Patent GB-1074502-A (GB)
Title: Binary encoding of analog image signals
Inventor: YOUNG WILLIAM RAE
IPC: H04N3/12
Filed: 1964-08-04
Abstract: 1,074,502. Copying by scanning. WESTERN ELECTRIC CO. Inc. Aug. 4, 1964 [July 24, 1963], No. 30682/64. Heading H4F. The invention relates to a binary encoding apparatus, for use in a facsimile transmission system, which comprises scanning means for providing analogue image signals, means for establishing a threshold level within an intermediate range of possible levels of the analogue image signals and means for providing binary signals the digital values of which represent whether the levels of analogue image signals are greater or less than the threshold level, the threshold level being varied from time to time by adding threshold function signals, according to a prescribed schedule, to the analogue image signals whereby binary signals representative of analogue image signals in the intermediate range, i.e. " grey signals, are reproducable as half-tone shades, i.e. a sequence of black and white signals according to said prescribed schedule. As shown, Fig. 2, an analogue signal, voltage or current, from a facsimile scanner 20 and a special shaped voltage or current function generated by function generator 22 are supplied to combining network 21, and the algebraic combination of the two is supplied to threshold network 23. Threshold network 23 produces one output signal value for any positive input and a second, different, output value for any negative input, i.e. " white " and " black " output signals. These signals actuate sample-and-hold network 24 and thus a digital signal appears at the transmission output terminal 25. Bit clock 26 controls the timing of the output bit stream and the timing of the function generator 22 and the bit time is established to represent the smallest elemental picture segment that the system can accommodate. The threshold function signal developed by generator 22 is changed on a programmed basis, and a typical set of threshold patterns (which represent the dynamic threshold valves in terms of the scanner output) suitable for developing a multishade reproduction of a multishade original is shown in Fig. 3. In the example of Fig. 3, five different levels of shading are possible, each encoded differently, region a-b is always responsible for a " white " signal, and region e-f is always responsible for a " black " signal. The function pattern shown in the Figure for each line is repetitive in four-bit intervals across that scan line, and it differs from line to line and repeats the four line pattern over the entire field. The threshold function pattern for line 1, Fig. 3, ensures tha analogue signals which lie above level b will encode for bit 1 as a white signal and below level b as a black signal; for bit 2, analogue signals above level d will encode white, below d as black &c. The resulting dot patterns for the several regions using the illustrative four-line threshold function of Fig. 3 is shown in Fig. 4. Thus, for regions between levels band c one bit area in every four is shaded along the line and the pattern occurs every fourth line. The Specification includes a detailed description of a function generator 22 (Fig. 5, not shown) for developing the four-line repetitive pattern with five shades illustrated in Fig. 3. Continuous threshold functions, e.g. sawtooth or triangular waves, may be used instead of discontinuous threshold functions.