TitleDacom 450/500 facsimile data transcoding
AuthorA. Agarwal, M.J. O'Connor, D.L. Mills
DateNovember 1981
Format:TXT, HTML

RFC 803

          Dacom 450/500 Facsimile Data Transcoding
         A. Agarwal, M. J. O'Connor and D. L. Mills
                      2 November 1981

1.  Introduction

     As part of our effort in support of  the  DARPA  Internet  Program,
software  modules  to encode and decode facsimile data for the Dacom 450
and 500 models Computerfax facsimile  machines  have  been  constructed.
The  purpose of these modules is to map the data representations used by
these machines to and from bit-map  and  run-length  representations  in
programs  for editing, transmission and archiving facsimile images.  The
modules are written in the PDP-11 MACRO-11 assembly language and can  be
incorporated into programs for, among others, the RT-11 operating system
and the DCNET BOS or VOS operating systems.

     The first part of this report describes in  detail  the  Dacom  450
data compression algorithm and is an update and correction to an earlier
memorandum [2].  Following this, the encoding  and  decoding  algorithms
are  described  along  with  the  supporting  programs and file formats.
Reference  [3]  describes  another  implementation   of   the   decoding
algorithm.   Grateful  acknowledgment  is made to E.  A.  Poe of Rapicom
for his assistance in this effort.

     The second part of this report describes briefly the Dacom 500 data
compression  algorithm  as used by the INTELPOST electronic-mail network
under  development  by  the  US  Postal  Service  and  several   foreign
administrations.    These  machines  conform  to  the  CCITT  T.4  Draft
Recommendation, described in [5].  Supporting programs and file  formats
are described.

2.  Dacom 450 Data Compression Principles

     The encoding algorithm for the Dacom 450 processes lines scanned by
the  machine  to  produce a two-dimensional run-length code described by
Weber [1]; however,  this  article  contains  a  number  of  errors  and
omissions,  many  of  which  were  discovered  only  after  considerable
analysis  and  experimentation  [2,3].   The  machine  operates  over  a
coordinate   space   of   l726   by  approximately  2200  pels  when  in
high-resolution (detail) mode.  In normal (quality)  mode  the  vertical
resolution is halved, so that about 1100 lines are transmitted, while in
express mode about 733 lines are transmitted (missed lines are filled in
on playback by replicating previous lines).

Dacom 450/500 Facsimile Data Transcoding                        PAGE   2

                    |          |          |
                ... |  x(1,j)  | x(1,j+1) | ...
                ... |  x(2,j)  | x(2,j+1) | ...
                    |          |          |
                      Direction of scan ->

               Figure 1. Data Representation

     For each j the vector (x(1,j),x(2,j)) represents  the  contents  of
the  jth  column, where x(i,j) can take on values of zero (white) or one
(black).  Each of the four possible vectors ranging  over  these  values
will  be  called a state (Dacom calls these "modes") with the succession
of transitions between these states determined by the picture content of
the  particular line-pair.  Scanning of the line-pairs follows one after
the other with no special end-of-line code in the data itself.  For  the
purpose  of later discussion and comparison with the published data, the
following conventions will be used (note: the pels read top-bottom):

                Pels    Vector  State
                W-W     (0,0)   0
                B-W     (1,0)   1
                W-B     (0,1)   2
                B-B     (1,1)   3

     The algorithm used by Dacom to generate the transmitted data as the
columns   are   scanned   can  be  described  as  the  non-deterministic
finite-state automaton (nfsa) shown in Figure 2.  Conceptually, the nfsa
starts  at  the beginning of a page in a designated state and at a point
just after scanning the jth column in the jth state.  It then scans  the
(j + 1)th column and enters that state while emitting the string of bits
shown in the figure.

Dacom 450/500 Facsimile Data Transcoding                        PAGE   4

     Field-length values are constrained not to exceed  seven,  so  that
runs  exceeding  l27 with n = 7 will be encoded as a separate 7-bit word
of one-bits for each run of l27  except  the  last,  which  must  always
contain  at  least one zero-bit.  The field length n is decreased by one
under the following circumstances: the current run has been encoded as a
single  n-bit  field,  and for n in the range four through seven the two
high-order bits are zero or for n equal to three the  single  high-order
bit  is  zero.   The field length is not allowed to be reduced below two

     The encoding algorithm starts in state 0 with  both  field  lengths
set to 7.

2.1.  Dacom 450 Decoding Algorithm

     For reasons of speed and simplicity it is desirable that the  Dacom
450  decoding  algorithm  be  modeled  on  the  basis of a deterministic
finite-state automaton (dfsa).  Using straightforward formal procedures,
the  dfsa  of Figure 3 can be constructed.  This machine makes one state
transition for every bit, except for the W-W (0)  and  B-B  (3)  states,
which  must be treated specially in any case.  The states are labeled in
such a way as to correspond to those of Figure  2  for  states  numbered
from zero to three.

Dacom 450/500 Facsimile Data Transcoding                        PAGE   6

2.2.  Formatting Considerations

     Data are encoded for transmission  by  the  Dacom  450  in  585-bit
frames,  consisting  of  a  24-bit  synchronization code, 37-bit leader,
512-bit information area and l2-bit checksum.  There are  two  kinds  of
frames  distinguished  by leader format, one for setup or initialization
and the other for the data itself.  Serial binary image data are  placed
in the data area of succeeding data frames.

     The header of each frame is shown in Figure 4.  The various  fields
are defined in Table 1 following the Figure.


   | Sync Code | Leader |        Data       | CRC Code |
        24    /    37    \       512             12
     .-------'            \----------------------.
    /                                             \
   | Flags | Count | X Pos | Black | White | State |
   |   7    \ 10      12       3       3       2
   |         \--------------------------.
   |                                     \
   | Seq | RUN | COFB | RPT | Spare | SUB |
      2     1     1      1      1      1

Dacom 450/500 Facsimile Data Transcoding                        PAGE   7

             Table 1. Header Field Definitions 


Field   Width   Function                Setup   Data
       (bits)                           Block   Block

Sync Code  24   Synchronization         30474730 (octal)

Seq         2   Sequence number         00      00,01,10,11

RUN         1   Initialize-start        0       1

COFB        1   Unknown                 0       0

RPT         1   Unknown                 1       0

Spare       1   Unknown                 0       0

SUB         1   Indicates setup frame   1       0

Count      10   Number of bits in data  All 1's
                field (0 - 512)

X Pos      12   Current position on     All 1's
                scan line (0 - 1725)

Black       3   Current black field     All 1's
                length (2 - 7)

White       3   Current white field     All 1's
                length (2 - 7)

State       2   Current state (0 - 3)   All 1's

Data      512   Data (0 - 512 bits)

Dacom 450/500 Facsimile Data Transcoding                        PAGE   8

     Setup frames have additional information in  the  data  field;  the
various fields and their functions are described in Table 2.

        Table 2. Field Definitions for Setup Frame.

Field       Width       Function

Start bit       1       Always zero

Speed bit       1       Set if express mode

Detail bit      1       Set if detail mode (speed and detail
                        bits both zero for quality mode)

14 inch         1       Set if 14-inch paper 

5 inch          1       Set if 5-inch inch paper (14-inch
                        and 5-inch inch paper bits both zero
                        for 11-inch paper)

Paper present   1       Set if paper present in scanner

Spare           5       Can have any value

Multi-page      1       Set if multi-page mode

               20       All 0's

              480       Alternate 1's and 0's

     The tailing setup frames differ from the leading setup frames  only
in  bits  which  indicate  whether  the system is operating in single or
multiple page mode and whether paper is present in the scanner.

     All n-bit numeric fields (except Seq) are transmitted by the  Dacom
450  machine  least-significant-bit  (LSB)  first  (i.e.   Count, X Pos,
Black,  White,  State, CRC, and run length words  in  the  data  field).
All other fields are transmitted left-most bit first.

Dacom 450/500 Facsimile Data Transcoding                        PAGE   9

follow exactly that described in the previous section.

     This requirement for every data  frame  to  be  self-synchronizing,
leads  to  a  few  subtleties in the encoding algorithm which seem quite
natural, but were not very obvious in the beginning.

1.  Transition bits(s) labeling the arcs on the state transition diagram
    in  Figure  2  are  not broken across frames.  Similarly, individual
    run-length words are not broken across frames.

2.  If a frame ends with a transition, the  header  of  the  next  frame
    contains the state to which the transition is made.

3.  If a frame ends with a transition out of state  0  or  3,  then  the
    transition  bit (0 or 1) is inserted at the end of the current frame
    (not at the beginning of the next frame).

4.  The field lengths for black and white runs  in  the  header  include
    changes that may have been caused at the end of the previous frame.

5.  If a frame begins with a white  or  black  run,  then  this  run  is
    treated  (for  purpose of decreasing its field length) as if it were
    the beginning of a new run, since there is  no  information  in  the
    header to indicate otherwise.

     The decoding algorithm is  initialized  at  the  first  data  frame
received  after  the  sequence  of  setup  frames  at  the  beginning of
transmission.  The first data frame has a count of zero,  indicating  no
data  bits  are  in  the frame.  The second data frame begins the actual
document; however, its X position appears to be irrelevant.  Instead, we
assume the initial X position at this time is one pel to the left of the
right margin  (-l  mod  l726).   With  these  assumptions  succeeding  X
positions of the algorithm and the frame headers agree.

2.3.  The Decoding Program

     The decoding algorithm described above has been implemented in  the
PDP-11  MACRO-11 assembly language for the RT-11 operating system.  This
program contains extensive features for selectively dumping  frames  and
tracing  the operation of the algorithm.  It is designed to operate on a
file containing the raw data generated  by  the  machine  and  does  not
depend  upon  any  prior  reformatting  of  the  data.  However, it will
operate also on files in the so-called UCL format [4],  which  has  been
adopted  as  the standard for use in the Internet Program.  The existing
DCNET supporting software for the Dacom 450  uses  the  UCL  format  and
operates  normally  to  copy  data  directly between the machine and the
file, with decoding operations done at a later time.  However, there  is
no  intrinsic factor, except processing-rate limitations, why input data
could not be decoded directly from the machine.

Dacom 450/500 Facsimile Data Transcoding                        PAGE  10

synchronization  pattern  is  found,  the  header  and data portions are
extracted  and  the  various  state  variable  checked  and  reset,   if
necessary.    Checksum   verification  is  performed  according  to  the
polynomial 1 + x**3 + x**5 + x**7 + x**8 + x**12.  In the case of  setup
frames  the  format  (detail, quality, express), page length (14, 8-l/2,
5-l/4) and multiple-page indicators are extracted from  the  data  area.
Finally,  under  control  of  specified  options,  the  header  and data
portions of the frame are printed with appropriate headings.

     The decoding algorithm itself is called for each  data  frame.   It
produces  an  output  consisting of a sequence of run-length pairs which
can be used to form bit maps and  other  representations  of  the  data.
Optionally, a printed trace of the operations performed by the algorithm
can be produced.

2.4.  The Encoding Program

     The encoding algorithm has been implemented in the PDP-11  MACRO-11
assembly  language  for the RT-11 operating system.  The program accepts
facsimile data in 16-bit run-length format or bit-map format.  The input
data  would normally be in a file, possibly obtained by translating some
other representation (e.g., T.4 format) to run-length or bit-map format.
The  program  produces  an output consisting of data compressed in Dacom
450 format and packed in 585-bit frames  along  with  the  corresponding
header and checksum information.

     The encoding program needs to be careful about how  to  break  data
across  frames  and  how many bits of data to insert in each frame.  The
rules mentioned in section 2.2.  help to solve the first  problem.   The
second  problem is a little less understood.  The problem arises because
data bits are required by the printing mechanism at a constant rate, but
successive  frames  transmitted  at  the line rate can contain different
amounts of decoded information, leading to  buffer  overrun  in  extreme

     In order to compensate for the rate mismatch,  it  has  been  found
sufficient  to  control  the  size  of  the  data  portion  of the frame
according to a simple set of empirical rules which produce results quite
similar  to  the  scanner  iteslf.  According to these rules, a frame is
"full" when it contains more than 500 bits of  data  or  when  the  data
represents more than 4800*X pels (or columns) of information,

where   X = 2 for transmission rate 2.4 kbs,
        X = 1 for transmission rate 4.8 kbs,
        X = 1/2 for transmission rate 9.6 kbs.

2.5.  Dacom 450 File Formats

Dacom 450/500 Facsimile Data Transcoding                        PAGE  11

end.  The command byte is coded as follows:

a.  56 (70 octal): The data field contains  a  setup  frame  (the  first
    record of the file).  The length byte is 76 (114 octal).

b.  57 (71 octal): The data field contains a data frame  (the  remaining
    records  in  the  file  except the last one).  The length byte is 76
    (114 octal).

c.  58 (72 octal): End of file (the last frame of the file).   There  is
    no data field and the length byte is 2.

2.6.  Run-Length and Bit-Map File Formats

     The decode program produces 16-bit run length words as its  output.
Each  run  is encoded in a 16-bit word, with white in positive and black
in negative two's complement values.  A zero word terminates each  line,
with the trailing white run suppressed if present.  An all-white line is
encoded as a single run of length one followed by a zero word.  The file
is terminated by a line of length zero, that is, a single zero word.

     Bit-map files consist of a four-byte header followed by  the  data.
The  header  consists  of  two  2-byte  quantities,  the  first of which
represents the number of pels in a line and the  second  the  number  of
lines  in  the  page.   Each  scanning line of data is represented in an
integral number of bytes,  the  last  byte  of  a  line  zero-filled  if

3.  Dacom 500 Data Compression Principles

     The Dacom 500 machines are high-speed versions  of  the  Dacom  450
machines  and  operate  in  the  50-Kbps range using the T.4 compression
algorithm.  This algorithm, described in the [5], is  a  one-dimensional
one,  rather  than  the  two-dimensional  one  used in the Dacom 450 and
described in previous sections.  Since this algorithm is well known  and
the  subject  of  an  international  standard,  it  will  not be further
discussed here.

3.1.  Dacom 500 Decoding Algorithm

     The decoding program has been implemented in  the  PDP-11  MACRO-11
assembly  language  for  the  DCNET  and  RT-11  operating  systems.  It
operates on a file containing  facsimile  data  encoded  using  the  T.4
algorithm and produces a file in bit-map format.

Dacom 450/500 Facsimile Data Transcoding                        PAGE  12

     The tables for black and  white  run-length  codes  are  stored  as
separate  binary  trees in the decoding program.  The decoding algorithm
starts by initializing an accumulator at zero.  It then  begins  at  the
root  of  the  corresponding  tree and traverses the tree as it consumes
bits one-by-one from the input.  When a terminal node  is  reached,  the
value  stored  at  that  node is added to the accumulator.  If a make-up
node is reached, the value at that node is added to the accumulator  and
the  search  is  resumed  with  the  same tree to obtain the terminating
value; otherwise, the accumulator represents the current run length  and
the search resumes with the alternate tree.

3.2.  Dacom 500 Encoding Program

     The encoding program is also implemented  in  the  PDP-11  MACRO-11
assembly  language  for the DCNET and RT-11 operating systems.  It scans
the bit-map input and encodes each run of  black  or  white  pels  by  a
simple  table  lookup  of  the  Huffman  codes.   It  operates on a file
containing facsimile data in bit-map format and produces a file  in  T.4
format.   The T.4 specifications [5] require a minimum transmission time
per scan line of 4.3 milliseconds, which at 50-Kbps corresponds  to  242
bits  (DATA bits plus any required FILL bits plus the EOL bits equal 242
bits minimum).

3.3.  Dacom 500 File Formats

     The file consists of a number of  512-byte  blocks,  the  first  of
which  is  the  header.  The header contains a list of two-byte entries,
the first of which contains the number of pages and  the  remaining  the
lengths  (in  blocks) of each page in turn.  The remaining blocks of the
file contain the data for each page in T.4 format.  The  data  for  each
page   is   preceded   by  a  page-setup  command  and  succeeded  by  a
page-end-of-record command, as transmitted by the Dacom 500.  The format
of  both  commands  consists  of  the 12-bit T.4 EOL code (000000000001)
repeated six times and followed  by  a  special  4-bit  code  word  also
repeated  six  times.  The special code word consists of bits B1 through
B4 as defined below.

    0 = 7.7 lines per millimeter
    1 = future option, not implemented 

    0 = short length (Letter size)
    1 = long length (Legal size)

    0 = no document present (end of page)
    1 = document present (beginning of page)

Dacom 450/500 Facsimile Data Transcoding                        PAGE  13

3.4.  Comparison of Facsimile Formats and Transcoding Speeds

     Four different file formats are presently used in  our  system  for
facsimile  data  storage:  Dacom 450, Dacom 500 (T.4), 16-bit run-length
and bit-map.  The sizes of typical files (in megabits) in these  formats
are shown below for comparison.


        File    Dacom   Dacom   16-bit
                450     500     run-length

        PNGUIN  0.22    0.5     0.27
        INTELP  0.62    0.77    3.3
        PANDA   1.02    2.03    6.41

The file called PNGUIN is  a  block  drawing  of  dancing  penguins  and
represents  a  "small"  file.   The  file  called  INTELP is a composite
INTELPOST test image with text and graphics and  represents  a  "medium"
file.    Finally,  the  file  called  PANDA  is  a  half-tone  newspaper
photograph of a giant panda and represents a "monster" file  (this  file
was  recorded  on  the  Dacom 450 in quality mode and is therefore about
half the size it would be in detail mode).  The size of the bit-map file
for all these images is 3.8 megabits.  

     Figure 5 shows the file sizes (in 512-byte blocks) and  transcoding
times  (in  seconds)  for  the  INTELPOST test page.  The file sizes are
indicated on the boxes, while the transcoding times are indicated on the
arrows.  All times were obtained on the LSI-11/23 processor.

             193                      925
        +-----------+     95     +-----------+
        |           |----------->|           |
        |    T.4    |            |  Bit-map  |
        |           |<-----------|           |
        +-----------+    165     +-----------+
                                     A   |
                          60         |   |
              .----------------------'   |110
              |                          |
              |                          V
        +-----------+     89     +-----------+
        |           |----------->|           |
        |Run-length |            | Dacom 450 |
        |           |<-----------|           |
        +-----------+    153     +-----------+
             413                      155

Dacom 450/500 Facsimile Data Transcoding                        PAGE  14

4.  References

1.  Weber, D.R.  An adaptive  run-length  encoding  algorithm.   ICC-75,
    IEEE, San Francisco, California, June 1975.

2.  Palmer, L.C.  Final Report, COMSAT Participation in the DARPA Packet
    Satellite  Internetworking  and Speech Applications Program.  COMSAT
    Laboratories, July 1980.

3.  Katz, A.  Decoding Facsimile  Data  from  the  Rapicom  450.   DARPA
    Network  Working  Group  Report  RFC-798,  USC/Information  Sciences
    Institute, September 1981.

4.  Postel, J.  Rapicom 450  Facsimile  File  Formats.   DARPA   Network
    Working Group Report RFC-769,   USC/Information  Sciences Institute,
    September 1980.