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
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.
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
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.
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.
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:
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
CCITT Draft Recommendation T.4 PAGE 12
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.
