International organisation for standardisation

Bitstream Issues

Max Neuendorf,

m20069

Follow-up on proposed revision of USAC bit stream syntax

Max Neuendorf, Markus Multrus, Stefan Döhla, Heiko Purnhagen, Frans de Bont, Julien Robilliard, Philippe Gournay,

The contribution proposes a number of revisions to the USAC syntax. Changes can be divided into three categories:

• 
  Interface to Systems
  
• 
  Configuration
  
• 
  Payload
  

This is more clearly shown in the following figure:

It proposes to re-structure the configuration structure: it is more modular and includes a general extension mechanism (which signals type and length). This permits decoders that are not aware of extension type to skip the extension. Extensions already defined are:

• 
  MPEG Surround
  
• 
  SAOC
  
• 
  Fill Bytes
  

The proposed changes make eSBR an integrated part of the USAC bitstream, whereas previously it’s syntax operated as if it was an optional post-processor.

It is proposed to add to each payload one or two additional bits that permit decoding of the current frame even when the previous frame was not received (i.e. due to channel errors or channel jitter). This increase in rate is offset by the adoption of the SBR default header, SbrDfltHeader(), structure. The presenter noted that these are the only changes that alter the payload “bit budget.”

The contribution proposes to implement all proposed changes into the USAC encoder/decoder that will be used in the USAC Verification Test.

The presenter noted that the impact on break-in on streaming or broadcast content would be minimal as the additional configuration bits (if any) are “amortized” over the entire file or over the set of coded frames corresponding to the break-in interval.

The UsacConfig() is now stand-alone, and does not require any MPEG-4 config. However, there is an interface define such that USAC can be carried under the MPEG-4 “umbrella” and hence use the existing MPEG-4 infrastructure for carriage in e.g. MPEG-2 Systems.

It was the consensus of the Audio Subgroup to adopt the changes proposed in the contribution plus the verbally propsed change:

To add a usacExtElementPresent bit in the payload for each Extension Element that was signalled as active in the usacConfig().

into the Study on USAC DIS.

Kristofer Kjörling, Dolby, presented

m20035

Proposed profiles for MPEG-D USAC

Bernhard Grill, Max Neuendorf, Roch Lefebvre, Kristofer Kjörling, Heiko Purnhagen, Werner Oomen

The contribution reviewed the role of profiles in MPEG and the history of the successful AAC family of profiles in MPEG-4. The contribution proposes two profiles: Baseline USAC Profile and HE-AAC V3 Profile.