
below is the CCITT spec for Group 3 encoding and transmission.
it says "draft recommendation", but i think it's pretty accurate.
it's about 16k. 


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
 
     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
    (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.
 
     Format: 000000000001 ... ... ... ... 000000000001
                     (total of 6 times)

 
         Table 1a. Terminating White Codes
         Code            Lng     Run
         ---------------------------
         00110101        8       0
         000111          6       1
         0111            4       2
         1000            4       3
         1011            4       4
         1100            4       5
         1110            4       6
         1111            4       7
         10011           5       8
         10100           5       9
         00111           5       10
         01000           5       11
         001000          6       12
         000011          6       13
         110100          6       14
         110101          6       15
         101010          6       16
         101011          6       17
         0100111         7       18
         0001100         7       19
         0001000         7       20
         0010111         7       21
         0000011         7       22
         0000100         7       23
         0101000         7       24
         0101011         7       25
         0010011         7       26
         0100100         7       27
         0011000         7       28
         00000010        8       29
         00000011        8       30
         00011010        8       31
         00011011        8       32
         00010010        8       33
         00010011        8       34
         00010100        8       35
         00010101        8       36
         00010110        8       37
         00010111        8       38
         00101000        8       39
         00101001        8       40
         00101010        8       41
         00101011        8       42
         00101100        8       43
         00101101        8       44
         00000100        8       45
         00000101        8       46
         00001010        8       47
         00001011        8       48
         01010010        8       49
         01010011        8       50
         01010100        8       51
         01010101        8       52
         00100100        8       53
         00100101        8       54
         01011000        8       55
         01011001        8       56
         01011010        8       57
         01011011        8       58
         01001010        8       59
         01001011        8       60
         00110010        8       61
         00110011        8       62
         00110100        8       63
 
 
         Table 1b. Make Up White Codes
         Code            Lng     Run
         ---------------------------
         11011           5       64
         10010           5       128
         010111          6       192
         0110111         7       256
         00110110        8       320
         00110111        8       384
         01100100        8       448
         01100101        8       512
         01101000        8       576
         01100111        8       640
         011001100       9       704
         011001101       9       768
         011010010       9       832
         011010011       9       896
         011010100       9       960
         011010101       9       1024
         011010110       9       1088
         011010111       9       1152
         011011000       9       1216
         011011001       9       1280
         011011010       9       1344
         011011011       9       1408
         010011000       9       1472
         010011001       9       1536
         010011010       9       1600
         011000          6       1664
         010011011       9       1728

 
         Table 2a. Terminating Black Codes
         Code            Lng     Run
         ---------------------------
         0000110111      10      0
         010             3       1
         11              2       2
         10              2       3
         011             3       4
         0011            4       5
         0010            4       6
         00011           5       7
         000101          6       8
         000100          6       9
         0000100         7       10
         0000101         7       11
         0000111         7       12
         00000100        8       13
         00000111        8       14
         000011000       9       15
         0000010111      10      16
         0000011000      10      17
         0000001000      10      18
         00001100111     11      19
         00001101000     11      20
         00001101100     11      21
         00000110111     11      22
         00000101000     11      23
         00000010111     11      24
         00000011000     11      25
         000011001010    12      26
         000011001011    12      27
         000011001100    12      28
         000011001101    12      29
         000001101000    12      30
         000001101001    12      31
         000001101010    12      32
         000001101011    12      33
         000011010010    12      34
         000011010011    12      35
         000011010100    12      36
         000011010101    12      37
         000011010110    12      38
         000011010111    12      39
         000001101100    12      40
         000001101101    12      41
         000011011010    12      42
         000011011011    12      43
         000001010100    12      44
         000001010101    12      45
         000001010110    12      46
         000001010111    12      47
         000001100100    12      48
         000001100101    12      49
         000001010010    12      50
         000001010011    12      51
         000000100100    12      52
         000000110111    12      53
         000000111000    12      54
         000000100111    12      55
         000000101000    12      56
         000001011000    12      57
         000001011001    12      58
         000000101011    12      59
         000000101100    12      60
         000001011010    12      61
         000001100110    12      62
         000001100111    12      63

 
         Table 2b. Make Up Black Codes
         Code            Lng     Run
         ---------------------------
         0000001111      10      64
         000011001000    12      128
         000011001001    12      192
         000001011011    12      256
         000000110011    12      320
         000000110100    12      384
         000000110101    12      448
         0000001101100   13      512
         0000001101101   13      576
         0000001001010   13      640
         0000001001011   13      704
         0000001001100   13      768
         0000001001101   13      832
         0000001110010   13      896
         0000001110011   13      960
         0000001110100   13      1024
         0000001110101   13      1088
         0000001110110   13      1152
         0000001110111   13      1216
         0000001010010   13      1280
         0000001010011   13      1344
         0000001010100   13      1408
         0000001010101   13      1472
         0000001011010   13      1536
         0000001011011   13      1600
         0000001100100   13      1664
         0000001100101   13      1728
 
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
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.

-- 
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