VLCDecodeEscBlock_AAC

Syntax

Parameters

Description

The function ippsVLCDecodeEscBlock_AAC is declared in the ippac.h header file. The function parses the bitstream and decodes variable length code using the VLCDecoder specification structure built by the function ippsVLCDecodeInit_32s or ippsVLCDecodeInitAlloc_32s from data compression domain, and resets the pointers to new positions. The pointer pBitStream points to the 8-bit value and the bit offset may vary from one to 32. After processing, the pointer and bit offset are changed and their new values are returned. To use the function ippsVLCDecodeEscBlock_AAC, rearrange inputTable before initialization as follows:

value = (first_sample_value << 8) + (second_sample_value + 128).

code = (((code_ini << (first_sample_value != 0)) + (first_sample_value < 0)) << (second_sample_value != 0)) + (first_sample_value < 0).

length = length_ini + (first_sample_value != 0) + (second_sample_value != 0).

AACEsc VLC codebook is an unsigned two-dimensional codebook with a large absolute value is equal to 16.

From one record of initial inputTable you can obtain from one to four records of rearranged inputTable. For example,

Initial input table record - {159, 0x1ad, 9}
abs(first_sample_value) = 159/17 = 9;
abs(second_sample_value) = 159 - 17*9 = 6;

code_ini = 0x1ad
length_ini = 9;

record 1: first_sample_value = -9, second_sample_value = -6;
		value = (-9 << 8) + (-6 + 128) = -2182;
		code = (((0x1ad << 1) + 1) << 1) + 1 = 0x6b7;
		length = 9 + 1 + 1 = 11;
		{-2182, 0x6b7, 11)

record 2: first_sample_value = -9, second_sample_value = 6;
		value = (-9 << 8) + (6 + 128) = -2170;
		code = (((0x1ad << 1) + 1) << 1) + 0 = 0x6b6;
		length = 9 + 1 + 1 = 11;
		{-2170, 0x6b6, 11)

record 3: first_sample_value = 9, second_sample_value = -6;
		value = (9 << 8) + (-6 + 128) = 2426;
		code = (((0x1ad << 1) + 0) << 1) + 1 = 0x6b5;
		length = 9 + 1 + 1 = 11;
		{2426, 0x6b7, 11)

record 4: first_sample_value = 9, second_sample_value = 6;
		value = (9 << 8) + (6 + 128) = 2438;
		code = (((0x1ad << 1) + 0) << 1) + 0 = 0x6b4;
		length = 9 + 1 + 1 = 11;
		{2438, 0x6b7, 11)

The function ippsVLCDecodeEscBlock_AAC is used in the float-point and fixed-point versions of AAC decoder included into IPP Samples. See introduction to this section.

Return Values