1.2 Spatial audio reproduction

One area where audio systems have recently gained the potential of delivering higher quality is their spatial realism. By increasing the number of audio channels from two (stereophonic reproduction) to five or six (as in many home cinema setups), the spatial properties of a sound scene can be captured and reproduced more accurately. Initially, multi-channel signal representations were almost the exclusive domain of cinemas, but the advent of DVDs and SACDs have made multi-channel audio available in living rooms as well. Interestingly, although multi-channel audio has now been widely adopted on such storage media, broadcast systems for radio and television are still predominantly operating in stereo. The fact that broadcast chains still operate in the two-channel domain has several reasons. One important aspect is that potential ‘upgrades’ of broadcast systems to multi-channel audio should ensure backward compatibility with existing devices that expect (and are often limited to) stereo content only. Secondly, an increase in the number of audio channels from two to five will result in an increase in the amount of information that has to be transmitted by a factor of about 2.5. In many cases, this increase is undesirable or in some cases simply unavailable. With the technology that is currently being used in broadcast environments it is very difficult to overcome these two major limitations.

But besides the home cinema, high-quality multi-channel audio has made its way to mobile applications as well. Music, movie material, and television broadcasts are received, stored, and reproduced by mobile phones or mobile audio/video players. On such devices, an upgrade from stereo to multi-channel audio faces two additional challenges on top of those mentioned above. The first is that the audio content is often reproduced over headphones, making multi-channel reproduction more cumbersome. Secondly, these devices are often operating on batteries. Decoding and reproduction of five audio channels requires more processing and hence battery power than two audio channels, which has a negative effect on a very important aspect of virtually all mobile devices: their battery life. Furthermore, especially in the field of mobile communication, every transmitted bit has a relatively high price tag and hence high efficiency of the applied compression algorithm is a must.