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Update twolame to 0.3.13.

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<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<meta name="generator" content="AsciiDoc 7.1.2" />
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<title>TwoLAME: MPEG Audio Layer II VBR</title>
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<body class="article">
<div id="header">
<h1>TwoLAME: MPEG Audio Layer II VBR</h1>
<span id="revision">version 0.3.11</span>
<span id="revnumber">version 0.3.13</span>
</div>
<h2>Contents</h2>
<div id="content">
<div class="sect1">
<h2 id="_contents">Contents</h2>
<div class="sectionbody">
<ul>
<div class="ulist"><ul>
<li>
<p>
Introduction
@@ -33,7 +40,7 @@ Bitrate Ranges for various Sampling frequencies
</li>
<li>
<p>
Why can't the bitrate vary from 32kbps to 384kbps for every file?
Why can&#8217;t the bitrate vary from 32kbps to 384kbps for every file?
</p>
</li>
<li>
@@ -51,29 +58,33 @@ Long Answer
Tech Stuff
</p>
</li>
</ul>
</ul></div>
</div>
<h2>Introduction</h2>
</div>
<div class="sect1">
<h2 id="_introduction">Introduction</h2>
<div class="sectionbody">
<p>VBR mode works by selecting a different bitrate for each frame. Frames
which are harder to encode will be allocated more bits i.e. a higher bitrate.</p>
<p>LayerII VBR is a complete hack - the ISO standard actually says that decoders are not
<div class="paragraph"><p>VBR mode works by selecting a different bitrate for each frame. Frames
which are harder to encode will be allocated more bits i.e. a higher bitrate.</p></div>
<div class="paragraph"><p>LayerII VBR is a complete hack - the ISO standard actually says that decoders are not
required to support it. As a hack, its implementation is a pain to try and understand.
If you're mega-keen to get full range VBR working, either (a) send me money (b) grab the
ISO standard and a C compiler and email me.</p>
If you&#8217;re mega-keen to get full range VBR working, either (a) send me money (b) grab the
ISO standard and a C compiler and email me.</p></div>
</div>
<h2>Usage</h2>
</div>
<div class="sect1">
<h2 id="_usage">Usage</h2>
<div class="sectionbody">
<div class="literalblock">
<div class="content">
<pre><tt>twolame -v [level] inputfile outputfile.</tt></pre>
</div></div>
<p>A level of 5 works very well for me.</p>
<p>The level value can is a measurement of quality - the higher
<div class="paragraph"><p>A level of 5 works very well for me.</p></div>
<div class="paragraph"><p>The level value can is a measurement of quality - the higher
the level the higher the average bitrate of the resultant file.
[See TECH STUFF for a better explanation of what the value does]</p>
<p>The confusing part of my implementation of LayerII VBR is that it's different from MP3 VBR.</p>
<ul>
See TECH STUFF for a better explanation of what the value does.</p></div>
<div class="paragraph"><p>The confusing part of my implementation of LayerII VBR is that it&#8217;s different from MP3 VBR.</p></div>
<div class="ulist"><ul>
<li>
<p>
The range of bitrates used is controlled by the input sampling frequency. (See below "Bitrate ranges")
@@ -84,17 +95,19 @@ The range of bitrates used is controlled by the input sampling frequency. (See b
The tendency to use higher bitrates is governed by the &lt;level&gt;.
</p>
</li>
</ul>
<p>E.g. Say you have a 44.1kHz Stereo file. In VBR mode, the bitrate can range from 192 to 384 kbps.</p>
<p>Using "-v -5" will force the encoder to favour the lower bitrate.</p>
<p>Using "-v 5" will force the encoder to favour the upper bitrate.</p>
<p>The value can actually be <strong>any</strong> int. -27, 233, 47. The larger the number, the greater
the bitrate bias.</p>
</ul></div>
<div class="paragraph"><p>E.g. Say you have a 44.1kHz Stereo file. In VBR mode, the bitrate can range from 192 to 384 kbps.</p></div>
<div class="paragraph"><p>Using "-v -5" will force the encoder to favour the lower bitrate.</p></div>
<div class="paragraph"><p>Using "-v 5" will force the encoder to favour the upper bitrate.</p></div>
<div class="paragraph"><p>The value can actually be <strong>any</strong> int. -27, 233, 47. The larger the number, the greater
the bitrate bias.</p></div>
</div>
<h2>Bitrate Ranges</h2>
</div>
<div class="sect1">
<h2 id="_bitrate_ranges">Bitrate Ranges</h2>
<div class="sectionbody">
<p>When making a VBR stream, the bitrate is only allowed to vary within
set limits</p>
<div class="paragraph"><p>When making a VBR stream, the bitrate is only allowed to vary within
set limits</p></div>
<div class="literalblock">
<div class="content">
<pre><tt>48kHz
@@ -111,10 +124,12 @@ Stereo: 192-384kbps Mono: 96-192kbps</tt></pre>
Stereo/Mono: 8-160kbps</tt></pre>
</div></div>
</div>
<h2>Why doesn't the VBR mode work the same as MP3VBR? The Short Answer</h2>
</div>
<div class="sect1">
<h2 id="_why_doesn_8217_t_the_vbr_mode_work_the_same_as_mp3vbr_the_short_answer">Why doesn&#8217;t the VBR mode work the same as MP3VBR? The Short Answer</h2>
<div class="sectionbody">
<p><strong>Why can't the bitrate vary from 32kbps to 384kbps for every file?</strong></p>
<p>According to the standard (ISO/IEC 11172-3:1993) Section 2.4.2.3</p>
<div class="paragraph"><p><strong>Why can&#8217;t the bitrate vary from 32kbps to 384kbps for every file?</strong></p></div>
<div class="paragraph"><p>According to the standard (ISO/IEC 11172-3:1993) Section 2.4.2.3</p></div>
<div class="literalblock">
<div class="content">
<pre><tt>"In order to provide the smallest possible delay and complexity, the
@@ -130,16 +145,19 @@ Stereo/Mono: 8-160kbps</tt></pre>
<div class="content">
<pre><tt>"For Layer II, not all combinations of total bitrate and mode are allowed."</tt></pre>
</div></div>
<p>Hence, most LayerII coders would not have been written with VBR in mind, and
<div class="paragraph"><p>Hence, most LayerII coders would not have been written with VBR in mind, and
LayerII VBR is a hack. It works for limited cases. Getting it to work to
the same extent as MP3-style VBR will be a major hack.</p>
<p>(If you <strong>really</strong> want better bitrate ranges, read "The Long Answer" and submit your mega-patch.)</p>
the same extent as MP3-style VBR will be a major hack.</p></div>
<div class="paragraph"><p>(If you <strong>really</strong> want better bitrate ranges, read "The Long Answer" and submit your mega-patch.)</p></div>
</div>
<h2>Why doesn't the VBR mode work the same as MP3VBR? The Long Answer</h2>
</div>
<div class="sect1">
<h2 id="_why_doesn_8217_t_the_vbr_mode_work_the_same_as_mp3vbr_the_long_answer">Why doesn&#8217;t the VBR mode work the same as MP3VBR? The Long Answer</h2>
<div class="sectionbody">
<p><strong>Why can't the bitrate vary from 32kbps to 384kbps for every file?</strong></p>
<h3>Reason 1: The standard limits the range</h3>
<p>As quoted above from the standard for 48/44.1/32kHz:</p>
<div class="paragraph"><p><strong>Why can&#8217;t the bitrate vary from 32kbps to 384kbps for every file?</strong></p></div>
<div class="sect2">
<h3 id="_reason_1_the_standard_limits_the_range">Reason 1: The standard limits the range</h3>
<div class="paragraph"><p>As quoted above from the standard for 48/44.1/32kHz:</p></div>
<div class="literalblock">
<div class="content">
<pre><tt>"For Layer II, not all combinations of total bitrate and mode are allowed. See
@@ -164,22 +182,24 @@ the same extent as MP3-style VBR will be a major hack.</p>
320 stereo only
384 stereo only</tt></pre>
</div></div>
<p>So based upon this table alone, you <strong>could</strong> have VBR stereo encoding which varies
<div class="paragraph"><p>So based upon this table alone, you <strong>could</strong> have VBR stereo encoding which varies
smoothly from 96 to 384kbps. Or you could have have VBR mono encoding which varies from
32 to 192kbps. But since the top and bottom bitrates don't apply to all modes, it would
be impossible to have a stereo file encoded from 32 to 384 kbps.</p>
<p>But this isn't what is really limiting the allowable bitrate range - the bit allocation
tables are the major hurdle.</p>
<h3>Reason 2: The bit allocation tables don't allow it</h3>
<p>From the standard, Section 2.4.3.3.1 "Bit allocation decoding"</p>
32 to 192kbps. But since the top and bottom bitrates don&#8217;t apply to all modes, it would
be impossible to have a stereo file encoded from 32 to 384 kbps.</p></div>
<div class="paragraph"><p>But this isn&#8217;t what is really limiting the allowable bitrate range - the bit allocation
tables are the major hurdle.</p></div>
</div>
<div class="sect2">
<h3 id="_reason_2_the_bit_allocation_tables_don_8217_t_allow_it">Reason 2: The bit allocation tables don&#8217;t allow it</h3>
<div class="paragraph"><p>From the standard, Section 2.4.3.3.1 "Bit allocation decoding"</p></div>
<div class="literalblock">
<div class="content">
<pre><tt>"For different combinations of bitrate and sampling frequency, different bit
allocation tables exist.</tt></pre>
</div></div>
<p>These bit allocation tables are pre-determined tables (in Annex B of the standard) which
indicate</p>
<ul>
<div class="paragraph"><p>These bit allocation tables are pre-determined tables (in Annex B of the standard) which
indicate</p></div>
<div class="ulist"><ul>
<li>
<p>
how many bits to read for the initial data (2,3 or 4)
@@ -191,11 +211,11 @@ these bits are then used as an index back into the table to
find the number of quantize levels for the samples in this subband
</p>
</li>
</ul>
<p>But the table used (and hence the number of bits and the calculated index) are different
for different combinations of bitrate and sampling frequency.</p>
<p>I will use TableB.2a as an example.</p>
<p>Table B.2a Applies for the following combinations.</p>
</ul></div>
<div class="paragraph"><p>But the table used (and hence the number of bits and the calculated index) are different
for different combinations of bitrate and sampling frequency.</p></div>
<div class="paragraph"><p>I will use TableB.2a as an example.</p></div>
<div class="paragraph"><p>Table B.2a Applies for the following combinations.</p></div>
<div class="literalblock">
<div class="content">
<pre><tt>Sampling Freq Bitrates in (kbps/channel) [emphasis: this is a PER CHANNEL bitrate]
@@ -203,17 +223,17 @@ for different combinations of bitrate and sampling frequency.</p>
44.1 56, 64, 80
32 56, 64, 80</tt></pre>
</div></div>
<p>If we have a STEREO 48kHz input file, and we use this table, then the bitrates
we could calculate from this would be 112, 128, 160, 192, 224, 256, 320 and 384 kbps.</p>
<p>This table contains no information on how to encode stuff at bitrates less than 112kbps
<div class="paragraph"><p>If we have a STEREO 48kHz input file, and we use this table, then the bitrates
we could calculate from this would be 112, 128, 160, 192, 224, 256, 320 and 384 kbps.</p></div>
<div class="paragraph"><p>This table contains no information on how to encode stuff at bitrates less than 112kbps
(for a stereo file). You would have to load allocation table B.2c to encode stereo at
64kbps and 128kbps.</p>
<p>Since it would be a MAJOR piece of hacking to get the different tables shifted in and out
during the encoding process, once an allocation table is loaded <strong>IT IS NOT CHANGED</strong>.</p>
<p>Hence, the best table is picked at the start of the encoding process, and the encoder
is stuck with it for the rest of the encode.</p>
<p>For twolame-02j, I have picked the table it loads for different
sampling frequencies in order to optimize the range of bitrates possible.</p>
64kbps and 128kbps.</p></div>
<div class="paragraph"><p>Since it would be a MAJOR piece of hacking to get the different tables shifted in and out
during the encoding process, once an allocation table is loaded <strong>IT IS NOT CHANGED</strong>.</p></div>
<div class="paragraph"><p>Hence, the best table is picked at the start of the encoding process, and the encoder
is stuck with it for the rest of the encode.</p></div>
<div class="paragraph"><p>For twolame-02j, I have picked the table it loads for different
sampling frequencies in order to optimize the range of bitrates possible.</p></div>
<div class="literalblock">
<div class="content">
<pre><tt>48 kHz - Table B.2a
@@ -235,21 +255,26 @@ sampling frequencies in order to optimize the range of bitrates possible.</p>
bitrate over the entire range.</tt></pre>
</div></div>
</div>
<h2>Tech Stuff</h2>
<div class="sectionbody">
<p>The VBR mode is mainly centered around the main_bit_allocation() and
a_bit_allocation() routines in encode.c.</p>
<p>The limited range of VBR is due to my particular implementation which restricts
ranges to within one alloc table (see tables B.2a, B.2b, B.2c and B.2d in ISO 11172).
The VBR range for 32/44.1khz lies within B.2b, and the 48khz VBR lies within table B.2a.</p>
<p>I'm not sure whether it is worth extending these ranges down to lower bitrates.
The work required to switch alloc tables <strong>during</strong> the encoding is major.</p>
<p>In the case of silence, it might be worth doing a quick check for very low signals
and writing a pre-calculated <strong>blank</strong> 32kpbs frame. [probably also a lot of work].</p>
</div>
<h2>How CBR works</h2>
</div>
<div class="sect1">
<h2 id="_tech_stuff">Tech Stuff</h2>
<div class="sectionbody">
<ul>
<div class="paragraph"><p>The VBR mode is mainly centered around the main_bit_allocation() and
a_bit_allocation() routines in encode.c.</p></div>
<div class="paragraph"><p>The limited range of VBR is due to my particular implementation which restricts
ranges to within one alloc table (see tables B.2a, B.2b, B.2c and B.2d in ISO 11172).
The VBR range for 32/44.1khz lies within B.2b, and the 48khz VBR lies within table B.2a.</p></div>
<div class="paragraph"><p>I&#8217;m not sure whether it is worth extending these ranges down to lower bitrates.
The work required to switch alloc tables <strong>during</strong> the encoding is major.</p></div>
<div class="paragraph"><p>In the case of silence, it might be worth doing a quick check for very low signals
and writing a pre-calculated <strong>blank</strong> 32kpbs frame. [probably also a lot of work].</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_how_cbr_works">How CBR works</h2>
<div class="sectionbody">
<div class="ulist"><ul>
<li>
<p>
Use the psycho model to determine the MNRs for each subband
@@ -281,11 +306,13 @@ This mode does not guarentee that all the subbands are without noise
ie there may still be subbands with MNR less than 0.0 (noisy!)
</p>
</li>
</ul>
</ul></div>
</div>
<h2>How VBR works</h2>
</div>
<div class="sect1">
<h2 id="_how_vbr_works">How VBR works</h2>
<div class="sectionbody">
<ul>
<div class="ulist"><ul>
<li>
<p>
pretend we have lots of bits to spare, and work out the bits which would
@@ -309,14 +336,16 @@ VBR "guarantees" that all subbands have MNR &gt; VBRLEVEL or that we have
reached the maximum bitrate.
</p>
</li>
</ul>
</ul></div>
</div>
<h2>FUTURE</h2>
</div>
<div class="sect1">
<h2 id="_future">FUTURE</h2>
<div class="sectionbody">
<ul>
<div class="ulist"><ul>
<li>
<p>
with this VBR mode, we know the bits aren't going to run out, so we can
with this VBR mode, we know the bits aren&#8217;t going to run out, so we can
just assign them "greedily".
</p>
</li>
@@ -325,12 +354,15 @@ with this VBR mode, we know the bits aren't going to run out, so we can
VBR_a_bit_allocation() is yet to be written :)
</p>
</li>
</ul>
</ul></div>
</div>
</div>
</div>
<div id="footnotes"><hr /></div>
<div id="footer">
<div id="footer-text">
Version 0.3.11<br />
Last updated 09-Jan-2008 11:45:18 BST
Version 0.3.13<br />
Last updated 2011-01-01 22:51:38 GMT
</div>
</div>
</body>