@@ -141,73 +141,73 @@ There are three possible operating modes for the proposed prototype:
</list>
Each of these modes supports a number of difference frame sizes and sampling
rates. In order to distinguish between the various modes and configurations,
we need to define a simple header that can used in the transport layer
we define a single-byte table-of-contents (TOC) header that can used in the transport layer
(e.g RTP) to signal this information. The following describes the proposed
header.
TOC byte.
</t>
<t>
The LP mode supports the following configurations (numbered from 00000...01011 in binary):
The LP mode supports the following configurations (numbered from 0 to 11):
<liststyle="symbols">
<t>8 kHz: 10, 20, 40, 60 ms (00000...00011)</t>
<t>12 kHz: 10, 20, 40, 60 ms (00100...00111)</t>
<t>16 kHz: 10, 20, 40, 60 ms (01000...01011)</t>
<t>8 kHz: 10, 20, 40, 60 ms (0..3)</t>
<t>12 kHz: 10, 20, 40, 60 ms (4..7)</t>
<t>16 kHz: 10, 20, 40, 60 ms (8..11)</t>
</list>
for a total of 12 configurations.
</t>
<t>
The hybrid mode supports the following configurations (numbered from 01100...01111):
The hybrid mode supports the following configurations (numbered from 12 to 15):
<liststyle="symbols">
<t>32 kHz: 10, 20 ms (01100...01101)</t>
<t>48 kHz: 10, 20 ms (01110...01111)</t>
<t>32 kHz: 10, 20 ms (12..13)</t>
<t>48 kHz: 10, 20 ms (14..15)</t>
</list>
for a total of 4 configurations.
</t>
<t>
The MDCT-only mode supports the following configurations (numbered from 10000...11101):
The MDCT-only mode supports the following configurations (numbered from 16 to 31):
<liststyle="symbols">
<t>8 kHz: 2.5, 5, 10, 20 ms (10000...10011)</t>
<t>16 kHz: 2.5, 5, 10, 20 ms (10100...10111)</t>
<t>32 kHz: 2.5, 5, 10, 20 ms (11000...11011)</t>
<t>48 kHz: 2.5, 5, 10, 20 ms (11100...11111)</t>
<t>8 kHz: 2.5, 5, 10, 20 ms (16..19)</t>
<t>16 kHz: 2.5, 5, 10, 20 ms (20..23)</t>
<t>32 kHz: 2.5, 5, 10, 20 ms (24..27)</t>
<t>48 kHz: 2.5, 5, 10, 20 ms (28..31)</t>
</list>
for a total of 16 configurations.
</t>
<t>
There is thus a total of 32 configurations, so 5 bits are necessary to
indicate the mode, frame size and sampling rate (MFS). This leaves 3 bits for the number of frames per packets (codes 0 to 7):
There is thus a total of 32 configurations, encoded in 5 bits. On bit is used to signal mono vs stereo, which leaves 2 bits for the number of frames per packets (codes 0 to 3):
<liststyle="symbols">
<t>0-2: 1-3 frames in the packet, each with equal compressed size</t>
<t>3: arbitrary number of frames in the packet, each with equal compressed size (one size needs to be encoded)</t>
<t>4-5: 2-3 frames in the packet, with different compressed sizes, which need to be encoded (except the last one)</t>
<t>6: arbitrary number of frames in the packet, with different compressed sizes, each of which needs to be encoded</t>
<t>7: The first frame has this MFS, but others have different MFS. Each compressed size needs to be encoded.</t>
<t>0: 1 frames in the packet</t>
<t>1: 2 frames in the packet, each with equal compressed size</t>
<t>2: arbitrary number of frames in the packet, each with equal compressed size</t>
<t>3: arbitrary number of frames in the packet, with different compressed sizes</t>
</list>
When code 7 is used and the last frames of a packet have the same MFS, it is
allowed to switch to another code for them.
For codes 2 and 3, the TOC byte is followed by the number of frames in the packet.
For code 3, the byte indicating the number of frames is followed by N-1 frame
lengths encoded as described below. As an additional limit, the audio duration contained
within a packet may not exceed 120 ms.
</t>
<t>
The compressed size of the frames (if needed) is indicated -- usually -- with one byte, with the following meaning:
<liststyle="symbols">
<t>0: No frame (DTX or lost packet)</t>
<t>1-251: Size of the frame in bytes</t>
<t>252-255: A second byte is needed. The total size is (size[1]*4)+(size[0]%4)+252</t>
<t>1-251: Size of the frame in bytes</t>
<t>252-255: A second byte is needed. The total size is (size[1]*4)+size[0]</t>
</list>
</t>
<t>
The maximum size representable is 255*4+3+252=1275 bytes. For 20 ms frames, that
The maximum size representable is 255*4+255=1275 bytes. For 20 ms frames, that
represents a bit-rate of 510 kb/s, which is really the highest rate anyone would want
to use in stereo mode (beyond that point, lossless codecs would be more appropriate).
</t>
<sectionanchor="examples"title="Examples">
<t>
Simplest case: one packet
Simplest case: one narrowband mono 20-ms SILK frame
