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
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.
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:
(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.
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)
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. arrangement of interworking between equipment conforming to Recommendation T.4 and these existing equipments is subject to further study.