3 Definitions, symbols and abbreviations

26.4043GPPEnhanced aacPlus encoder Spectral Band Replication (SBR) partEnhanced aacPlus general audio codecGeneral audio codec audio processing functionsRelease 17TS

3.1 Definitions

For the purposes of this TS, the following definitions apply:

band: (as in limiter band, noise floor band, etc.) a group of consecutive QMF subbands

chirp factor: the bandwidth expansion factor of the formants described by a LPC polynomial

Down Sampled SBR: the SBR Tool with a modified synthesis filterbank resulting in a down sampled output signal with the same sample rate as the input signal to the SBR Tool. May be used whenever a lower sample rate output is desired.

envelope scalefactor: an element representing the averaged energy of a signal over a region described by a frequency band and a time segment

frequency band: interval in frequency, group of consecutive QMF subbands

frequency border: frequency band delimiter, expressed as a specific QMF subband

noise floor: a vector of noise floor scalefactors

noise floor scalefactor: an element associated with a region described by a frequency band and a time segment, representing the ratio between the energy of the noise to be added to the envelope adjusted HF generated signal and the energy of the same

patch: a number of adjoining QMF subbands moved to a different frequency location

SBR envelope: a vector of envelope scalefactors

SBR frame: time segment associated with one SBR extension data element

SBR range: the frequency range of the signal generated by the SBR algorithm

subband: a frequency range represented by one row in a QMF matrix, carrying a subsampled signal

time border: time segment delimiter, expressed as a specific time slot

time segment: interval in time, group of consecutive time slots

time / frequency grid: a description of SBR envelope time segments and associated frequency resolution tables as well as description of noise floor time segments

time slot: finest resolution in time for SBR envelopes and noise floors. One time slot equals two subsamples in the QMF domain

3.2 Symbols

For the purposes of this TS, the following symbols apply:

Description of variables defined in one sub clause and used in other subclasses.

ch is the current channel, and when used as index in vectors left channel is represented by ch= 0 and right channel is represented ch= 1.

E_Orig has L_E columns where each column is of length N_Low or N_High depending on the frequency resolution for each SBR envelope. The elements in E_Orig contains the envelope scalefactors of the original signal.

has two column vectors containing the frequency border tables for low and high frequency resolution.

internal sampling frequency of the SBR Tool, twice the sampling frequency of the core coder (after sampling frequency mapping, Table 4.55). The sampling frequency of the SBR enhanced output signal is equal to the internal sampling frequency of the SBR Tool, unless the SBR Tool is operated in downsampled mode. If the SBR Tool is operated in downsampled mode, the output sampling frequency is equal to the sampling frequency of the core coder.

f_Master is of length N_Master+1 and contains QMF master frequency grouping information.

f_TableHigh is of length N_High+1 and contains frequency borders for high frequency resolution SBR envelopes.

f_TableLow is of length N_Low+1 and contains frequency borders for low frequency resolution SBR envelopes.

f_TableNoise is of length N_Q+1 and contains frequency borders used by noise floors.

k_x the first QMF subband in the SBR range.

k₀ the first QMF subband in the f_Master table.

L_E number of SBR envelopes.

L_Q number of noise floors.

M number of QMF subbands in the SBR range.

middleBorder points to a specific time border.

N_L number of limiter bands.

N_Master number of frequency bands in the master frequency resolution table.

N_Q number of noise floor bands.

number of frequency bands for low and high frequency resolution.

numPatches a variable indicating the number of patches in the SBR range.

numTimeSlots number of SBR envelope time slots that exist within an AAC frame, 16 for a 1024 AAC frame .

offset-values for the SBR envelope and noise floor data, when using coupled channels.

patchBorders a vector containing the frequency borders of the patches.

patchNumSubbands a vector holding the number of subbands in every patch.

Q_Orig has L_Q columns where each column is of length N_Q and contains the noise floor scalefactors.

frequency resolution for all SBR envelopes in the current SBR frame, zero for low resolution, one for high resolution.

reset a variable in the encoder and the decoder set to one if certain bitstream elements have changed from the previous SBR frame, otherwise set to zero.

t_E is of length L_E+1 and contains start and stop time borders for all SBR envelopes in the current SBR frame.

t_HFAdj offset for the envelope adjuster module.

t_HFGen offset for the HF-generation module.

t_Q is of length L_Q+1 and contains start and stop time borders for all noise floors in the current SBR frame.

3.3 Abbreviations

For the purposes of this TS, the following abbreviations apply.

NA Not Applicable

aacPlus Combination of MPEG-4 AAC and MPEG-4 Bandwidth extension (SBR)

Enhanced aacPlus Combination of MPEG-4 AAC, MPEG-4 Bandwidth extension (SBR) and MPEG-4 Parametric Stereo

QMF Quadrature Mirror Filter

SBR Spectral Band Replication