Dacom 450/500 facsimile data transcoding (RFC0803)
Original Publication Date: 1981-Nov-01
Included in the Prior Art Database: 2019-Feb-14
Internet Society Requests For Comment (RFCs)
A. Agarwal: AUTHOR [+2]
The first part of this RFC describes in detail the Dacom 450 data compression algorithms and is an update and correction to an earlier memorandum. The second part of this RFC 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 administrators.
Dacom 450/500 Facsimile Data Transcoding A. Agarwal, M. J. O’Connor and D. L. Mills 2 November 1981
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 . Following this, the encoding and decoding algorithms are described along with the supporting programs and file formats. Reference  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 . 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 ; 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).
Data are encoded two rows at a time using a two-dimensional run-length code. Each row-pair is scanned left-to-right and the line-pairs themselves processed top-to-bottom of the document. Figure 1 shows how the pels are represented.
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 transiti...