Frequency Domain Prediction Functions

MPEG-2, 4 AAC encoder uses prediction in frequency domain (FDP) to decrease redundancy in the audio signal and ensure more effective coding. For each spectral line, input signals are filtered through the second order adaptive FIR filter called a predictor. Then, instead of processing the signal, the difference between the original signal and the filter output (that is, the prediction error), is passed for further processing. The decoder derives the original signal from the prediction error using a symmetrical block.

You should regularly reset the predictors to their initial state to reduce accumulated calculation error. You may also need to reset the predictors in special cases discussed and described in the ISO-144963 standard. You can reset predictors for the entire spectrum, several scale factor bands, or a selected group of spectral lines.

For more information on the algorithm of filter coefficient adaptation and FDP usage see ISO-144963, clause 6.5.3.2.

To use the FDP prediction tool functions described in this section, follow these steps:

• Call the function ippsFDPInitAlloc to allocate memory and initialize predictor state.
• Call the function ippsFDPFwd for each frame to calculate prediction error or call the function ippsFDPInv to retrieve the original signal.
• Call the functions ippsFDPReset, ippsFDPResetSfb, or ippsFDPResetGroup to reset predictors in certain spectral lines at any time after creating the state.
• Call the function ippsFDPFree to free the memory allocated by ippsFDPInitAlloc.

• FDPInitAlloc
  Creates and initializes predictor state.
• FDPInit
  Initializes predictor state.
• FDPFree
  Closes FDP state.
• FDPGetSize
  Gets size of FDP state structure.
• FDPReset
  Resets predictors for all spectral lines.
• FDPResetSfb
  Resets predictor-specific information in some scale factor bands.
• FDPResetGroup
  Resets predictors for group of spectral lines.
• FDPFwd
  Performs frequency domain prediction procedure and calculates prediction error.
• FDPInv
  Retrieves input signal from prediction error, using frequency domain prediction procedure.

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