Patent GB-1154531-A (GB)
Title: Facsimile Line Skipping System
IPC: H04N1/17
Filed: 1966-07-12
Abstract: 1,154,531. Facsimile systems; selective transistor amplifiers. XEROX CORP. 12 July, 1966 [14 July, 1965], No. 31351/66. Headings H3T and H4F. In a facsimile transmission system information bearing portions of the original are scanned at a slower rate than non-information bearing portions and video signals are transmitted only during the slower scan. The invention is described mainly in connection with the facsimile transmission of type-written or printed (black) information on paper (white) but an adaption suitable for the transmission of a wider range of tonal values, e.g. documents containing pictures &c., is also disclosed the object in both cases being to reduce the total transmission time required. In Fig. 1 the original to be transmitted is advanced via rollers 100, 102 driven by a stepping device 110 past a first detector switch 112 and then past a second detector switch 116 the latter being adjacent a slit 120 in a platen 104. Operation of switch 112 results, via the closure of associated contacts 114a, 114b in the connection of master synchronizing pulses (B) to the output 146 connected to the transmission line to bring remote receivers into synchronism and to the stepping device 110 which advances the original. When the latter reaches switch 116 the associated contacts 118a, 118b operate to open the two previously made circuits. In this position the orginal, which is illuminated via the slit with light from sources 122, is scanned in a line transverse to its length by means of a mirror 124 oscillated at the higher of the two scanning speeds by means of scan generator 136. The latter, the circuit of which is described in connection with Fig. 8, comprises a three stage active filter with a twin T network and operates to produce a 30 c.p.s. sine wave output in response to a 60 c.p.s. square wave input derived from a synchronizing arrangement (Fig. 2, not shown) which also supplies various other control signals (Fig. 3, not shown) to similarly identified terminals in Fig. 1. The original is thus scanned at the higher rate from right-to-left and from left-to-right the scanning mirror directing the light to a photo-electric device 130 which feeds a squaring amplifier 132 delivering a two level output corresponding to black or white portions of the original. The operation is then as follows. During the higher rate scan if no black areas (i.e. information bearing print &c.) are detected the white representative level of the output of amplifier 132 inhibits the transmission of video signals but causes the stepping device 110 to advance the original and also allows the transmission of advance control signals to step on associated receivers, such advance occurring at the end of each scan. When during the higher rate scan black areas are detected the second level output of amplifier 132 (1) inhibits the advance signals (2) gates off the higher speed scan generator 136 and gates on a slower speed saw-tooth scan generator 134 (3) initiates the transmission of a pre-video alterting signal (4) transmits video signals generated during the forward trace of the slower speed scan and (5) advances the original and transmits advance control signals but no video signals during the (rapid) retrace period of the slower speed scan. If black (information bearing) areas are detected during this retrace period a further slower speed scan is effected but if no such areas occur an advance operation is effected and the system reverts to the higher scan rate and advance operations until black areas are again detected. At a co-operating receiver (Fig. 4, not shown) the received video signals are supplied to a recording head 172 which is advanced via lead screw 178 driven by a stepping device 110 to mark a recording sheet carried by a drum 174 rotating in synchronism with the slower scan generator 134 of Fig. 1. When a wider range of tonal values is required the transmitter is modified by the addition of a linear amplifier (190, Fig. 5, not shown) an analog gate (192) and a linear adder (194) and the receiver is modified by the addition of an analog gate (196, Fig. 6, not shown). The slower scan saw-tooth wave generator may take the form depicted in Fig. 7 (not shown).