Group 3 fax: CCITT draft recommendation T.4
The CCITT standard that lets a fax machine squeeze a scanned page into a stream of short code words.
International Telegraph and Telephone Consultative Committee of the International Telecommunication Union, "CCITT draft recommendation T.4", RFC 804, DOI 10.17487/RFC0804, January 1981, <https://www.rfc-editor.org/info/rfc804>.
@misc{rfc804,
series = {Request for Comments},
number = 804,
howpublished = {RFC 804},
publisher = {RFC Editor},
doi = {10.17487/RFC0804},
url = {https://www.rfc-editor.org/info/rfc804},
author = {International Telegraph and Telephone Consultative Committee of the International Telecommunication Union},
title = {{CCITT draft recommendation T.4}},
pagetotal = 12,
year = 1981,
month = jan,
} RFC text
Contents
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:
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.
CCITT Draft Recommendation T.4 PAGE 4
Format: 000000000001 ... ... ... ... 000000000001
(total of 6 times)
CCITT Draft Recommendation T.4 PAGE 5
Table 1a. Terminating White Codes
Code Lng Run
---------------------------
00110101 8 0
000111 6 1
0111 4 2
1000 4 3
1011 4 4
1100 4 5
1110 4 6
1111 4 7
10011 5 8
10100 5 9
00111 5 10
01000 5 11
001000 6 12
000011 6 13
110100 6 14
110101 6 15
101010 6 16
101011 6 17
0100111 7 18
0001100 7 19
0001000 7 20
0010111 7 21
0000011 7 22
0000100 7 23
0101000 7 24
0101011 7 25
0010011 7 26
0100100 7 27
0011000 7 28
00000010 8 29
00000011 8 30
00011010 8 31
00011011 8 32
00010010 8 33
00010011 8 34
00010100 8 35
00010101 8 36
00010110 8 37
00010111 8 38
00101000 8 39
00101001 8 40
00101010 8 41
00101011 8 42
00101100 8 43
00101101 8 44
00000100 8 45
00000101 8 46
00001010 8 47
00001011 8 48
01010010 8 49
CCITT Draft Recommendation T.4 PAGE 6
01010011 8 50
01010100 8 51
01010101 8 52
00100100 8 53
00100101 8 54
01011000 8 55
01011001 8 56
01011010 8 57
01011011 8 58
01001010 8 59
01001011 8 60
00110010 8 61
00110011 8 62
00110100 8 63
CCITT Draft Recommendation T.4 PAGE 7
Table 1b. Make Up White Codes
Code Lng Run
---------------------------
11011 5 64
10010 5 128
010111 6 192
0110111 7 256
00110110 8 320
00110111 8 384
01100100 8 448
01100101 8 512
01101000 8 576
01100111 8 640
011001100 9 704
011001101 9 768
011010010 9 832
011010011 9 896
011010100 9 960
011010101 9 1024
011010110 9 1088
011010111 9 1152
011011000 9 1216
011011001 9 1280
011011010 9 1344
011011011 9 1408
010011000 9 1472
010011001 9 1536
010011010 9 1600
011000 6 1664
010011011 9 1728
CCITT Draft Recommendation T.4 PAGE 8
Table 2a. Terminating Black Codes
Code Lng Run
---------------------------
0000110111 10 0
010 3 1
11 2 2
10 2 3
011 3 4
0011 4 5
0010 4 6
00011 5 7
000101 6 8
000100 6 9
0000100 7 10
0000101 7 11
0000111 7 12
00000100 8 13
00000111 8 14
000011000 9 15
0000010111 10 16
0000011000 10 17
0000001000 10 18
00001100111 11 19
00001101000 11 20
00001101100 11 21
00000110111 11 22
00000101000 11 23
00000010111 11 24
00000011000 11 25
000011001010 12 26
000011001011 12 27
000011001100 12 28
000011001101 12 29
000001101000 12 30
000001101001 12 31
000001101010 12 32
000001101011 12 33
000011010010 12 34
000011010011 12 35
000011010100 12 36
000011010101 12 37
000011010110 12 38
000011010111 12 39
000001101100 12 40
000001101101 12 41
000011011010 12 42
000011011011 12 43
000001010100 12 44
000001010101 12 45
000001010110 12 46
000001010111 12 47
000001100100 12 48
000001100101 12 49
CCITT Draft Recommendation T.4 PAGE 9
000001010010 12 50
000001010011 12 51
000000100100 12 52
000000110111 12 53
000000111000 12 54
000000100111 12 55
000000101000 12 56
000001011000 12 57
000001011001 12 58
000000101011 12 59
000000101100 12 60
000001011010 12 61
000001100110 12 62
000001100111 12 63
CCITT Draft Recommendation T.4 PAGE 10
Table 2b. Make Up Black Codes
Code Lng Run
---------------------------
0000001111 10 64
000011001000 12 128
000011001001 12 192
000001011011 12 256
000000110011 12 320
000000110100 12 384
000000110101 12 448
0000001101100 13 512
0000001101101 13 576
0000001001010 13 640
0000001001011 13 704
0000001001100 13 768
0000001001101 13 832
0000001110010 13 896
0000001110011 13 960
0000001110100 13 1024
0000001110101 13 1088
0000001110110 13 1152
0000001110111 13 1216
0000001010010 13 1280
0000001010011 13 1344
0000001010100 13 1408
0000001010101 13 1472
0000001011010 13 1536
0000001011011 13 1600
0000001100100 13 1664
0000001100101 13 1728
CCITT Draft Recommendation T.4 PAGE 11
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:
Table 3. Extended Make Up Codes (Black and White)
Code Lng Run
---------------------------
00000001000 11 1792
00000001100 11 1856
00000001101 11 1920
000000010010 12 1984
000000010011 12 2048
000000010100 12 2112
000000010101 12 2176
000000010110 12 2240
000000010111 12 2304
000000011100 12 2368
000000011101 12 2432
000000011110 12 2496
000000011111 12 2560
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.
ISO Plain Language
On this page
§1The Goal and the Scanning Track
The CCITT set out the reasons for a third generation of fax apparatus. Recommendation T.2 covered group 1 type apparatus for ISO A4 document transmission over a telephone-type circuit in approximately six minutes, and Recommendation T.3 covered group 2 type apparatus for the same job in approximately three minutes. There was a demand for group 3 apparatus that would transmit an ISO A4 document over a telephone-type circuit in approximately one minute. For a large number of applications, black and white reproduction was sufficient. The service might be requested either together with a telephone conversation or when either or both stations were unattended, and in both cases the facsimile operation would follow Recommendation T.30.
The group 3 apparatus for use on the general switched telephone network and international leased circuits was therefore to be designed and operated according to the standards in this document.
The first of those standards is the scanning track. The message area should be scanned in the same direction in the transmitter and the receiver. Viewed in a vertical plane, the scanning direction should be from left to right, and subsequent scans should be adjacent to and below the previous scan.
The point of a third generation of fax machines was speed: about one minute for an A4 page, against about six minutes for the first generation and about three for the second. The document then fixes how the page is read, so that the sending and receiving machines agree on which direction the scan runs and where each line starts.
§2Dimensions of the Apparatus
Four dimensions are specified. The standard definition and an optional higher definition are 3.85 and 7.7 line per millimetre respectively in the vertical direction. There are 1728 black and white picture elements along the scanned line, and the scanning line length is 215 mm. Other scanning line lengths may be employed, in which case the scanning density should be changed to maintain the correct picture proportions. Input documents up to a minimum of ISO A4 size should be accepted.
A standard page is scanned at 3.85 lines per millimetre, or 7.7 for the optional finer setting, and each line holds 1728 picture elements across a width of 215 mm. That fixes both the shape of the image and the amount of data a line has to carry.
§3Transmission Time per Scanning Line
A total scanning line is defined as the sum of the data bits, any required fill bits and the end-of-line bits. Four minimum transmission times are specified for it. The recommended standard is 20 milliseconds, with an optional fall-back to the 40 millisecond option. A 10 millisecond option is recognised, with a mandatory fall-back to the 20 millisecond standard. A 5 millisecond option is recognised, with a mandatory fall-back to the 10 millisecond option and then to the 20 millisecond standard. A 40 millisecond option is also recognised.
The identification and choice of the minimum transmission time is 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 standard line takes at least 20 milliseconds to send. Faster settings of 10 and 5 milliseconds are allowed, but each has to fall back to the next slower one if the line fails to get through, so that a receiver which cannot keep up still receives the page. The two ends agree on the setting before the page starts.
| Option | Minimum time per line | Fall-back |
|---|---|---|
| Recommended standard THIS RFC | 20 milliseconds | Optional fall-back to 40 milliseconds |
| Recognised option | 10 milliseconds | Mandatory fall-back to the 20 millisecond standard |
| Recognised option | 5 milliseconds | Mandatory fall-back to 10 then 20 milliseconds |
| Recognised option | 40 milliseconds | None |
§4The Coding Scheme: DATA, EOL, FILL and RTC
The one-dimensional run length coding scheme is the core of the recommendation. A line of data is composed of a series of variable length code words, each representing a run length of either all white or all black, with white runs and black runs alternating. A total of 1728 picture elements represent one horizontal scanning line of a standard A4 document. All data lines begin with a white run length code word, so that the receiver maintains colour synchronisation; if the actual line begins with a black run, a white run length of zero is sent. Black and white run lengths, up to a maximum of one scanning line of 1728 pels, are defined by the code words in Tables 1 and 2, and there is a different list of code words for black and for white.
The code words are of two types. A run length in the range of 0 to 63 pels is encoded with its appropriate terminating code word. A run length in the range of 64 to 1728 pels is encoded first by the make-up code word representing the run length that is equal to or shorter than the one required, followed by the terminating code word representing the difference between the required run length and the run length represented by the make-up code.
Three control signals complete the scheme. The end-of-line code word follows each line of data. It is a unique code word that can never be found within a valid line of data, so resynchronisation after an error burst is possible, and the signal also occurs before the first data line of a page. Its format is 000000000001. Fill may be placed in the message flow by transmitting a variable length string of zeros; it may be inserted between a line of data and its end-of-line, but never within a line of data, and it must be added to insure that each line of data, fill and end-of-line exceeds the minimum transmission time established in the premessage control procedure. The maximum length for a single line of fill is 5 seconds, after which the receiver may disconnect. The end of a document transmission is indicated by sending six consecutive end-of-line signals, and after that the transmitter sends the post-message commands in the standard T.30 blocked format at the data rate.
Each line of the page is sent as a list of runs of black or white, and the runs alternate. A short run has one code word. A long run has two: a make-up code for the biggest run that still fits, followed by a terminating code for the remainder. A fixed bit pattern marks the end of a line, so a receiver that missed some bits can pick up again at the next line.
- 000000000001EOL — End of line. Never appears inside a line of data, and also comes before the first data line of a page.
- 0FILL — A variable length string of zeros, inserted between a line of data and its end-of-line.
- 001101010 white pels — Terminating code word for a white run of zero, sent when a line starts with black.
- 01112 white pels — Terminating code word for a white run of two.
- 1101164 white pels — Make-up code word for a white run of 64.
- 00001101110 black pels — Terminating code word for a black run of zero.
- 0101 black pel — Terminating code word for a black run of one.
- 112 black pels — Terminating code word for a black run of two.
- 000000111164 black pels — Make-up code word for a black run of 64.
- 000000010001792 pels — Extended make-up code, for black and white alike, for machines taking wider paper.
§5Two-Dimensional Coding and the Line Modulation
The one-dimensional coding scheme may be extended as an option to a two-dimensional scheme, which the document says is the subject of further study. The choice between the standard code table and the extended code table, which adds make-up codes for runs of 1792 to 2560 pels for machines that accept wider paper, is made in the pre-message, Phase B, portion of the T.30 control procedures.
For the line itself, it was provisionally agreed that group 3 apparatus operating on the general switched telephone network should use the modulation scrambler, equalisation and timing signals defined in Recommendation V.27ter, specifically in the preamble and in procedures 2, 3, 6, 7, 9 and 10. The data signalling rates to be used are those recommended in Recommendation V.27ter, 4800 and 2400 bit per second. The document notes that some administrations said it would not be possible to guarantee the service at a data signalling rate higher than 2400 bit per second, and that there are equipments in service using other modulation methods, with the arrangement for interworking between equipment conforming to T.4 and those existing equipments subject to further study. For higher speed operation, such as may be possible on leased circuits, it was provisionally agreed that group 3 apparatus may use the signals specifically defined in procedures 1, 2, 3, 4, 7, 8, 9, 11 and 12 of Recommendation V.29.
The bits of each code word are put on the line using the modem rules of Recommendation V.27ter, at 4800 or 2400 bits per second. On leased lines, where the connection is good enough, the faster signals of Recommendation V.29 may be used. A two-dimensional coding scheme was still being studied when this draft was written.