CCITT draft recommendation T.4
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RFC 804

               CCITT Draft Recommendation T.4

  As Defined In: CCITT STUDY GROUP XIV, CONTRIBUTION No.25-E

SOURCE: DRAFTING GROUP ON DRAFT RECOMMENDATION T.4
TITLE: DRAFT RECOMMENDATION T.4 - STANDARDIZATION OF GROUP 3
    FACSIMILE   APPARATUS   FOR  DOCUMENT  TRANSMISSION  (as
    amended and agreed by Working Party XIV/2)

Draft Recommendation T.4

STANDARDIZATION OF GROUP 3 FACSIMILE APPARATUS FOR  DOCUMENT
TRANSMISSION

The CCITT, considering

a.  that Recommendation T.2 refers to Group 1 type apparatus
    for  ISO  A4 document transmission over a telephone-type
    circuit in approximately six minutes;
b.  that Recommendation T.3 refers to Group 2 type apparatus
    for  ISO  A4 document transmission over a telephone-type
    circuit in approximately three minutes:
c.  that there is a  demand  for  Group  3  apparatus  which
    enables  an  ISO  A4  document  to be transmitted over a
    telephone-type circuit in approximately one minute;
d.  that for a large number of applications black and  white
    reproduction is sufficient;
e.  that  such   a   service   may   be   requested   either
    alternatively   with  telephone  conversation,  or  when
    either or both stations are not attended; in both cases,
    the facsimile operation will follow Recommendation T.30,

(unanimously) declares  the  view  that  Group  3  facsimile
apparatus  for use on the general switched telephone network
and international leased circuits  should  be  designed  and
operated according to the following standards:

1.  Scanning track

     The  message  area  should  be  scanned  in  the   same
direction  in  the  transmitter  and  receiver.  Viewing the
message area in a vertical  plane,  the  scanning  direction
should  be from left to right and subsequent scans should be
adjacent and below the previous scan.

2.  Dimensions of apparatus


CCITT Draft Recommendation T.4                      PAGE   2

     The following dimensions should be used:

a.  A normal definition  standard  and  an  optional  higher
    definition standard of 3.85 and 7.7 line/mm respectively
    in vertical direction;
b.  1728 black and white picture elements along the  scanned
    line;
c.  A scanning line length of 215 mm.  Other  scanning  line
    lengths  may  be  employed  in  which  case the scanning
    density  should  be  changed  to  maintain  the  correct
    picture proportions;
d.  Input documents up to a minimum of ISO A4 size should be
    accepted.

3.  Transmission time per scanning line

     A total scanning line is defined as  the  sum  of  DATA
bits  plus  any  required  FILL bits plus the EOL bits.  The
minimum transmission times of the total scanning line should
conform to the following:

a.  20 milliseconds recommended standard  with  an  optional
    fall-back to the 40 milliseconds option;
b.  10  milliseconds  recognized  option  with  a  mandatory
    fall-back to the 20 milliseconds standard;
c.  5  milliseconds  recognized  option  with  a   mandatory
    fall-back  to  the  10  milliseconds  option  and the 20
    milliseconds standard;
d.  40 milliseconds recognized option.

     The  identification  and   choice   of   this   minimum
transmission time is to be made in the pre-message (Phase B)
portion  of  the  T.30  control  procedure.    The   maximum
transmission  time of any total scanning line should be less
than 5 seconds.

4.  Coding scheme

     The  one-dimensional   run   length   encoding   scheme
recommended for Group 3 apparatus is as follows:

a.  DATA
    A line of data is  composed  of  a  series  of  variable
    length  code  words.   Each  code  word represents a run
    length of either all white or all black.  White runs and
    black  runs alternate.  A total of 1728 picture elements
    represent one horizontal scanning line of  the  document
    of  standard  A4  size.   In  order  to  insure that the
    receiver maintains color synchronization, all DATA lines
    will  begin  with  a white run length code word.  If the
    actual scanning line begins with a black  run,  a  white
    run  length  of  zero  will be sent.  Black or white run
    lengths, up to a maximum length  of  one  scanning  line


CCITT Draft Recommendation T.4                      PAGE   3

    (1728  picture elements or pels) are defined by the code
    words in Tables 1 and 2.  The  code  words  are  of  two
    types:  Terminating  Code  words and Make Up Code words.
    Each run length is represented by either one Terminating
    Code  word  or  one  Make  Up  Code  word  followed by a
    Terminating Code word.

    Run lengths in the range of 0 to  63  pels  are  encoded
    with their appropriate Terminating Code word.  Note that
    there is a different list of code words  for  black  and
    white run lengths.

    Run lengths in the range of 64 to 1728 pels are  encoded
    first  by  the  Make  Up  Code word representing the run
    length which is equal to or shorter than that  required.