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:
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.
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.
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.
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.