RTP MPEG2-AAC Payload Format Proposal - PowerPoint by epmd

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									RTP MPEG2-AAC
Payload Format Proposal




    Mathias Kretschmer
    AT&T Labs Research
ISO/IEC MPEG2-AAC

 Variable Frame Length
 Four Frame Types Complicates Strategy
      Quality Degradation Depends on Frame Type & Context
       => Importance/Priority
      Interleaver Should Be Aware of Priorities
 Error Concealment Interpolates Across Lost
  Frames to Give Best Reconstruction



2/11/2009                                                    2
Frame Priority/Importance

 Perceptual Entropy of AAC Frames Depends on the
  Audio Signal
      Transient Signals Hard to Reconstruct
       => Higher Priority
      Stationary Signals Easier to Reconstruct
       => Lower Priority
 Frame Importance Is Dynamic ( Static Priority)
      Depends on Frame Priority and Nearby Loss
       => Decoder Plays Important Role
 Multiple Ways to Interpret Priority Information
2/11/2009                                           3
Priority Vector

 Contains Priority Information About the Current
  and Previously Sent RTP Packets
       P(t)   P(t-1)   P(t-2)   P(t-3)   P(t-4)   P(t-5)



 Enables the Decoder to Choose Among Repair
  Mechanisms to Balance Quality Against Network
  Characteristics
      Limited Bandwidth Channels


2/11/2009                                                  4
Priority Based Interleaving

 Flexible Interleaving Based on Frame Distance
  AND Priority
      This Choice of Grouping Equal Priorities Allows Same
       Decoder Repair Mechanisms
      Priorities Could Be Used effectively for Media Delivery
      Variable Number of AAC Frames May be Grouped for
       Lower Overhead/Optimal Sizing for Anticipated Network
       Characteristics



2/11/2009                                                        5
Repair Information

 Contains Essential Information Of Original Frame
      Worse than Original - But Much Better than Any
       Reconstructed Frame (Transient Signals)
      Way Better Than Muting
 Current Proposal: 16 kBit/s Mono AAC Frame
      Just One Codec Needed
      Doesn’t Break Concealment Scheme
 Enables the Transmitter to Adapt the RTP Stream
  to Known Network Characteristics

2/11/2009                                               6
RTP AAC Payload Format
            0                   1                   2                   3
            0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            |X|REPAIRLEN    |PRI VECTOR                                     | Header
            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            |PRI VECTOR (continued), if X==1                                |
            +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
            |RSEQ           |RLEN           |REPAIRDATA 1                   |
            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
            |                               .                               | Repair
            |                               .                               | Data
            |                               .                               |
            |               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            |               |RSEQ           |RLEN           |REPAIRDATA N   |
            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               |
            |                                                               |
            |                                                               |
            |                                                               |
            +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
            |SEQ            |LEN                    |UBITS |AAC FRAME 1     |
            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               |
            |                               .                               |
            |                               .                               |
            |                               .                               | AAC
            |               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Frames
            |               |SEQ            |LEN                    |UBITS |
            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            |AAC FRAME N                                                    |
            |                                                               |
2/11/2009   |                                                               |          7
            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

								
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