Implements stage 2 of the MP3 hybrid analysis filterbank.
Syntax
IppStatus ippsMDCTFwd_MP3_32s(const Ipp32s* pSrc, Ipp32s* pDst, int blockType, int mixedBlock, IppMP3FrameHeader* pFrameHeader, Ipp32s* pOverlapBuf);
Parameters
pSrc |
Pointer to the start of the 576-element block PQMF analysis output vector containing 18 consecutive blocks of 32 subband samples that are indexed as follows: pDstS[32* i+ sb], where i = 0,1,...,17 is time series index, sb = 0,1,...,31 is the subband index. All coefficients are represented using the Q7.24 format. |
blockType |
Block type indicator: |
| 0 0 stands for normal block 1 stands for start block 2 stands for short block 3 stands for stop block. | |
mixedBlock |
Mixed block indicator:0 stands for not mixed 1 stands for mixed. |
pFrameHeader |
Pointer to the IppMP3FrameHeader structure that contains the header associated with the current frame. Only MPEG‑1 (id = 1) is supported. |
pOverlapBuf |
Pointer to the MDCT overlap buffer that contains a copy of the most recent 576-element block of PQMF bank outputs. Prior to processing a new audio stream with the analysis filterbank, all elements of this buffer must be initialized to the constant value 0. |
pDst |
Pointer to the 576-element spectral coefficient output vector generated by the analysis filterbank. |
Description
The function is declared in the ippac.h file. This function implements the second stage of MP3 hybrid analysis filterbank by performing the following operations:
- Forward MDCT. An appropriately arranged set of 12-point and/or 36-point forward Modified Discrete Cosine Transforms (MDCTs) is applied to the 18-sample spectral coefficient blocks generated on each of the 32 PQMF subbands during the first stage analysis.
- Aliasing reduction butterflies. The butterflies specified in ISO/IEC 11172-3 are applied to the MDCT outputs in order to mitigate the aliasing artifacts introduced by cascading two critically sampled analysis filterbanks. Each of these introduces some non-negligible amount of interband aliasing.
The function ippsMDCTFwd_MP3_32s updates the 576-element MDCT overlap buffer pMDCTOverlap[], the contents of which must be preserved between calls to facilitate coherent block processing. The function must be applied once per granule on each channel (that is, applied twice per channel on each frame).
Note
Input coefficients are represented in the Q7.24 format. Output coefficients are represented in the Q5.26 format.
Return Values
ippStsNoErr |
Indicates no error. |
ippStsNullPtrErr |
Indicates an error when any of the specified pointers is NULL. |