selection of free MP3 Surround music tracks:

www.all4MP3.com

My own web site has lots of information on surround sound, as well as downloadable tracks in 5.1 DTS and MP3 surround:

www.bilocation.co.uk

The Acoustics Dept. of the University of Salford has some useful web pages:

www.acoustics.salford.ac.uk/acoustics_info/

Some very impressive software for creating 3D sound using HRTFs:

www.holistiks.com/amphiotik