RFC 1890                       AV Profile                   January 1996   For sample-based encodings producing one or more octets per sample,   samples from different channels sampled at the same sampling instant   are packed in consecutive octets. For example, for a two-channel   encoding, the octet sequence is (left channel, first sample), (right   channel, first sample), (left channel, second sample), (right   channel, second sample), .... For multi-octet encodings, octets are   transmitted in network byte order (i.e., most significant octet   first).   The packing of sample-based encodings producing less than one octet   per sample is encoding-specific.4.3 Guidelines for Frame-Based Audio Encodings   Frame-based encodings encode a fixed-length block of audio into   another block of compressed data, typically also of fixed length. For   frame-based encodings, the sender may choose to combine several such   frames into a single message. The receiver can tell the number of   frames contained in a message since the frame duration is defined as   part of the encoding.   For frame-based codecs, the channel order is defined for the whole   block. That is, for two-channel audio, right and left samples are   coded independently, with the encoded frame for the left channel   preceding that for the right channel.   All frame-oriented audio codecs should be able to encode and decode   several consecutive frames within a single packet. Since the frame   size for the frame-oriented codecs is given, there is no need to use   a separate designation for the same encoding, but with different   number of frames per packet.Schulzrinne                 Standards Track                     [Page 7]RFC 1890                       AV Profile                   January 19964.4 Audio Encodings           encoding    sample/frame    bits/sample    ms/frame           ____________________________________________________           1016        frame           N/A            30           DVI4        sample          4           G721        sample          4           G722        sample          8           G728        frame           N/A            2.5           GSM         frame           N/A            20           L8          sample          8           L16         sample          16           LPC         frame           N/A            20           MPA         frame           N/A           PCMA        sample          8           PCMU        sample          8           VDVI        sample          var.                 Table 1: Properties of Audio Encodings   The characteristics of standard audio encodings are shown in Table 1   and their payload types are listed in Table 2.4.4.1 1016   Encoding 1016 is a frame based encoding using code-excited linear   prediction (CELP) and is specified in Federal Standard FED-STD 1016   [2,3,4,5].   The U. S. DoD's Federal-Standard-1016 based 4800 bps code excited   linear prediction voice coder version 3.2 (CELP 3.2) Fortran and C   simulation source codes are available for worldwide distribution at   no charge (on DOS diskettes, but configured to compile on Sun SPARC   stations) from: Bob Fenichel, National Communications System,   Washington, D.C. 20305, phone +1-703-692-2124, fax +1-703-746-4960.4.4.2 DVI4   DVI4 is specified, with pseudo-code, in [6] as the IMA ADPCM wave   type. A specification titled "DVI ADPCM Wave Type" can also be found   in the Microsoft Developer Network Development Library CD ROM   published quarterly by Microsoft. The relevant section is found under   Product Documentation, SDKs, Multimedia Standards Update, New   Multimedia Data Types and Data Techniques, Revision 3.0, April 15,   1994. However, the encoding defined here as DVI4 differs in two   respects from these recommendations:Schulzrinne                 Standards Track                     [Page 8]RFC 1890                       AV Profile                   January 1996        o The header contains the predicted value rather than the first         sample value.        o IMA ADPCM blocks contain odd number of samples, since the         first sample of a block is contained just in the header         (uncompressed), followed by an even number of compressed         samples. DVI4 has an even number of compressed samples only,         using the 'predict' word from the header to decode the first         sample.   Each packet contains a single DVI block. The profile only defines the   4-bit-per-sample version, while IMA also specifies a 3-bit-per-sample   encoding.   The "header" word for each channel has the following structure:     int16  predict;  /* predicted value of first sample                         from the previous block (L16 format) */     u_int8 index;    /* current index into stepsize table */     u_int8 reserved; /* set to zero by sender, ignored by receiver */   Packing of samples for multiple channels is for further study.   The document, "IMA Recommended Practices for Enhancing Digital Audio   Compatibility in Multimedia Systems (version 3.0)", contains the   algorithm description.  It is available from:   Interactive Multimedia Association   48 Maryland Avenue, Suite 202   Annapolis, MD 21401-8011   USA   phone: +1 410 626-13804.4.3 G721   G721 is specified in ITU recommendation G.721. Reference   implementations for G.721 are available as part of the CCITT/ITU-T   Software Tool Library (STL) from the ITU General Secretariat, Sales   Service, Place du Nations, CH-1211 Geneve 20, Switzerland. The   library is covered by a license.4.4.4 G722   G722 is specified in ITU-T recommendation G.722, "7 kHz audio-coding   within 64 kbit/s".   G728 is specified in ITU-T recommendation G.728, "Coding of speech at   16 kbit/s using low-delay code excited linear prediction".Schulzrinne                 Standards Track                     [Page 9]RFC 1890                       AV Profile                   January 19964.4.6 GSM   GSM (group speciale mobile) denotes the European GSM 06.10   provisional standard for full-rate speech transcoding, prI-ETS 300   036, which is based on RPE/LTP (residual pulse excitation/long term   prediction) coding at a rate of 13 kb/s [7,8,9]. The standard can be   obtained from   ETSI (European Telecommunications Standards Institute)   ETSI Secretariat: B.P.152   F-06561 Valbonne Cedex   France   Phone: +33 92 94 42 00   Fax: +33 93 65 47 164.4.7 L8   L8 denotes linear audio data, using 8-bits of precision with an   offset of 128, that is, the most negative signal is encoded as zero.4.4.8 L16   L16 denotes uncompressed audio data, using 16-bit signed   representation with 65535 equally divided steps between minimum and   maximum signal level, ranging from -32768 to 32767. The value is   represented in two's complement notation and network byte order.4.4.9 LPC   LPC designates an experimental linear predictive encoding contributed   by Ron Frederick, Xerox PARC, which is based on an implementation   written by Ron Zuckerman, Motorola, posted to the Usenet group   comp.dsp on June 26, 1992.4.4.10 MPA   MPA denotes MPEG-I or MPEG-II audio encapsulated as elementary   streams. The encoding is defined in ISO standards ISO/IEC 11172-3 and   13818-3. The encapsulation is specified in work in progress [10],   Section 3. The authors can be contacted at   Don Hoffman   Sun Microsystems, Inc.   Mail-stop UMPK14-305   2550 Garcia Avenue   Mountain View, California 94043-1100   USA   electronic mail: don.hoffman@eng.sun.comSchulzrinne                 Standards Track                    [Page 10]RFC 1890                       AV Profile                   January 1996   Sampling rate and channel count are contained in the payload. MPEG-I   audio supports sampling rates of 32000, 44100, and 48000 Hz (ISO/IEC   11172-3, section 1.1; "Scope"). MPEG-II additionally supports ISO/IEC   11172-3 Audio...").4.4.11 PCMA   PCMA is specified in CCITT/ITU-T recommendation G.711. Audio data is   encoded as eight bits per sample, after logarithmic scaling. Code to   convert between linear and A-law companded data is available in [6].   A detailed description is given by Jayant and Noll [11].4.4.12 PCMU   PCMU is specified in CCITT/ITU-T recommendation G.711. Audio data is   encoded as eight bits per sample, after logarithmic scaling. Code to   convert between linear and mu-law companded data is available in [6].   PCMU is the encoding used for the Internet media type audio/basic.  A   detailed description is given by Jayant and Noll [11].4.4.13 VDVI   VDVI is a variable-rate version of DVI4, yielding speech bit rates of   between 10 and 25 kb/s. It is specified for single-channel operation   only. It uses the following encoding:                    DVI4 codeword    VDVI bit pattern                    __________________________________                                0    00                                1    010                                2    1100                                3    11100                                4    111100                                5    1111100                                6    11111100                                7    11111110                                8    10                                9    011                               10    1101                               11    11101                               12    111101                               13    1111101                               14    11111101                               15    11111111Schulzrinne                 Standards Track                    [Page 11]RFC 1890                       AV Profile                   January 19965.  Video   The following video encodings are currently defined, with their   abbreviated names used for identification:5.1 CelB   The CELL-B encoding is a proprietary encoding proposed by Sun   Microsystems.  The byte stream format is described in work in   progress [12].  The author can be contacted at   Michael F. Speer   Sun Microsystems Computer Corporation   2550 Garcia Ave MailStop UMPK14-305   Mountain View, CA 94043   United States   electronic mail: michael.speer@eng.sun.com5.2 JPEGThe encoding is specified in ISO Standards 10918-1 and 10918-2. TheRTP payload format is as specified in work in progress [13].  Furtherinformation can be obtained from   Steven McCanne   Lawrence Berkeley National Laboratory   M/S 46A-1123   One Cyclotron Road   Berkeley, CA 94720   United States   Phone: +1 510 486 7520   electronic mail: mccanne@ee.lbl.gov5.3 H261   The encoding is specified in CCITT/ITU-T standard H.261. The   packetization and RTP-specific properties are described in work in   progress [14]. Further information can be obtained from   Thierry Turletti   Office NE 43-505   Telemedia, Networks and Systems   Laboratory for Computer Science   Massachusetts Institute of Technology   545 Technology Square   Cambridge, MA 02139   United States   electronic mail: turletti@clove.lcs.mit.eduSchulzrinne                 Standards Track                    [Page 12]RFC 1890                       AV Profile                   January 1996