Television – Image signal processing circuitry specific to television – Special effects
Patent
1992-02-05
1997-05-13
Burgess, Glenton B.
Television
Image signal processing circuitry specific to television
Special effects
348588, 348 15, 379 98, H04N 974
Patent
active
056297416
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention concerns a scan reversing unit for video signals. Although not limited thereto, one application of scan reversal is in video telephone or conferencing systems where it is desired to display to the user his own (i.e. the transmitted) picture; it is subjectively more acceptable for this picture to be laterally inverted, because it is this image of himself in a mirror that the user is accustomed to seeing. Although this effect can be achieved on a CRT display simply by reversing the polarity of the line scanning coils, this involves switching of high peak voltages; moreover, reversal can not always be readily implemented on modern solid-state display devices.
2. Description of the Related Art
The invention is concerned with apparatus for processing the video signal so that the resulting signal results in a laterally inverted display on a standard monitor. Previous such apparatus is described in U.S. Pat. No. 4,727,411 and U.S. Pat. No. 4,220,965. It is particularly concerned with video signals in which luminance and chrominance samples are interleaved, where the desired sample sequence is not simply the reverse of the input line.
SUMMARY OF THE INVENTION
Each line of a television signal has a sequence of interleaved luminance and chrominance samples according to a given format (Y.sub.o U.sub.o Y.sub.1 V.sub.o etc.) which are written into a one line store using a first address sequence and then read out using a second address sequence such that the luminance samples are in reverse order, (and likewise the chrominance); however their interleaving conforms to the given format. The second address sequence is used also for writing in the next incoming line, which is then read out using the first sequence, the two sequences alternating throughout a field period. The technique is especially useful in videotelephony and videoconferencing.
BRIEF DESCRIPTION OF THE DRAWINGS
Some embodiments of the invention will now be described with reference to the accompanying drawings, in which:
FIG. 1 is a block diagram of one form of image reversing unit according to the invention;
FIG. 2 is a timing diagram for the apparatus of FIG. 1;
FIG. 3 shows the contents of the memory 1 of the apparatus of FIG. 1 at the end of the first line of an incoming frame; FIGS. 4a and 4b when placed side-by-side show the occupancy of the above locations at each clock period; and
FIGS. 5a and 5b illustrate schematically a user terminal for video communication incorporating the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The scan reversal circuit shown in FIG. 1 is, for ease of illustration, designed to process a digital television signal having eight picture elements per line. Any practical system will of course have considerably more elements than this. Each pair of picture elements n, n+1 is represented by respective luminance values Yn, Yn+1 and common chrominance (colour difference) values Un and Vn--i.e. the horizontal chrominance resolution is half that of the luminance.
Each line of the input signal consists of a sequence of sixteen digital valises, in sequence, viz: blanking period.
The main part of the apparatus is a read-write memory 1 having sufficient locations to accommodate an entire line of the picture--i.e. in this example, sixteen locations a0 . . . . a15. Its addresses are provided by an address generator 2, and system timing is provided by two-phase clock generator 3 synchronous with the incoming digital values--i.e. the clock rate is twice the video pixel rate. During each clock period it produces clock pulses .phi.R and .phi.W during which, respectively, read and write operations to the memory take place.
The object of this arrangement is to write each line into the memory whilst reading out the preceding line in a sequence such that the resulting picture is laterally inverted when viewed. Note that the YUYV format under consideration means that the readout sequence is not the reverse of the write sequence. The resolution of this probl
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Viewphone, Tele Conference. The Business Communications Magazine, vol. 9, No. 4., Nov. 90.
British Telecommunications public limited company
Burgess Glenton B.
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