The MPEG Surround decoder structure is outlined in Figure 6.8. The down-mix is first processed by a pre-gain, which is the inverse of the post-gain of the MPEG Surround encoder. Subsequently, the input signals are processed by an analysis filterbank that is identical to the filterbank described in Section 6.3.3. A spatial decoder regenerates multichannel audio by reinstating the spatial properties described by the decoded parameters. Finally, applying a set of synthesis filterbanks and post-gains (the inverse of the encoder pre-gains) results in the time domain multi-channel output signals.
The spatial decoder generates multi-channel output signals from the down-mixed input signal by reinstating the spatial cues captured by the spatial parameters. The spatial synthesis of OTT decoding blocks employs so-called decorrelators and matrix operations in a similar fashion as parametric stereo decoders . In a OTT decoding block, two output signals with the correct spatial cues are generated by mixing a mono input signal with the output of a decorrelator that is fed with that mono input signal.