GB1471899A - Coding video signals - Google Patents

Abstract

1471899 Facsimile systems; analogue-todigital conversion ROBERT BOSCH GmbH 11 Dec 1974 [24 Dec 1973] 53532/74 Headings H4F H3H and H4P A video signal representing successive points in successive scan lines of a scanned field is differentially encoded and the encoded signal quantized, the quantization characteristic being changed during a scan line n if a large difference signal is detected in the preceding scan line, such signal representing a substantial change of contrast, e.g. the edge of an object in the scanned field. The new quantization characteristic is maintained during scan line n for a period that embraces the difference signal representing the portion of the edge in that scan line, so ensuring that it is subjected to the new quantization even if the edge has a substantial slope. The difference signal comprises a series of digital or analog values each representing the difference of the values of the video signal at two adjacent picture points. Successive difference values at 11 are passed to a threshold unit 15, Fig. 2, and to a quantizer comprising threshold units 12-14. If a difference value exceeds, for example, 15% of the maximum possible difference value, and therefore represents the presence of an edge in the scanned field, unit 15 passes an edge signal to OR gate 29 and serially connected delays 20-23. Each delay 20-23 is equivalent to the scanning interval between two picture points. The edge signal causes delay 30 to reverse switches 16-18 after an interval corresponding to one scan line minus two picture points. The switches remain reversed for 5 points since delays 20-23 are connected to the OR gate input. The switches change the thresholds of units 12-14 such that coarse instead of fine quantization takes place. The coarse quantization lasts for 5 points and so exists when the corresponding edge occurs in the next line, even if the edge is substantially inclined to the scan lines. Unit 31 receives the output from each threshold unit and develops a signal for transmission.