ISO/IEC 13818-7:2006

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Norsk tittel: Information technology — Generic coding of moving pictures and associated audio information — Part 7: Advanced Audio Coding (AAC)
Engelsk tittel: Information technology — Generic coding of moving pictures and associated audio information — Part 7: Advanced Audio Coding (AAC)
Varetype: Standard
Språk: Engelsk
Utgave: 4 (2006-01-09)
Erstatter: ISO/IEC 13818-7:2004 Alert Tilbaketrukket
ISO/IEC 13818-7:2004/Cor 1:2005 Alert Tilbaketrukket
Antall sider: 194
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Omfang: <sec id="sec_1.1"><label>1.1</label><title>General</title><p>This International Standard describes the MPEG-2 audio non-backwards compatible standard called MPEG-2 Advanced Audio Coding, AAC [1], a higher quality multichannel standard than achievable while requiring MPEG-1 backwards compatibility. This MPEG-2 AAC audio standard allows for ITU-R “indistinguishable” quality according to <xref ref-type="bibr" rid="biblref_2">[2]</xref> at data rates of 320 kbit/s for five full-bandwidth channel audio signals.</p><p>The AAC decoding process makes use of a number of required tools and a number of optional tools. <xref ref-type="table" rid="tab_1">Table 1</xref> lists the tools and their status as required or optional. Required tools are mandatory in any possible profile. Optional tools may not be required in some profiles.</p><table-wrap id="tab_1"><label>Table 1</label><caption><title>AAC decoder tools</title></caption><table cellpadding="0" cellspacing="0" frame="box" rules="all" style="border-collapse:collapse;border:solid 2px;" width="488"><colgroup><col align="center" style="border-right:solid 2px;" valign="top" width="61%"/><col align="center" valign="top" width="39%"/></colgroup><thead style="border-bottom:solid 2px;"><tr><th style="padding:0 7.20px 0 7.20px;">Tool Name</th><th style="padding:0 7.20px 0 7.20px;">Required / Optional</th></tr></thead><tbody><tr><td style="padding:0 7.20px 0 7.20px;">Bitstream Formatter</td><td style="padding:0 7.20px 0 7.20px;">Required</td></tr><tr><td style="padding:0 7.20px 0 7.20px;">Noiseless Decoding</td><td style="padding:0 7.20px 0 7.20px;">Required</td></tr><tr><td style="padding:0 7.20px 0 7.20px;">Inverse quantization</td><td style="padding:0 7.20px 0 7.20px;">Required</td></tr><tr><td style="padding:0 7.20px 0 7.20px;">Rescaling</td><td style="padding:0 7.20px 0 7.20px;">Required</td></tr><tr><td style="padding:0 7.20px 0 7.20px;">M/S</td><td style="padding:0 7.20px 0 7.20px;">Optional</td></tr><tr><td style="padding:0 7.20px 0 7.20px;">Prediction</td><td style="padding:0 7.20px 0 7.20px;">Optional</td></tr><tr><td style="padding:0 7.20px 0 7.20px;">Intensity</td><td style="padding:0 7.20px 0 7.20px;">Optional</td></tr><tr><td style="padding:0 7.20px 0 7.20px;">Dependently switched coupling</td><td style="padding:0 7.20px 0 7.20px;">Optional</td></tr><tr><td style="padding:0 7.20px 0 7.20px;">TNS</td><td style="padding:0 7.20px 0 7.20px;">Optional</td></tr><tr><td style="padding:0 7.20px 0 7.20px;">Filterbank / block switching</td><td style="padding:0 7.20px 0 7.20px;">Required</td></tr><tr><td style="padding:0 7.20px 0 7.20px;">Gain control</td><td style="padding:0 7.20px 0 7.20px;">Optional</td></tr><tr><td style="padding:0 7.20px 0 7.20px;">Independently switched coupling</td><td style="padding:0 7.20px 0 7.20px;">Optional</td></tr></tbody></table></table-wrap></sec><sec id="sec_1.2"><label>1.2</label><title>MPEG-2 AAC Tools Overview</title><p>The basic structure of the MPEG-2 AAC system is shown in <xref ref-type="fig" rid="fig_1">Figure 1</xref> and <xref ref-type="fig" rid="fig_2">Figure 2</xref>. As is shown in <xref ref-type="table" rid="tab_1">Table 1</xref>, there are both required and optional tools in the decoder. The data flow in this diagram is from left to right, top to bottom. The functions of the decoder are to find the description of the quantized audio spectra in the bitstream, decode the quantized values and other reconstruction information, reconstruct the quantized spectra, process the reconstructed spectra through whatever tools are active in the bitstream in order to arrive at the actual signal spectra as described by the input bitstream, and finally convert the frequency domain spectra to the time domain, with or without an optional gain control tool. Following the initial reconstruction and scaling of the spectrum reconstruction, there are many optional tools that modify one or more of the spectra in order to provide more efficient coding. For each of the optional tools that operate in the spectral domain, the option to “pass through” is retained, and in all cases where a spectral operation is omitted, the spectra at its input are passed directly through the tool without modification.</p><p>The input to the <underline>bitstream demultiplexer tool</underline> is the MPEG-2 AAC bitstream. The demultiplexer separates the parts of the MPEG-AAC data stream into the parts for each tool, and provides each of the tools with the bitstream information related to that tool.</p><p>The outputs from the bitstream demultiplexer tool are:</p><list list-type="bullet"><list-item><label>•</label><p>The sectioning information for the noiselessly coded spectra,</p></list-item><list-item><label>•</label><p>The noiselessly coded spectra,</p></list-item><list-item><label>•</label><p>The M/S decision information (optional),</p></list-item><list-item><label>•</label><p>The predictor state information (optional),</p></list-item><list-item><label>•</label><p>The intensity stereo control information and coupling channel control information (both optional),</p></list-item><list-item><label>•</label><p>The temporal noise shaping (TNS) information (optional),</p></list-item><list-item><label>•</label><p>The filterbank control information, and</p></list-item><list-item><label>•</label><p>The gain control information (optional).</p></list-item></list><p>The <underline>noiseless decoding tool</underline> takes information from the bitstream demultiplexer, parses that information, decodes the Huffman coded data, and reconstructs the quantized spectra and the Huffman and DPCM coded scalefactors.</p><p>The inputs to the noiseless decoding tool are:</p><list list-type="bullet"><list-item><label>•</label><p>The sectioning information for the noiselessly coded spectra, and</p></list-item><list-item><label>•</label><p>The noiselessly coded spectra.</p></list-item></list><p>The outputs of the Noiseless Decoding tool are:</p><list list-type="bullet"><list-item><label>•</label><p>The decoded integer representation of the scalefactors, and</p></list-item><list-item><label>•</label><p>The quantized values for the spectra.</p></list-item></list><p>The <underline>inverse quantizer tool</underline> takes the quantized values for the spectra, and converts the integer values to the non-scaled, reconstructed spectra. This quantizer is a non-uniform quantizer.</p><p>The input to the Inverse Quantizer tool is:</p><list list-type="bullet"><list-item><label>•</label><p>The quantized values for the spectra.</p><p>The output of the inverse quantizer tool is:</p></list-item><list-item><label>•</label><p>The un-scaled, inversely quantized spectra.</p></list-item></list><p>The <underline>rescaling tool</underline> converts the integer representation of the scalefactors to the actual values, and multiplies the un-scaled inversely quantized spectra by the relevant scalefactors.</p><p>The inputs to the rescaling tool are:</p><list list-type="bullet"><list-item><label>•</label><p>The decoded integer representation of the scalefactors, and</p></list-item><list-item><label>•</label><p>The un-scaled, inversely quantized spectra.</p></list-item></list><p>The output from the scalefactors tool is:</p><list list-type="bullet"><list-item><label>•</label><p>The scaled, inversely quantized spectra.</p></list-item></list><p>The <underline>M/S tool</underline> converts spectra pairs from Mid/Side to Left/Right under control of the M/S decision information in order to improve coding efficiency.</p><p>The inputs to the M/S tool are:</p><list list-type="bullet"><list-item><label>•</label><p>The M/S decision information, and</p></list-item><list-item><label>•</label><p>The scaled, inversely quantized spectra related to pairs of channels.</p></list-item></list><p>The output from the M/S tool is:</p><list list-type="bullet"><list-item><label>•</label><p>The scaled, inversely quantized spectra related to pairs of channels, after M/S decoding.</p></list-item></list><non-normative-note><label>Note</label><p>The scaled, inversely quantized spectra of individually coded channels are not processed by the M/S block, rather they are passed directly through the block without modification. If the M/S block is not active, all spectra are passed through this block unmodified.</p></non-normative-note><p>The <underline>prediction tool</underline> reverses the prediction process carried out at the encoder. This prediction process re-inserts the redundancy that was extracted by the prediction tool at the encoder, under the control of the predictor state information. This tool is implemented as a second order backward adaptive predictor. The inputs to the prediction tool are:</p><list list-type="bullet"><list-item><label>•</label><p>The predictor state information, and</p></list-item><list-item><label>•</label><p>The scaled, inversely quantized spectra.</p></list-item></list><p>The output from the prediction tool is:</p><list list-type="bullet"><list-item><label>•</label><p>The scaled, inversely quantized spectra, after prediction is applied.</p></list-item></list><non-normative-note><label>Note</label><p>If the prediction is disabled, the scaled, inversely quantized spectra are passed directly through the block without modification.</p></non-normative-note><p>The <underline>intensity stereo tool</underline> implements intensity stereo decoding on pairs of spectra.</p><p>The inputs to the intensity stereo tool are:</p><list list-type="bullet"><list-item><label>•</label><p>The inversely quantized spectra, and</p></list-item><list-item><label>•</label><p>The intensity stereo control information.</p></list-item></list><p>The output from the intensity stereo tool is:</p><list list-type="bullet"><list-item><label>•</label><p>The inversely quantized spectra after intensity channel decoding.</p></list-item></list><non-normative-note><label>Note</label><p>The scaled, inversely quantized spectra of individually coded channels are passed directly through this tool without modification, if intensity stereo is not indicated. The intensity stereo tool and M/S tool are arranged so that the operation of M/S and intensity stereo are mutually exclusive on any given scalefactor band and group of one pair of spectra.</p></non-normative-note><p>The <underline>coupling tool</underline> for <underline>dependently switched</underline> coupling channels adds the relevant data from dependently switched coupling channels to the spectra, as directed by the coupling control information.</p><p>The inputs to the coupling tool are:</p><list list-type="bullet"><list-item><label>•</label><p>The inversely quantized spectra, and</p></list-item><list-item><label>•</label><p>The coupling control information.</p></list-item></list><p>The output from the coupling tool is:</p><list list-type="bullet"><list-item><label>•</label><p>The inversely quantized spectra coupled with the dependently switched coupling channels.</p></list-item></list><non-normative-note><label>Note</label><p>The scaled, inversely quantized spectra are passed directly through this tool without modification, if coupling is not indicated. Depending on the coupling control information, dependently switched coupling channels might either be coupled before or after the TNS processing.</p></non-normative-note><p>The <underline>coupling tool</underline> for <underline>independently switched</underline> coupling channels adds the relevant data from independently switched coupling channels to the time signal, as directed by the coupling control information.</p><p>The inputs to the coupling tool are:</p><list list-type="bullet"><list-item><label>•</label><p>The time signal as output by the filterbank, and</p></list-item><list-item><label>•</label><p>The coupling control information.</p></list-item></list><p>The output from the coupling tool is:</p><list list-type="bullet"><list-item><label>•</label><p>The time signal coupled with the independently switched coupling channels.</p></list-item></list><non-normative-note><label>Note</label><p>The time signal is passed directly through this tool without modification, if coupling is not indicated.</p></non-normative-note><p>The <underline>temporal noise shaping (TNS) tool</underline> implements a control of the fine time structure of the coding noise. In the encoder, the TNS process has flattened the temporal envelope of the signal to which it has been applied. In the decoder, the inverse process is used to restore the actual temporal envelope(s), under control of the TNS information. This is done by applying a filtering process to parts of the spectral data.</p><p>The inputs to the TNS tool are:</p><list list-type="bullet"><list-item><label>•</label><p>The inversely quantized spectra, and</p></list-item><list-item><label>•</label><p>The TNS information.</p></list-item></list><p>The output from the TNS block is:</p><list list-type="bullet"><list-item><label>•</label><p>The inversely quantized spectra.</p></list-item></list><non-normative-note><label>Note</label><p>If this block is disabled, the inversely quantized spectra are passed through without modification.</p></non-normative-note><p>The <underline>filterbank / block switching tool</underline> applies the inverse of the frequency mapping that was carried out in the encoder. An inverse modified discrete cosine transform (IMDCT) is used for the filterbank tool. The IMDCT can be configured to support either one set of 128 or 1024, or four sets of 32 or 256 spectral coefficients.</p><p>The inputs to the filterbank tool are:</p><list list-type="bullet"><list-item><label>•</label><p>The inversely quantized spectra, and</p></list-item><list-item><label>•</label><p>The filterbank control information.</p></list-item></list><p>The output(s) from the filterbank tool is (are):</p><list list-type="bullet"><list-item><label>•</label><p>The time domain reconstructed audio signal(s).</p></list-item></list><p>When present, the <underline>gain control tool</underline> applies a separate time domain gain control to each of four frequency bands that have been created by the gain control PQF filterbank in the encoder. Then, it assembles four frequency bands and reconstructs the time waveform through the gain control tool’s filterbank.</p><p>The inputs to the gain control tool are:</p><list list-type="bullet"><list-item><label>•</label><p>The time domain reconstructed audio signal(s), and</p></list-item><list-item><label>•</label><p>The gain control information.</p></list-item></list><p>The output(s) from the gain control tool is (are):</p><list list-type="bullet"><list-item><label>•</label><p>The time domain reconstructed audio signal(s).</p></list-item></list><p>If the gain control tool is not active, the time domain reconstructed audio signal(s) are passed directly from the filterbank tool to the output of the decoder. This tool is used for the scalable sampling rate (SSR) profile only.</p><fig id="fig_1"><label>Figure 1</label><caption><title>MPEG-2 AAC Encoder Block Diagram</title></caption><graphic xmlns:p1="http://www.w3.org/1999/xlink" p1:href="fig_1" p1:type="simple"/></fig><fig id="fig_2"><label>Figure 2</label><caption><title>MPEG-2 AAC Decoder Block Diagram</title></caption><graphic xmlns:p2="http://www.w3.org/1999/xlink" p2:href="fig_2" p2:type="simple"/></fig></sec>
Komité: SN/K 172
Fastsatt: 2006-01-09
ICS: 35.040.40 - Koding av lyd, video, multimedia- og hypermediainformasjon
35.040 - Tegnsett og informasjonskoding