GB1466281A - Facsimile transmission - Google Patents

Abstract

1466281 Data transmission; facsimile systems HASLER AG 6 June 1974 [8 June 1973] 25283/74 Headings H4P and H4F In a facsimile system, particularly incorporating redundancy reducing coding, each picture line received is tested for errors, which if found, is scrapped and the previous line already reproduced is duplicated on the next line position. A code word EOL is transmitted after the end of each line and on receipt of each EOL word, a test is made to ascertain whether the sum of picture points since the previous EOL word, derived from a decoder, corresponds to the known number of points per line. A beginning of line word BOL is also transmitted, any gaps in the line being filled with blanks. At the transmitter, a reader 1 controlled by 3 emits binary signals to shift register 2. At each line end, control 5 causes a BOL signal to be emitted from read only memory 7 and routed through OR 19 and modulator 8 to line followed by content of register 2 through coder 20. This includes a counter 10 which, between each change of brightness value, counts digits which count is dumped into register 12 for transmission, then proceeds to count the alternative value. Control 5 also detects when register 2 is empty, i.e. an end of line condition, hence EOL is emitted from memory 18 and next line is recorded in register 2. At the receiver after demodulation at 32 a circuit 57 synchronizes a clock with incoming signals which are clocked into register 34 having a decoder 35 which recognizes BOL (EOL) signals and provides an output on line 36 (37) which sets (resets) flip-flop 38. Via read only memory 39 a succession of code words in a section of register 34 is passed to counter 40 which are clocked to zero, when the counter stops and resets but pulses are continually read into register 45 and counter 47, which records actual number of points received per line plus the number of bits = EOL. A signal on line 49 via gates 50 tests whether EOL is in decoder 35 and if so a signal is given on line 51 to write control 53 causing emptying of register 45 to writing device 54 and subsequent advancement of one line space. Meanwhile register 46 is being filled through switch 56 reversed. Control 57 determines whether a code word signifies 0 or 1. If not so, a signal on line 52 prevents switch 56 being reversed, the register storing the previous line being read out again to the writing device and the subsequent line received from the transmitter is over-read into the register with the faulty line bits.