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Versions: 01 02                                                         
Internet Engineering Task Force          Audio-Video Transport Working Group
INTERNET-DRAFT                                                H. Schulzrinne
draft-ietf-avt-encodings-02.txt                       AT&T Bell Laboratories
                                                          September 17, 1993
                                                          Expires:  10/01/93

                              Media Encodings

Status of this Memo

This document is an Internet Draft.   Internet Drafts are working  documents
of the Internet Engineering  Task Force (IETF), its  Areas, and its  Working
Groups.   Note that other  groups may also  distribute working documents  as
Internet Drafts.

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Internet Drafts may be  updated, replaced, or  obsoleted by other  documents
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Distribution of this document is unlimited.


     This  document describes a possible structure of the media content
    for audio  and video  for Internet applications.    The definitions
    are independent  of the particular  transport mechanism used.   The
    descriptions  provide  pointers  to reference  implementations  and
    the  detailed  standards.     This  document is  meant  as  an  aid
    for implementors  of audio,  video  and other  real-time multimedia

INTERNET-DRAFT       draft-ietf-avt-encodings-02.txt      September 17, 1993

1 Audio

1.1 Encoding-independent recommendations

The  following  recommendations  are  default  operating  parameters.     An
applications should  be  prepared  to  handle other  values.     The  ranges
given  are  meant  to  give  guidance   to  application  writers,   allowing
a set  of  applications  conforming  to  these  guidelines  to  interoperate
without additional  negotiation.    These  guidelines are  not  intended  to
restrict operating parameters for  application that can  negotiate a set  of
interoperable parameters, e.g., through a conference control protocol.

For packetized  audio,  the default  packetization  interval should  have  a
duration of 20 ms,  unless otherwise noted  in Table 1.   The  packetization
interval determines the minimum end-to-end  delay; longer packets  introduce
less header overhead but higher delay and make packet loss more  noticeable.
For non-interactive  applications  such as  lectures  or links  with  severe
bandwidth constraints, a higher packetization delay may be appropriate.  For
frame-based encodings (marked as F in the  table 1 below) such as LPC,  CELP
and GSM, the  sender may choose  to combine several  frame intervals 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.

If multiple channels are used, the left channel information always  precedes
the right-channel information.  For  more than two channels, the  convention
followed by the  AIFF-C audio  interchange format should  be followed.    It
is listed in the  table below.   (The AIFF-C  specification is available  by
anonymous ftp at ftp.sgi.com in the file sgi/aiff-c.9.26.91.ps.)


stereo     left        right
3 channel  left        right        center
quad       front left  front right  rear left  rear right
4 channel  left        center       right      surround
6 channel  left        left center  center     right       right center  surround

The sampling frequency should be drawn from the set:  8, 11.025, 16,  22.05,
44.1 and 48 kHz.

1.2 Recommended Audio Encodings

The table 1 shows the names, types (sample vs.  frame oriented), per-channel
bit rates and default  sampling frequencies of recommended  encodings.   The
list is  partially  drawn  from the  document  ``Recommended  practices  for

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enhancing digital audio compatibility in multimedia systems'', published  by
the Interactive Multimedia Assocation, Version 3.00, Oct.  1992  (referenced
as [IMA]). The  names are for  identification only;  they correspond to  the
names used within the Real-Time Transport Protocol (RTP). Other applications
may choose different namings.  Note  that the L16 encoding may be used  with
different sampling rates.

  name  nom.  sampling   rate  type  frame  description
  L16               48    768  S            16-bit linear, 2's complement
                  44.1  705.6  S
                 22.05  352.8  S
                11.025  176.4  S
  G722              16     64  S            CCITT subband ADPCM
  PCMU               8     64  S            CCITT mu-law PCM
  PCMA               8     64  S            CCITT A-law PCM
  G721               8     32  S            CCITT ADPCM
  IDVI               8     32  S            Intel/DVI ADPCM [IMA]
  G723               8     24  S            CCITT ADPCM
  GSM                8     13  F     20     RTE/LTP GSM 06.10

                         Table 1:  Audio encodings

For multi-octet  encodings, octets  are transmitted  in network  byte  order
(i.e., most significant octet first).

A detailed description of  the encodings is  given below.   The names  shown
(L16, PCMU, etc.)   are limited to four characters  and suitable to be  used
for identification in protocols such as RTP (RFC TBD).

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.

PCMU: specified in  CCITT recommendation G.711.   Audio  data is encoded  as
    eight  bits per  sample, after  companding.    Code to  convert  between
    linear and mu-law companded data is available in the IMA document.

PCMA: specified in  CCITT recommendation G.711.   Audio  data is encoded  as
    eight  bits per  sample, after  companding.    Code to  convert  between
    linear and A-law companded data is available in the IMA document.

G721 through G729: specified  in  the corresponding  CCITT  recommendations.
    Reference implementations for G.721  and G.723 are available as part  of
    the CCITT Software Tool Library (STL) from the ITU  General Secretariat,
    Sales Service, Place  du Nations, CH-1211 Geneve  20, Switzerland.   The

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    library is covered  by a license and is  available for anonymous ftp  on
    gaia.cs.umass.edu, file pub/ccitt/ccitt_tools.tar.Z.

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.  A reference implementation was  written by
    Carsten Borman and Jutta  Degener (TU Berlin, Germany) and is  available
    for anonymous ftp from tub.cs.tu-berlin.de, directory tub/tubmik.

1016: uses  code-excited  linear  prediction  (CELP)  and  is  specified  in
    Federal Standard  FED-STD 1016,  published by the  Office of  Technology
    and Standards, Washington, DC 20305-2010.

    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.

    Example  input  and processed  speech  files,  a  technical  information
    bulletin, and  the official standard  ``Federal Standard 1016,  Telecom-
    munications:   Analog  to Digital  Conversion of  Radio Voice  by  4,800
    bit/second Code  Excited Linear Prediction (CELP)''  are included at  no
    charge.  According  to Vincent Cate (Carnegie Mellon), the  distribution
    is  also  available  for  anonymous  ftp  at   furmint.nectar.cs.cmu.edu
    ( in directory celp.audio.compression.

    The  following articles  describes  the  Federal-Standard-1016  4.8-kbps
    CELP coder:

    Campbell, Joseph  P. Jr., Thomas  E. Tremain and  Vanoy C. Welch,  ``The
    Proposed Federal  Standard 1016 4800  bps Voice Coder:   CELP,''  S_p_e_e_c_h_
    T_e_c_h_n_o_l_o_g_y_ M_a_g_a_z_i_n_e_, April/May 1990, p.  58-64.

    Campbell, Joseph  P. Jr., Thomas  E. Tremain and  Vanoy C. Welch,  ``The
    Federal  Standard 1016  4800  bps  CELP Voice  Coder,''  D_i_g_i_t_a_l_  S_i_g_n_a_l_
    P_r_o_c_e_s_s_i_n_g_, Academic Press, 1991, Vol.  1, No.  3, p.  145-155.

    Campbell, Joseph  P. Jr., Thomas  E. Tremain and  Vanoy C. Welch,  ``The
    DoD 4.8  kbps Standard (Proposed Federal  Standard 1016),'' in  A_d_v_a_n_c_e_s_
    i_n_ S_p_e_e_c_h_  C_o_d_i_n_g_,  ed.   Atal,  Cuperman  and Gersho,  Kluwer  Academic
    Publishers, 1991, Chapter 12, p.  121-133.

    Campbell, Joseph  P. Jr., Thomas  E. Tremain and  Vanoy C. Welch,  ``The
    Proposed Federal  Standard 1016 4800  bps Voice Coder:   CELP,''  S_p_e_e_c_h_
    T_e_c_h_n_o_l_o_g_y_ M_a_g_a_z_i_n_e_, April/May 1990, p.  58-64.

    Copies of the FS-1016 document are available for $2.50 each from:

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    GSA Rm 6654
    7th & D St SW
    Washington, D.C. 20407

DVI: is  specified in  the  ``Recommended  Practices for  Enhancing  Digital
    Audio  Compatibility   in  Multimedia   Systems'',   published  by   the
    Interactive Multimedia  Association (IMA), Annapolis,  MD. The  document
    also contains reference implementations for mu-law to 16-bit,  ADPCM and
    sample rate conversions.

For sample-based encodings,  a receiver should  accept packets  representing
between 0 and  200 ms of  audio data.(1)   Receivers should  be prepared  to
accept multi-channel audio, but may choose to only play a single channel.

1.3 Application Programming Interface for Audio Codecs

The application programming interface (API) for audio codecs described  here
is suggested, but not required for interoperability.  The API shown here  is
similar to the one used by SunOS 4.1.  The encoding types are drawn from the
standard names defined here.

typedef {AE_PCMU = 1, AE_PCMA, AE_L16} encoding_t;

typedef struct {
  unsigned sample_rate;            /* samples per second */
  unsigned samples_per_unit;       /* samples per unit */
  unsigned bytes_per_unit;         /* bytes per sample unit */
  unsigned channels;               /* # of interleaved channels */
  encoding_t encoding;             /* data encoding format */
  unsigned data_size;              /* length of data (optional) */
} audio_descr_t;

void *x_init(void *state, double period);

int x_encode(void *in_buf, int in_size, audio_descr_t *in_descr,
   void *out_buf, int *out_size, void *state);
int x_decode(void *in_buf, int in_size, audio_descr_t *out_descr,
   void *out_buf, int *out_size, void *state);

 1. This restriction allows reasonable buffer sizing for the receiver.

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x_init initializes a particular instance  of a codec.  If the argument  state
is zero, a memory area  sufficient to hold the  encoder or decoder state  is
allocated; if that argument is non-zero, the existing area is reinitialized.
The function returns a  pointer to the  area, zero if  the state area  could
not be  allocated.    The argument  period refers  to  the amount  of  audio
data in each  block, measured in  seconds.   It is  typically only used  for
block-oriented codecs.

The generic pointer to state refers to an area of storage whose structure is
opaque to the application program.  In the functions, 'x' is replaced by the
appropriate codec name, appropriately modified to conform to C syntax (e.g.,
g711, g721, etc).

The encoder and  decoder transform the  data contained in  the input  buffer
in_buf (in_size bytes) and  deposit the  result into the  output buffer  area
out_buf.    The variable  out_size  is set  to the  number of  bytes  actually
contained in the output buffer.   The ah arguments points to a structure  of
type audio_hdr_t,  which defines the given  input data format for the  encoder
and the desired output data format for the decoder.  The functions return  0
on success, a negative number if a failure occurred.

All block-oriented audio codecs should be able to encode and decode  several
consecutive blocks.

2 Video

The following video encodings are defined, with their abbreviated names used
for identification:

Bolt: The encoding is  implemented by the Bolter video codec [ED: need  more
    info on company, designation].

JPEG: The  encoding  is specified  in  ISO  Standards DIS  10918-1  and  DIS
    10918-2.    The data  is  formatted according  to  the JFIF  (JPEG  File
    Interchange Format) defined by C-Cube Microsystems.

H261: The encoding  is specified in ITU-T  (formerly CCITT) standard  H.261.
    The packetization and RTP-specific properties are described in RFC TBD.

nv: The encoding is implemented in the program 'nv' developed at  Xerox PARC
    by Ron Frederick.

CUSM: The  encoding is  implemented  in the  program CU-SeeMe  developed  at
    Cornell University  by  Dick Cogger,  Scott Brim,  Tim Dorcey  and  John

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PicW: The encoding is implemented in the program PictureWindow  developed at
    Bolt, Beranek and Newman (BBN).

3 Address of Author

Henning Schulzrinne
AT&T Bell Laboratories
MH 2A244
600 Mountain Avenue
Murray Hill, NJ 07974-0636
telephone:  +1 908 582 2262
facsimile:  +1 908 582 5809
electronic mail:  hgs@research.att.com

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