As described in the previous section, a BCC synthesis engine or MPEG Surround decoder can in principle be used to perform the combined process of source signal decoding and mixing. The MPEG audio subgroup is currently developing a new standard for ‘spatial audio object coding’ (SAOC) that aims at the combined process of joint source coding and mixing as described in the current chapter. Their approach is to develop a socalled transcoder that generates an MPEG Surround compatible bitstream from transmitted object parameters and mixing parameters (also referred to as ‘rendering’ parameters). The proposed scheme is outlined in Figure 9.7.
The use of an MPEG Surround decoder as rendering engine has several advantages. First, it provides a proven, and flexible rendering engine for many different output channel configurations. Second, the down-mix may comprise two channels as well. Third, it provides binaural rendering facilities for headphones. But besides the functionality provided by the MPEG Surround decoder, additional functionality is envisioned as well. The proposed system also comprises a pre-processing stage to send of one or more objects to external effect stages, such as reverberators, delays, modulation effects, dynamic range reduction, etc. (not shown in Figure 9.7).