Updating Opus to a pre-release of 1.1

doc/draft-valin-codec-opus-update.xml

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+<?xml version="1.0" encoding="US-ASCII"?>
+<!DOCTYPE rfc SYSTEM "rfc2629.dtd">
+<?rfc toc="yes"?>
+<?rfc tocompact="yes"?>
+<?rfc tocdepth="3"?>
+<?rfc tocindent="yes"?>
+<?rfc symrefs="yes"?>
+<?rfc sortrefs="yes"?>
+<?rfc comments="yes"?>
+<?rfc inline="yes"?>
+<?rfc compact="yes"?>
+<?rfc subcompact="no"?>
+<rfc category="std" docName="draft-valin-codec-opus-update-00"
+     ipr="trust200902">
+  <front>
+    <title abbrev="Opus Update">Updates to the Opus Audio Codec</title>
+
+<author initials="JM" surname="Valin" fullname="Jean-Marc Valin">
+<organization>Mozilla Corporation</organization>
+<address>
+<postal>
+<street>650 Castro Street</street>
+<city>Mountain View</city>
+<region>CA</region>
+<code>94041</code>
+<country>USA</country>
+</postal>
+<phone>+1 650 903-0800</phone>
+<email>jmvalin@jmvalin.ca</email>
+</address>
+</author>
+
+<author initials="T." surname="Terriberry" fullname="Timothy B. Terriberry">
+<organization>Mozilla Corporation</organization>
+<address>
+<postal>
+<street>650 Castro Street</street>
+<city>Mountain View</city>
+<region>CA</region>
+<code>94041</code>
+<country>USA</country>
+</postal>
+<phone>+1 650 903-0800</phone>
+<email>tterriberry@mozilla.com</email>
+</address>
+</author>
+
+<author initials="K." surname="Vos" fullname="Koen Vos">
+<organization>Skype Technologies S.A.</organization>
+<address>
+<postal>
+<street>Soder Malarstrand 43</street>
+<city>Stockholm</city>
+<region></region>
+<code>11825</code>
+<country>SE</country>
+</postal>
+<phone>+46 73 085 7619</phone>
+<email>koen.vos@skype.net</email>
+</address>
+</author>
+
+
+
+    <date day="12" month="July" year="2013" />
+
+    <abstract>
+      <t>This document addresses minor issues that were found in the specification
+      of the Opus audio codec in <xref target="RFC6716">RFC 6716</xref>.</t>
+    </abstract>
+  </front>
+
+  <middle>
+    <section title="Introduction">
+      <t>This document address minor issues that were discovered in the reference
+      implementation of the Opus codec that serves as the specification in 
+      <xref target="RFC6716">RFC 6716</xref>. Only issues affecting the decoder are
+      listed here. An up-to-date implementation of the Opus encoder can be found at
+      http://opus-codec.org/. The updated specification remains fully compatible with
+      the original specification and only one of the changes results in any difference
+      in the audio output.
+      </t>
+    </section>
+
+    <section title="Terminology">
+      <t>The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
+      "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
+      document are to be interpreted as described in <xref
+      target="RFC2119">RFC 2119</xref>.</t>
+    </section>
+
+    <section title="Stereo State Reset in SILK">
+      <t>The reference implementation does not reinitialize the stereo state
+      during a mode switch. The old stereo memory can produce a brief impulse
+      (i.e. single sample) in the decoded audio. This can be fixed by changing
+      silk/dec_API.c at line 72:
+      <figure>
+      <artwork><![CDATA[
+     for( n = 0; n < DECODER_NUM_CHANNELS; n++ ) {
+         ret  = silk_init_decoder( &channel_state[ n ] );
+     }
++    silk_memset(&((silk_decoder *)decState)->sStereo, 0, 
++                sizeof(((silk_decoder *)decState)->sStereo));
++    /* Not strictly needed, but it's cleaner that way */
++    ((silk_decoder *)decState)->prev_decode_only_middle = 0;
+ 
+     return ret;
+ }
+]]></artwork>
+</figure>
+     This change affects the normative part of the decoder. Fortunately,
+     the modified decoder is still compliant with the original specification because
+     it still easily passes the testvectors. For example, for the float decoder
+     at 48 kHz, the opus_compare (arbitrary) "quality score" changes from
+     from 99.9333% to 99.925%.
+      </t>
+    </section>
+
+    <section anchor="padding" title="Parsing of the Opus Packet Padding">
+      <t>It was discovered that some invalid packets of very large size could trigger
+      an out-of-bounds read in the Opus packet parsing code responsible for padding.
+      This is due to an integer overflow if the signaled padding exceeds 2^31-1 bytes
+      (the actual packet may be smaller). The code can be fixed by applying the following
+      changes at line 596 of src/opus_decoder.c:
+      <figure>
+      <artwork><![CDATA[
+       /* Padding flag is bit 6 */
+       if (ch&0x40)
+       {
+-         int padding=0;
+          int p;
+          do {
+             if (len<=0)
+                return OPUS_INVALID_PACKET;
+             p = *data++;
+             len--;
+-            padding += p==255 ? 254: p;
++            len -= p==255 ? 254: p;
+          } while (p==255);
+-         len -= padding;
+       }
+]]></artwork>
+</figure>
+      </t>
+      <t>This packet parsing issue is limited to reading memory up
+         to about 60 kB beyond the compressed buffer. This can only be triggered
+         by a compressed packet more than about 16 MB long, so it's not a problem
+         for RTP. In theory, it <spanx style="emph">could</spanx> crash a file
+         decoder (e.g. Opus in Ogg) if the memory just after the incoming packet
+         is out-of-range, but that could not be achieved when attempted in a production
+         application built using an affected version of the Opus decoder.</t>
+    </section>
+
+    <section anchor="resampler" title="Resampler buffer">
+      <t>The SILK resampler had the following issues:
+        <list style="numbers">
+    <t>The calls to memcpy() were using sizeof(opus_int32), but the type of the
+        local buffer was opus_int16.</t>
+    <t>Because the size was wrong, this potentially allowed the source
+        and destination regions of the memcpy overlap.
+          We <spanx style="emph">believe</spanx> that nSamplesIn is at least fs_in_khZ,
+          which is at least 8.
+       Since RESAMPLER_ORDER_FIR_12 is only 8,that should not be a problem once
+       the type size is fixed.</t>
+          <t>The size of the buffer used RESAMPLER_MAX_BATCH_SIZE_IN, but the
+        data stored in it was actually _twice_ the input batch size
+        (nSamplesIn&lt;&lt;1).</t>
+      </list></t>
+      <t>
+      The fact that the code never produced any error in testing (including when run under the
+      Valgrind memory debugger), suggests that in practice
+     the batch sizes are reasonable enough that none of the issues above
+     was ever a problem. However, proving that is non-obvious.
+    </t>
+    <t>The code can be fixed by applying the following changes to like 70 of silk/resampler_private_IIR_FIR.c:
+<figure>
+<artwork><![CDATA[
+     opus_int16                      out[],          /* O    Output signal               */
+     const opus_int16                in[],           /* I    Input signal                */
+     opus_int32                      inLen           /* I    Number of input samples     */
+ )
+ {
+     silk_resampler_state_struct *S = (silk_resampler_state_struct *)SS;
+     opus_int32 nSamplesIn;
+     opus_int32 max_index_Q16, index_increment_Q16;
+-    opus_int16 buf[ RESAMPLER_MAX_BATCH_SIZE_IN + RESAMPLER_ORDER_FIR_12 ];
++    opus_int16 buf[ 2*RESAMPLER_MAX_BATCH_SIZE_IN + RESAMPLER_ORDER_FIR_12 ];
+ 
+     /* Copy buffered samples to start of buffer */
+-    silk_memcpy( buf, S->sFIR, RESAMPLER_ORDER_FIR_12 * sizeof( opus_int32 ) );
++    silk_memcpy( buf, S->sFIR, RESAMPLER_ORDER_FIR_12 * sizeof( opus_int16 ) );
+ 
+     /* Iterate over blocks of frameSizeIn input samples */
+     index_increment_Q16 = S->invRatio_Q16;
+     while( 1 ) {
+         nSamplesIn = silk_min( inLen, S->batchSize );
+ 
+         /* Upsample 2x */
+         silk_resampler_private_up2_HQ( S->sIIR, &buf[ RESAMPLER_ORDER_FIR_12 ], in, nSamplesIn );
+ 
+         max_index_Q16 = silk_LSHIFT32( nSamplesIn, 16 + 1 );         /* + 1 because 2x upsampling */
+         out = silk_resampler_private_IIR_FIR_INTERPOL( out, buf, max_index_Q16, index_increment_Q16 );
+         in += nSamplesIn;
+         inLen -= nSamplesIn;
+ 
+         if( inLen > 0 ) {
+             /* More iterations to do; copy last part of filtered signal to beginning of buffer */
+-            silk_memcpy( buf, &buf[ nSamplesIn << 1 ], RESAMPLER_ORDER_FIR_12 * sizeof( opus_int32 ) );
++            silk_memmove( buf, &buf[ nSamplesIn << 1 ], RESAMPLER_ORDER_FIR_12 * sizeof( opus_int16 ) );
+         } else {
+             break;
+         }
+     }
+ 
+     /* Copy last part of filtered signal to the state for the next call */
+-    silk_memcpy( S->sFIR, &buf[ nSamplesIn << 1 ], RESAMPLER_ORDER_FIR_12 * sizeof( opus_int32 ) );
++    silk_memcpy( S->sFIR, &buf[ nSamplesIn << 1 ], RESAMPLER_ORDER_FIR_12 * sizeof( opus_int16 ) );
+ }
+]]></artwork>
+</figure>
+    </t>
+    </section>
+    
+    <section title="Downmix to Mono">
+      <t>The last issue is not strictly a bug, but it is an issue that has been reported
+      when downmixing Opus decoded stream to mono, whether this is done inside the decoder
+      or as a post-processing on the stereo decoder output. Opus intensity stereo allows
+      optionally coding the two channels 180-degrees out of phase on a per-band basis. 
+      This provides better stereo quality than forcing the two channels to be in phase,
+      but when the output is downmixed to mono, the energy in the affected bands is cancelled
+      sometimes resulting in audible artefacts.
+      </t>
+      <t>A possible work-around for this issue would be to optionally allow the decoder to
+      not apply the 180-degree phase shift when the output is meant to be downmixed (inside or
+      outside of the decoder).
+      </t>
+    </section>
+    <section anchor="IANA" title="IANA Considerations">
+      <t>This document makes no request of IANA.</t>
+
+      <t>Note to RFC Editor: this section may be removed on publication as an
+      RFC.</t>
+    </section>
+
+    <section anchor="Acknowledgements" title="Acknowledgements">
+      <t>We would like to thank Juri Aedla for reporting the issue with the parsing of
+      the Opus padding.</t>
+    </section>
+  </middle>
+
+  <back>
+    <references title="References">
+      <?rfc include="http://xml.resource.org/public/rfc/bibxml/reference.RFC.2119.xml"?>
+      <?rfc include="http://xml.resource.org/public/rfc/bibxml/reference.RFC.6716.xml"?>
+
+
+    </references>
+  </back>
+</rfc>