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<TD><A HREF="../index.html"><FONT SIZE="2" FACE="Arial, Helvetica"><B>MPEG Layer-3</B></FONT></A></TD>
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<TD><A HREF="../../aac/index.html"><FONT SIZE="2" FACE="Arial, Helvetica"><B>MPEG-2 AAC</B></FONT></A></TD>
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<TD COLSPAN="2"><FONT SIZE="4" FACE="Arial, Helvetica"><B>Frequently Asked Questions about MPEG Audio Layer-3</B></FONT></TD>
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<TD><FONT FACE="Arial, Helvetica"><B>Version 3.0</B> <BR>
(V3.0 is the latest version of this FAQ page from March 1998)</FONT></TD>
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<TD><FONT FACE="Arial, Helvetica">This page gives a comprehensive text about various topics related to audio compression.
Because many foreign pages link to this page we decided to maintain it and recommend it as an comprehensive overview
for off-line processing as printout. Please send questions and comments to </FONT><A HREF="mailto:amm_info@iis.fhg.de, clipplayer@iis.fhg.de"><FONT
FACE="Arial, Helvetica">amm_info@iis.fhg.de</FONT></A></TD>
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<TD COLSPAN="2"><FONT FACE="Arial, Helvetica"><B>Q: O.K., Layer-3 is obviously a key to many applications. Where are its limitations?</B></FONT></TD>
<TD> </TD>
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<TD><FONT FACE="Arial, Helvetica"><B>A:</B> Well, MPEG Layer-3 is a perceptual audio coding scheme, exploiting the properties
of the human ear, and trying to maintain the original sound quality as far as possible.<BR>
In contrast, a dedicated speech codec exploits the properties of the human vocal tract, trying to maintain the
intelligibility of the voice signals as far as possible. Advanced speech coding schemes (e.g., CS-ACELP [LD-CELP]
as standardised by ITU as G.723.1 [G.728]) achieve a useful voice reproduction at bitrates as low as 5.3 [16] kbps,
with a codec delay below 40 [1] ms. At such very low bitrates, they behave superior to MPEG Layer-3 for pure voice signals,
and they offer the low delay that is necessary for full- duplex voice communications.<BR>
In the framework of MPEG-4, scalable audio coding schemes are devised that combine speech coding and perceptual
audio coding.</FONT>
</TD>
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<TD COLSPAN="2"><FONT FACE="Arial, Helvetica"><B>Q: You mentioned the codec delay. May I have some figures?</B></FONT></TD>
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<TD><FONT FACE="Arial, Helvetica"><B>A:</B> Well, the standard gives some figures of the theoretical minimum delay:</FONT>
<UL>
<LI><FONT FACE="Arial, Helvetica">Layer-1: 19 ms (<50 ms)</FONT>
<LI><FONT FACE="Arial, Helvetica">Layer-2: 35 ms (100 ms)</FONT>
<LI><FONT FACE="Arial, Helvetica">Layer-3: 59 ms (150 ms)</FONT>
</UL>
<FONT FACE="Arial, Helvetica">Practical values are significantly above that. As they depend on the implementation,
precise figures are hard to give. So the numbers in brackets are just rough thumb values - real codecs may show
even higher values. So yes, there are certain applications that may suffer from such a delay (like feedback links
for remote reporter units). For many other applications (like the ones mentioned above), delay is of minor interest.</FONT>
<HR ALIGN="CENTER"><CENTER><A HREF="#top"><FONT SIZE="2">| TOP |</FONT></A></CENTER>
</TD>
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<TD COLSPAN="2"><FONT SIZE="3" FACE="Arial, Helvetica"><B>Q: What is "MPEG"?</B></FONT></TD>
<TD> </TD>
</TR>
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<TD><FONT FACE="Arial, Helvetica"><B>A:</B> MPEG is the "<B><FONT COLOR="#990000">M</FONT></B>oving <B><FONT COLOR="#990000">P</FONT></B>icture
<B><FONT COLOR="#990000">E</FONT></B>xperts <B><FONT COLOR="#990000">G</FONT></B>roup",
working under the joint direction of the International Standards Organization (ISO) and the International Electro-Technical
Commission (IEC). This group works on standards for the coding of moving pictures and audio. <A HREF="http://www.cselt.stet.it/mpeg/">
MPEG </A>has created its own homepage, providing information on the what, where, when and how of the standards.</FONT>
</TD>
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<TD COLSPAN="2"><FONT FACE="Arial, Helvetica"><B>Q: Are MPEG-3 and Layer-3 the same thing?</B></FONT></TD>
<TD> </TD>
</TR>
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<TD> </TD>
<TD><FONT FACE="Arial, Helvetica"><B>A:</B> No! Layer-3 is a powerful audio coding scheme which certainly is part
of the MPEG standard. Layer-3 is defined within the audio part of both existing international standards, MPEG-1
and MPEG-2.<BR>
<B>But:</B>There is no MPEG 3 defined.</FONT>
</TD>
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<TD COLSPAN="2"><FONT FACE="Arial, Helvetica"><B>Q: How do I get the MPEG documents?</B></FONT></TD>
<TD> </TD>
</TR>
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<TD> </TD>
<TD><FONT FACE="Arial, Helvetica"><B>A:</B> Well, you may contact <A HREF="http://www.iso.ch/index.html">ISO</A>, or you order it from your
national standards body. E.g., in Germany, please contact <A HREF="mailto:postmaster@din.de">DIN</A>.</FONT>
</TD>
<TD> </TD>
</TR>
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<TD COLSPAN="2"><FONT FACE="Arial, Helvetica"><B>Q: Is some public C source available?</B></FONT></TD>
<TD> </TD>
</TR>
<TR>
<TD> </TD>
<TD><FONT FACE="Arial, Helvetica"><B>A:</B> Well, there is "public C source" available on various sites, e.g. at
<A HREF="ftp://ftp.iis.fhg.de/pub/layer3/public_c/">ftp://ftp.iis.fhg.de/pub/layer3/public_c/</A>. This code has been written mainly for explanation
purposes, so do not expect too much performance.</FONT>
<HR ALIGN="CENTER"><CENTER><A HREF="#top"><FONT SIZE="2">| TOP |</FONT></A></CENTER>
</TD>
<TD> </TD>
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<TD COLSPAN="2"><FONT FACE="Arial, Helvetica"><B>What about Layer-1, Layer-2, Layer-3?</B></FONT></TD>
<TD> </TD>
</TR>
<TR>
<TD COLSPAN="2"><FONT FACE="Arial, Helvetica"><B>Q: Talking about MPEG audio, I always hear "Layer 1, 2 and 3". What
does it mean?</B></FONT>
</TD>
<TD> </TD>
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<TD> </TD>
<TD><FONT FACE="Arial, Helvetica"><B>A:</B> MPEG describes the compression of audio signals using high performance
perceptual coding schemes. It specifies a family of three audio coding schemes, simply called Layer-1, Layer-2,
and Layer-3. From Layer-1 to Layer-3, encoder complexity and performance (sound quality per bitrate) are increasing.<BR>
The three codecs are compatible in a hierarchical way, i.e. a Layer-N decoder may be able to decode bitstream data
encoded in Layer-N and all Layers below N (e.g., a Layer-3 decoder may accept Layer-1,-2,-3, whereas a Layer-2
decoder may accept only Layer-1 and -2.)</FONT>
</TD>
<TD> </TD>
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<TD COLSPAN="2"><FONT FACE="Arial, Helvetica"><B>Q: So we have a family of three audio coding schemes. What does the MPEG standard
define, exactly?</B></FONT>
</TD>
<TD> </TD>
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<TD> </TD>
<TD><FONT FACE="Arial, Helvetica"><B>A:</B> For each Layer, the standard specifies the bitstream format and the
decoder. To allow for future improvements, it does not specify the encoder, but an informative chapter gives an
example for an encoder for each Layer.</FONT>
</TD>
<TD> </TD>
</TR>
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<TD COLSPAN="2"><FONT FACE="Arial, Helvetica"><B>Q: What have the three audio Layers in common?</B></FONT></TD>
<TD> </TD>
</TR>
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<TD> </TD>
<TD><FONT FACE="Arial, Helvetica"><B>A:</B> All Layers use the same basic structure. The coding scheme can be described
as "perceptual noise shaping" or "perceptual subband / transform coding". The encoder analyzes
the spectral components of the audio signal by calculating a filterbank (transform) and applies a psychoacoustic
model to estimate the just noticeable noise-level. In its quantization and coding stage, the encoder tries to allocate
the available number of data bits in a way to meet both the bitrate and masking requirements.<BR>
The decoder is much less complex. Its only task is to synthesize an audio signal out of the coded spectral components.<BR>
All Layers use the same analysis filterbank (polyphase with 32 subbands). Layer-3 adds a MDCT transform to increase
the frequency resolution.<BR>
All Layers use the same "header information" in their bitstream, to support the hierarchical structure
of the standard.<BR>
All Layers have a similar sensitivity to biterrors. They use a bitstream structure that contains parts that are
more sensitive to biterrors ("header", "bit allocation", "scalefactors", "side
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