rfc803.txt

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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
cases.

     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 facsimile data is ordinarily stored as an RT-11  file  in
the  so-called  UCL  format  [4].  In this format, each 585-bit frame is
stored in a 76-byte record.  The first byte specifies the length of  the
record,  the  second  specifies  a  command  and  the remaining 72 bytes
contain the 585 bits of the original Dacom 450 frame zero-filled at  the

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
necessary.

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.

     The decoding program scans the input data bit-by-bit and recognizes
sequences  of  bits which form valid run-length codes (see the tables in
[5]).  The table of Huffman codes can be represented as  a  binary  tree
with  the  values of the run lengths (e.g.  1, 2, 64, 1728, etc.) at the
terminal nodes and each branch labeled 0 or 1.  The  code  for  any  run
length  then  is the sequence of branch labels on the path from the root
to the terminal node representing this length.

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.


B1: VERTICAL RESOLUTION
    0 = 7.7 lines per millimeter
    1 = future option, not implemented 

B2: OUTPUT PAPER LENGTH
    0 = short length (Letter size)
    1 = long length (Legal size)

B3: DOCUMENT IN SCANNER
    0 = no document present (end of page)
    1 = document present (beginning of page)

B4: ODD PARITY OVER B1-B4

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

         Figure 5. File Sizes and Transcoding Times

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.

5.  Draft Recommendation T.4 - Standardization of Group 3 Facsimile  for
    Document  Transmission.   CCITT  Study Group XIV Contribution #25-E,
    December 1977.  (Also in RFC-804).