VLC Functions

Variable length coding (VLC) is an audio data compression method that uses statistical modeling to define which values occur more frequently than others to build tables for subsequent encoding and decoding operations. Audio data in the bitstream is encoded with VLC tables so that the shortest codes correspond to the most frequent values and the longer codes correspond to the less frequent values. Every standard that uses variable length coding has corresponding tables named inputTable that list possible codes and their values.

static IppsVLCTable_32s inputTable[]=

{

   {value0, code0, length0};

   {value1, code1, length1};

    ...

{valueN, codeN, lengthN};

}

The use of the functions described in this section is demonstrated in Intel® IPP Samples downloadable from http://www.intel.com/cd/software/products/asmo-na/eng/220046.htm.

• VLCDecodeEscBlock_MP3
  Parses the bitstream and decodes variable length code for MP3 using signed VLC tables.
• VLCDecodeEscBlock_AAC
  Parses the bitstream and decodes variable length code for AAC using signed VLC tables.
• VLCDecodeUTupleEscBlock_MP3
  Parses the bitstream and decodes variable length code for MP3 using unsigned multi-tupled VLC tables.
• VLCDecodeUTupleEscBlock_AAC
  Parses the bitstream and decodes variable length code for AAC using unsigned multi-tupled VLC tables.
• VLCCountEscBits_MP3
  Calculates the number of bits necessary for encoding in MP3 format.
• VLCCountEscBits_AAC
  Calculates the number of bits necessary for encoding in AAC format.
• VLCEncodeEscBlock_MP3
  Encodes an array of values into destination bitstream in MP3 format and advances bitstream pointer.
• VLCEncodeEscBlock_AAC
  Encodes an array of values into destination bitstream in AAC format and advances bitstream pointer.

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