rfc3551.txt

完整的RTP RTSP代码库

   |0 0 0 0|1 1 0 0|1 1 0 0 0 0 0 0|0 0 0 0 0 0 0 0|0 0 0 0|1 1 0 0|
   |3 2 1 0|1 0 9 8|1 0 9 8 7 6 5 4|7 6 5 4 3 2 1 0|3 2 1 0|1 0 9 8|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   MSBPOS    |Z|POS|  MSBPOS   |     POS0      |POS|   POS0    |
   |             | | 0 |           |               | 1 |           |
   |0 0 0 0 0 0 0|0|0 0|1 1 1 0 0 0|0 0 0 0 0 0 0 0|0 0|1 1 1 1 1 1|
   |6 5 4 3 2 1 0| |1 0|2 1 0 9 8 7|9 8 7 6 5 4 3 2|1 0|5 4 3 2 1 0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     POS1      | POS2  | POS1  |     POS2      | POS3  | POS2  |
   |               |       |       |               |       |       |
   |0 0 0 0 0 0 0 0|0 0 0 0|1 1 1 1|1 1 0 0 0 0 0 0|0 0 0 0|1 1 1 1|
   |9 8 7 6 5 4 3 2|3 2 1 0|3 2 1 0|1 0 9 8 7 6 5 4|3 2 1 0|5 4 3 2|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     POS3      |   PSIG0   |POS|PSIG2|  PSIG1  |  PSIG3  |PSIG2|
   |               |           | 3 |     |         |         |     |
   |1 1 0 0 0 0 0 0|0 0 0 0 0 0|1 1|0 0 0|0 0 0 0 0|0 0 0 0 0|0 0 0|
   |1 0 9 8 7 6 5 4|5 4 3 2 1 0|3 2|2 1 0|4 3 2 1 0|4 3 2 1 0|5 4 3|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 1: G.723 (6.3 kb/s) bit packing

   For the 5.3 kb/s data rate, the header (HDR) bits are always "0 1",
   as shown in Fig. 2, to indicate operation at 5.3 kb/s.













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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    LPC    |HDR|      LPC      |      LPC      |   ACL0    |LPC|
   |           |   |               |               |           |   |
   |0 0 0 0 0 0|0 1|1 1 1 1 0 0 0 0|2 2 1 1 1 1 1 1|0 0 0 0 0 0|2 2|
   |5 4 3 2 1 0|   |3 2 1 0 9 8 7 6|1 0 9 8 7 6 5 4|5 4 3 2 1 0|3 2|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  ACL2   |ACL|A| GAIN0 |ACL|ACL|     GAIN0     |     GAIN1     |
   |         | 1 |C|       | 3 | 2 |               |               |
   |0 0 0 0 0|0 0|0|0 0 0 0|0 0|0 0|1 1 0 0 0 0 0 0|0 0 0 0 0 0 0 0|
   |4 3 2 1 0|1 0|6|3 2 1 0|1 0|6 5|1 0 9 8 7 6 5 4|7 6 5 4 3 2 1 0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | GAIN2 | GAIN1 |     GAIN2     |    GAIN3      | GRID  | GAIN3 |
   |       |       |               |               |       |       |
   |0 0 0 0|1 1 0 0|1 1 0 0 0 0 0 0|0 0 0 0 0 0 0 0|0 0 0 0|1 1 0 0|
   |3 2 1 0|1 0 9 8|1 0 9 8 7 6 5 4|7 6 5 4 3 2 1 0|4 3 2 1|1 0 9 8|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     POS0      | POS1  | POS0  |     POS1      |     POS2      |
   |               |       |       |               |               |
   |0 0 0 0 0 0 0 0|0 0 0 0|1 1 0 0|1 1 0 0 0 0 0 0|0 0 0 0 0 0 0 0|
   |7 6 5 4 3 2 1 0|3 2 1 0|1 0 9 8|1 0 9 8 7 6 5 4|7 6 5 4 3 2 1 0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | POS3  | POS2  |     POS3      | PSIG1 | PSIG0 | PSIG3 | PSIG2 |
   |       |       |               |       |       |       |       |
   |0 0 0 0|1 1 0 0|1 1 0 0 0 0 0 0|0 0 0 0|0 0 0 0|0 0 0 0|0 0 0 0|
   |3 2 1 0|1 0 9 8|1 0 9 8 7 6 5 4|3 2 1 0|3 2 1 0|3 2 1 0|3 2 1 0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 2: G.723 (5.3 kb/s) bit packing

   The packing of G.723.1 SID (silence) frames, which are indicated by
   the header (HDR) bits having the pattern "1 0", is depicted in Fig.
   3.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    LPC    |HDR|      LPC      |      LPC      |   GAIN    |LPC|
   |           |   |               |               |           |   |
   |0 0 0 0 0 0|1 0|1 1 1 1 0 0 0 0|2 2 1 1 1 1 1 1|0 0 0 0 0 0|2 2|
   |5 4 3 2 1 0|   |3 2 1 0 9 8 7 6|1 0 9 8 7 6 5 4|5 4 3 2 1 0|3 2|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 3: G.723 SID mode bit packing






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4.5.4  G726-40, G726-32, G726-24, and G726-16

   ITU-T Recommendation G.726 describes, among others, the algorithm
   recommended for conversion of a single 64 kbit/s A-law or mu-law PCM
   channel encoded at 8,000 samples/sec to and from a 40, 32, 24, or 16
   kbit/s channel.  The conversion is applied to the PCM stream using an
   Adaptive Differential Pulse Code Modulation (ADPCM) transcoding
   technique.  The ADPCM representation consists of a series of
   codewords with a one-to-one correspondence to the samples in the PCM
   stream.  The G726 data rates of 40, 32, 24, and 16 kbit/s have
   codewords of 5, 4, 3, and 2 bits, respectively.

   The 16 and 24 kbit/s encodings do not provide toll quality speech.
   They are designed for used in overloaded Digital Circuit
   Multiplication Equipment (DCME).  ITU-T G.726 recommends that the 16
   and 24 kbit/s encodings should be alternated with higher data rate
   encodings to provide an average sample size of between 3.5 and 3.7
   bits per sample.

   The encodings of G.726 are here denoted as G726-40, G726-32, G726-24,
   and G726-16.  Prior to 1990, G721 described the 32 kbit/s ADPCM
   encoding, and G723 described the 40, 32, and 16 kbit/s encodings.
   Thus, G726-32 designates the same algorithm as G721 in RFC 1890.

   A stream of G726 codewords contains no information on the encoding
   being used, therefore transitions between G726 encoding types are not
   permitted within a sequence of packed codewords.  Applications MUST
   determine the encoding type of packed codewords from the RTP payload
   identifier.

   No payload-specific header information SHALL be included as part of
   the audio data.  A stream of G726 codewords MUST be packed into
   octets as follows:  the first codeword is placed into the first octet
   such that the least significant bit of the codeword aligns with the
   least significant bit in the octet, the second codeword is then
   packed so that its least significant bit coincides with the least
   significant unoccupied bit in the octet.  When a complete codeword
   cannot be placed into an octet, the bits overlapping the octet
   boundary are placed into the least significant bits of the next
   octet.  Packing MUST end with a completely packed final octet.  The
   number of codewords packed will therefore be a multiple of 8, 2, 8,
   and 4 for G726-40, G726-32, G726-24, and G726-16, respectively.  An
   example of the packing scheme for G726-32 codewords is as shown,
   where bit 7 is the least significant bit of the first octet, and bit
   A3 is the least significant bit of the first codeword:






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          0                   1
          0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
         |B B B B|A A A A|D D D D|C C C C| ...
         |0 1 2 3|0 1 2 3|0 1 2 3|0 1 2 3|
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   An example of the packing scheme for G726-24 codewords follows, where
   again bit 7 is the least significant bit of the first octet, and bit
   A2 is the least significant bit of the first codeword:

          0                   1                   2
          0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
         |C C|B B B|A A A|F|E E E|D D D|C|H H H|G G G|F F| ...
         |1 2|0 1 2|0 1 2|2|0 1 2|0 1 2|0|0 1 2|0 1 2|0 1|
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   Note that the "little-endian" direction in which samples are packed
   into octets in the G726-16, -24, -32 and -40 payload formats
   specified here is consistent with ITU-T Recommendation X.420, but is
   the opposite of what is specified in ITU-T Recommendation I.366.2
   Annex E for ATM AAL2 transport.  A second set of RTP payload formats
   matching the packetization of I.366.2 Annex E and identified by MIME
   subtypes AAL2-G726-16, -24, -32 and -40 will be specified in a
   separate document.

4.5.5 G728

   G728 is specified in ITU-T Recommendation G.728, "Coding of speech at
   16 kbit/s using low-delay code excited linear prediction".

   A G.278 encoder translates 5 consecutive audio samples into a 10-bit
   codebook index, resulting in a bit rate of 16 kb/s for audio sampled
   at 8,000 samples per second.  The group of five consecutive samples
   is called a vector.  Four consecutive vectors, labeled V1 to V4
   (where V1 is to be played first by the receiver), build one G.728
   frame.  The four vectors of 40 bits are packed into 5 octets, labeled
   B1 through B5.  B1 SHALL be placed first in the RTP packet.

   Referring to the figure below, the principle for bit order is
   "maintenance of bit significance".  Bits from an older vector are
   more significant than bits from newer vectors.  The MSB of the frame
   goes to the MSB of B1 and the LSB of the frame goes to LSB of B5.







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                   1         2         3        3
         0         0         0         0        9
         ++++++++++++++++++++++++++++++++++++++++
         <---V1---><---V2---><---V3---><---V4---> vectors
         <--B1--><--B2--><--B3--><--B4--><--B5--> octets
         <------------- frame 1 ---------------->

   In particular, B1 contains the eight most significant bits of V1,
   with the MSB of V1 being the MSB of B1.  B2 contains the two least
   significant bits of V1, the more significant of the two in its MSB,
   and the six most significant bits of V2.  B1 SHALL be placed first in
   the RTP packet and B5 last.

4.5.6 G729

   G729 is specified in ITU-T Recommendation G.729, "Coding of speech at
   8 kbit/s using conjugate structure-algebraic code excited linear
   prediction (CS-ACELP)".  A reduced-complexity version of the G.729
   algorithm is specified in Annex A to Rec. G.729.  The speech coding
   algorithms in the main body of G.729 and in G.729 Annex A are fully
   interoperable with each other, so there is no need to further
   distinguish between them.  An implementation that signals or accepts
   use of G729 payload format may implement either G.729 or G.729A
   unless restricted by additional signaling specified elsewhere related
   specifically to the encoding rather than the payload format.  The
   G.729 and G.729 Annex A codecs were optimized to represent speech
   with high quality, where G.729 Annex A trades some speech quality for
   an approximate 50% complexity reduction [10].  See the next Section
   (4.5.7) for other data rates added in later G.729 Annexes.  For all
   data rates, the sampling frequency (and RTP timestamp clock rate) is
   8,000 Hz.

   A voice activity detector (VAD) and comfort noise generator (CNG)
   algorithm in Annex B of G.729 is RECOMMENDED for digital simultaneous
   voice and data applications and can be used in conjunction with G.729
   or G.729 Annex A.  A G.729 or G.729 Annex A frame contains 10 octets,
   while the G.729 Annex B comfort noise frame occupies 2 octets.
   Receivers MUST accept comfort noise frames if restriction of their
   use has not been signaled.  The MIME registration for G729 in RFC
   3555 [7] specifies a parameter that MAY be used with MIME or SDP to
   restrict the use of comfort noise frames.

   A G729 RTP packet may consist of zero or more G.729 or G.729 Annex A
   frames, followed by zero or one G.729 Annex B frames.  The presence
   of a comfort noise frame can be deduced from the length of the RTP
   payload.  The default packetization interval is 20 ms (two frames),
   but in some situations it may be desirable to send 10 ms packets.  An




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   example would be a transition from speech to comfort noise in the
   first 10 ms of the packet.  For some applications, a longer
   packetization interval may be required to reduce the packet rate.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |L|      L1     |    L2   |    L3   |       P1      |P|    C1   |
      |0|             |         |         |               |0|         |
      | |0 1 2 3 4 5 6|0 1 2 3 4|0 1 2 3 4|0 1 2 3 4 5 6 7| |0 1 2 3 4|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |       C1      |  S1   | GA1 |  GB1  |    P2   |      C2       |
      |          1 1 1|       |     |       |         |               |
      |5 6 7 8 9 0 1 2|0 1 2 3|0 1 2|0 1 2 3|0 1 2 3 4|0 1 2 3 4 5 6 7|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   C2    |  S2   | GA2 |  GB2  |
      |    1 1 1|       |     |       |
      |8 9 0 1 2|0 1 2 3|0 1 2|0 1 2 3|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 4: G.729 and G.729A bit packing