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ETSI ES 201 980 V4.1.2 (2017-04) |
Begin
Generate frameError Yes 
Concealing control
data
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No |
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prevFrame- |
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timeCompensate- |
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coupling = |
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Yes |
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ErrorFlag |
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FirstEnvelope |
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prevCoupling |
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No
addConcealing-
EnvelopeData
deltaToLinearPCMEnvelopeDecoding
deltaToLinearPCMEnvelope-
Decoding
check data
set frame error |
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requantise- |
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Yes |
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error |
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No |
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return |
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flag |
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EnvelopeData |
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Figure 9: SBR error concealment overview
5.4.4.5Parametric Stereo concealment
Parametric stereo concealment is based on the fact that the stereo image is quasi-stationary. The concealment strategy keeps the Parametric Stereo settings from the last valid frame until a new set of Parametric Stereo settings can be decoded from a valid frame.
5.4.5AAC + MPS
AAC frames and AAC + SBR frames can be enhanced with MPS data. The MPS part of the audio frame follows immediately the AAC part and the presence of MPS data is signalled in the SDC data entity 9. If the MPEG Surround mode is not 000 then MPS data is available in the audio frame. The MPS data bits are written/read in the same order as the AAC data bits.
Audio Frame n-1 |
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Audio Frame n |
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Audio Frame n+1 |
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AAC data |
MPS data |
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bit reading direction |
bit reading direction |
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stuffing bits |
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Figure 10: AAC + MPS frame
If both MPS and SBR data is present in an audio frame, stuffing bits are inserted between the MPS and SBR data.
ETSI
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45 |
ETSI ES 201 980 V4.1.2 (2017-04) |
Audio Frame n-1 |
Audio Frame n |
Audio Frame n+1 |
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AAC data |
MPS |
SBR |
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data |
data |
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bit reading direction
stuffing bits
Figure 11: AAC + SBR + MPS frame
The combination of AAC or AAC + SBR with MPEG Surround (MPS) uses the MPEG Surround tool as it is defined in ISO/IEC 23003-1 [10]. The method how to extract the MPS elements from the MPS data and thus the mapping to the format defined in the MPEG standard is given in table 16.
Table 16: Syntax of mps_aac_frame()
Syntax |
No. of bits |
Note |
mps_aac_frame() |
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{ |
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mps_crc_bits; |
8 |
see annex D |
ancType; |
2 |
uimsbf |
ancStart; |
1 |
uimsbf |
ancStop; |
1 |
uimsbf |
cnt = ancLenBytes; |
8 |
uimsbf |
if (cnt==255) { |
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cnt += ancLenBytesAdd; |
16 |
uimsbf |
} |
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for (i=0; i<cnt; i++) { |
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ancDataSegmentByte[i]; |
8 |
bslbf |
} |
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} |
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mps_crc_bits |
Cyclic redundancy checksum for the MPS bit stream part. The CRC algorithm covers all bits |
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in the mps_aac_frame() element excluding the mps_crc_bits itself. |
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ancType |
Indicates type of ancillary data, see table 17. |
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Table 17: Definition of ancType |
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ancType |
Meaning |
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0x0 |
SacDataFrame(0) (MPEG Surround frame) |
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0x1 |
SacDataFrame(1) (MPEG Surround header and MPEG Surround frame) |
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0x2, 0x3 |
(reserved) |
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NOTE: SacDataFrame() is defined in [10]. |
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ancStart |
Indicates if data segment begins a data block. |
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ancStop |
Indicates if data segment ends a data block. |
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ancLenBytes |
Number of bytes in data segment. |
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ancLenBytesAdd |
Additional number of bytes in data segment, needed if the data segments contains 255 or |
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more bytes. |
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ancDataSegmentByte |
The concatenation of all ancDataSegmentByte from consecutive mps_aac_frame(), starting |
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from the mps_aac_frame() with ancStart==1 up to and including the mps_aac_frame() with |
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ancStop==1 forms one data block. In case a complete data block is contained in one |
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mps_aac_frame(), it has ancStart==1 and ancStop==1. If ancType==0x0 or ancType==0x1 |
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then this data block constitutes one SacDataFrame() syntax element, padded at the end to |
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obtain an integer number of bytes. |
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ETSI