Spatial Audio Processing: MPEG Surround and Other Applications

6.3 MPEG Surround encoder

6.3.1 Structure

The structure of the MPEG Surround encoder is shown in Figure 6.4. A multi-channel input signal is first processed by a channel-dependent pre-gain. These gains enable adjustment of the level of certain channels (for example LFE and surround) within the transmitted down mix. Subsequently, the input signals are decomposed into time/frequency tiles using an analysis filter bank. A spatial encoder generates a down-mix signal and (encoded) spatial parameters for each time/frequency tile. These parameters are quantized and encoded into a parameter bitstream by a parameter encoder (Q). The down-mix is converted to the time domain using a synthesis filterbank. Finally, a post-gain is applied to control the overall signal level of the down-mix.

Figure 6.4 Structure of the MPEG Surround encoder. Reproduced by permission of the Audio Engineering Society, Inc, New York, USA.

6.3.2 Pre- and post-gains

In the process of down-mixing a multi-channel signal to a stereo signal, it is often desirable to have unequal weights for the different input channels. For example, the surround channels are often attenuated by 3 dB prior to the actual down-mix process. MPEG Surround supports user-controllable pre-gains between 0 and −6 dB, in steps of 1.5 dB. For the LFE, these weights are adjustable between 0 and −20 dB in steps of 5 dB.

The level of the generated down-mix ...