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.
    This  is  then  followed  by  the  Terminating Code word
    representing the difference  between  the  required  run
    length  and  the  run  length represented by the Make Up
    Code.

b.  END OF LINE (EOL)

    This code word follows each  line  of  DATA.   It  is  a
    unique  code word that can never be found within a valid
    line of  DATA;  therefore,  resynchronization  after  an
    error burst is possible.

    In addition, this signal will occur prior to  the  first
    DATA line of a page.

                    Format: 000000000001

c.  FILL

    A  pause  may  be  placed  in  the   message   flow   by
    transmitting  FILL.  FILL may be inserted between a line
    of DATA and an EOL, but never within  a  line  of  DATA.
    Fill  must  be  added  to insure that each line of DATA,
    FILL, and EOL exceeds the minimum transmission time of a
    total   scanning  line  established  in  the  premessage
    control procedure.  The maximum length for a single line
    of  FILL  is  5  seconds,  after  which the receiver may
    disconnect.

        Format: variable length string of 0's.      

d.  RETURN TO CONTROL (RTC)

    The end of  a  document  transmission  is  indicated  by
    sending   six  consecutive  EOL's.   Following  the  RTC
    signal, the  transmitter  will  send  the  post  message
    commands in the standard T.30 blocked format at the data
    rate.

CCITT Draft Recommendation T.4                      PAGE  11

Note: It is recognized that machines exist which accommodate
larger   paper   widths   whilst  maintaining  the  standard
horizontal resolution.  This option has been provided for by
the addition of the Make Up Code Set defined as follows:

     
     
         Table 3. Extended Make Up Codes (Black and White)
         Code            Lng     Run
         ---------------------------
         00000001000     11      1792
         00000001100     11      1856
         00000001101     11      1920
         000000010010    12      1984
         000000010011    12      2048
         000000010100    12      2112
         000000010101    12      2176
         000000010110    12      2240
         000000010111    12      2304
         000000011100    12      2368
         000000011101    12      2432
         000000011110    12      2496
         000000011111    12      2560

     The identification and choice of  either  the  standard
code  table  or the extended code table is to be made in the
pre-message  (Phase  B)  portion   of   the   T.30   control
procedures.

4.2 Two dimensional coding scheme

     The one-dimensional coding scheme defined in 4.1 may be
extended  as an option to a two-dimensional scheme.  This is
the subject of further study.

5.  Modulation and demodulation method

     It is  provisionally  agreed  that  Group  3  apparatus
operating  on  the  general switched telephone network shall
utilize the modulation scrambler,  equalization  and  timing
signals  defined  in Recommendation V.27ter, specifically in
the preamble and 2, 3, 6, 7, 9 and 10.

     The  data  signalling  rates  to  be  used  are   those
recommended  in  Recommendation V.27ter, i.e.  4800 and 2400
bit/s.

Note 1: Some administrations pointed out that it  would  not
be  possible  to  guarantee the service at a data signalling
rate higher than 2400 bit/s.

Note 2: It should be noted  that  there  are  equipments  in
service  using,  inter  alia, other modulation methods.  The

CCITT Draft Recommendation T.4                      PAGE  12

arrangement of interworking between equipment conforming  to
Recommendation  T.4 and these existing equipments is subject
to further study.

Note 3: For higher speed operation, such as may be  possible
on  leased circuits, it is provisionally agreed that Group 3
apparatus may utilize the signals specifically defined in 1,
2, 3, 4, 7, 8, 9, 11, and 12 of Recommendation V.29.