shithub: opus

--- a/doc/draft-ietf-codec-opus.xml

+++ b/doc/draft-ietf-codec-opus.xml

@@ -53,7 +53,7 @@

 </address>

 </author>

-<date day="16" month="June" year="2011" />

+<date day="7" month="July" year="2011" />

 <area>General</area>

@@ -3121,9 +3121,69 @@

 </section>

+<section anchor="switching" title="Mode Switching">

+<t>

+Switching between the Opus coding modes requires careful consideration. More

+specifically, the transitions that cannot be easily handled are the ones where

+the lower frequencies have to switch between the SILK LP-based model and the CELT

+transform model. If nothing is done, a glitch will occur for these transitions.

+On the other hand, switching between the SILK-only modes and the hybrid mode

+does not require any special treatment.

+</t>

+<t>

+There are two ways to avoid or reduce glitches during the problematic mode

+transitions: with, or without side information. Only transitions with side

+information are normatively specified. For transitions with no side

+information, it is RECOMMENDED for the decoder to use a concealment technique

+(e.g. make use of the PLC algorithm) to "fill in"

+the gap or the discontinuity caused by the mode transition. Note that this

+concealment MUST NOT be applied when switching between the SILK mode and the

+hybrid mode or vice versa. Similarly, it MUST NOT be applied when merely

+changing the bandwidth within the same mode.

+</t>

+<section anchor="side-info" title="Switching Side Information">

+<t>

+Switching with side information involves transmitting in-band a 5-ms

+"redundant" CELT frame within the Opus frame.

+This frame is designed to fill-in the gap or discontinuity without requiring

+the decoder to conceal it. For transitons from a CELT-only frame to a

+SILK-only or hybrid frame, the redundant frame is inserted in the frame

+following the transition (i.e. the SILK-only/hybrid frame). For transitions

+from a SILK-only/hybrid frame to a CELT-only frame, the redundant frame is

+inserted in the first frame. For all SILK-only and hybrid frames (not only

+those involved in a mode transition), a binary symbol of probability 2^-12

+needs to be decoded just after the SILK part of the bit-stream. When the

+symbol value is 1, then the frame includes an embedded redundant frame. The

+redundant frame always starts and ends on byte boundaries. For SILK-only

+frames, the number of bytes is simply the number of whole remaining bytes.

+For hybrid frames, the number of bytes is equal to 2, plus a decoded unsigned

+integer (ec_dec_uint()) between 0 and 255. For hybrid frames, the redundant

+frame is placed at the end of the frame, after the CELT layer of the

+hybrid frame. The redundant frame is decoded like any other CELT-only frame,

+with the exception that it does not contain a TOC byte. The bandwidth

+is instead set to the same bandwidth of the current frame (for mediumband

+frames, the redundant frame is set to wideband).

+</t>

+<t>

+For CELT-only to SILK-only/hybrid transitions, the first

+2.5 ms of the redundant frame is used as-is for the reconstructed

+output. The remaining 2.5 ms is overlapped and added (cross-faded using

+the square of the MDCT power-complemantary window) to the decoded SILK/hybrid

+signal, ensuring a smooth transition. For SILK-only/hyrid to CELT-only

+transitions, only the second half of the 5-ms decoded redundant frame is used.

+In that case, only a 2.5-ms cross-fade is applied, still using the

+power-complemantary window.

+</t>

 </section>

+</section>

+</section>

 <!--  ******************************************************************* -->

 <!--  **************************   OPUS ENCODER   *********************** -->

 <!--  ******************************************************************* -->

@@ -3993,9 +4053,21 @@

<t>

 Compliance with this specification means that a decoder's output MUST be

 within the thresholds specified compared to the reference implementation

-using the opus_compare.m tool in Appendix <xref

+using the opus_compare.m tool in <xref

 target="opus-compare"></xref>.

 </t>

+<t>

+To complement the Opus specification, the "Opus Custom" codec is defined to

+handle special sampling rates and frame rates that are not supported by the

+main Opus specification. Use of Opus Custom is discouraged for all but very

+special applications for which a frame size different from 2.5, 5, 10, 20 ms is

+needed (for either complexity or latency reasons). Such applications will not

+be compatible with the "main" Opus codec. In Opus Custom operation,

+only the CELT later is available, which is available using the celt_* function

+calls in celt.h.

+</t>

 </section>

 <section anchor="security" title="Security Considerations">