Commit 8ff61171 authored by Jean-Marc Valin's avatar Jean-Marc Valin
Browse files

redundant frames

parent 39fb1932
......@@ -53,7 +53,7 @@
</address>
</author>
<date day="16" month="June" year="2011" />
<date day="7" month="July" year="2011" />
<area>General</area>
......@@ -3121,6 +3121,66 @@ in celt_decode_lost() (mdct.c).
</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>
......@@ -3993,9 +4053,21 @@ the latter shall take precedence.
<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">
......
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