Zone Plate Signal Generator

The invention concerns the generator of circular zone plate signals for testing digital television signals, in particular in accordance with CCIR-REC.601.

Such known generators have horizontal frequency counter and a vertical frequency counter for generating an orthogonal raster and ROMs for providing the horizontal and vertical frequency phase values, an addition stage for putting these sets of phase values together and further ROM facilities for converting phase values into amplitude values.

The signals generated by a zone plate generator are test signals that are often used for investigating and experimenting with digital television signals. Full details regarding the characteristics of the zone plate test signal and especially its use in the development of television techniques and refinements are to be found in the article by M. Weston, "The Zone Plate: Its Principals and Applications" published in the periodical EBU Review--Technical, Number 195 of October 1982, pages 218 ff.

A generator of the type of the above described is known from the FIG. 14 diagram on page 7 with the BBC-Research Report (BBC RD 1978/23) of J. O. Drewry, entitled "The Zone Plate As a Television Pattern". In that known circuit the generator can produce only a luminance signal as the zone plate signal and even that signal is not compatible with the CCIR-REC. 601-Standard. Furthermore, in this known zone plate generator no frequency calibration is provided.

From U.S. Pat. No. 4,322,749, a zone plate generator signal is known in which the currently known phase values are produced by means of a so called accumulator circuit. This generator, however, has a very complicated and expensive circuit.

An object of the present invention to provide a zone plate generator of the above described kind by which a circular zone plate display can be provided for both the luminance and the chrominance transmission in accordance with CCIR-REC. 601-Standard wherein frequency calibration marks can be produced from the display.

Briefly, the output of the addition stage which combines the horizontal and vertical frequency phase values to produce a two dimensional pattern, a first post-addition ROM is provided for generating a luminance signal Y according to the equation y=-cos .phi. and a second post-addition ROM is provided for generating a chrominance signal C in accordance with the equation C=sin .phi.. The output signals of these ROMs, after being clocked at appropriate intervals, for example in respective registers, are supplied to a 4:2:2 decoder for bit-parallel digital video signals. The horizontal and vertical frequency counters that drive the phase value PROMs also drive PROMs for providing the calibration marks which can be made to show up on the zone plate pattern by modulating the luminance under control of separate switches for vertical and horizontal marks.

The invention has the advantage that zone plate signals for both luminance and chrominance can be produced which are compatible with the CCIR-Rec. 601 Standard and are obtainable economically. There is a further advantage that both horizontal and vertical frequency marks can be made to show up on the screen for calibration purposes.

The invention is further described by the way of illustrative example with reference to the annexed drawing, the single FIGURE of which is a block circuit diagram of a zone plate generator according to the invention.

The drawing shows a block circuit diagram of a zone plate generator according to the invention. A pixel counter 1 is loaded with each horizontal pulse and is caused to count at the sampling frequency f.sub.a of 13.5 MHz. A line counter 2, loaded with each of the vertical pulses is made to count the pulses of horizontal frequency. The output signals of the counter 1 (10-bit) and of the counter 2 (9-bit) may be used to produce an orthogonal raster or pattern. A first ROM 3 is connected at the output of the counter 2 and is so programmed that the phase values of the horizontal and vertical wobble signals are generated. On account of the orthogonality of the scanning pattern these phase values can be linearly superposed in a following addition stage 5. It is possible to feed a temporary component, for example a timewise wobble signal or a constant motion frequency through a third input 6 of the addition stage 5.

The output of the addition stage 5 is on the one hand connected to a third ROM 7 for the luminance components Y and on the other hand to a fourth ROM 8 for the chrominance components C. These ROMs 7 and 8 convert the supplied phase values into amplitude values of a cosine function for the luminance Y or a sine function or the chrominance C. The output of the memories 7 and 8 the respective amplitude value signals are available in 8 bit parallel form. These signals then go to the respective D registers 9 and 10 from which they are clocked out at 13.5 MHz for the luminance signal Y and at 6.75 MHz for the chrominance signal C and supplied to a conventional 4:2:2 decoder (not shown) for bit parallel digital video signals conforming to the standard EBU TECHn. 32 46-E. Since according to the CCIR-Rec. 601 Standard the chrominance signal C has only the half band width of the luminance signal Y, the chrominance wobbling is useful only up to half the sampling frequency (i.e. up to 3.37 MHz). Beyond this frequency limit there arise what are known as mirror frequencies which are visible in the picture as alias disturbances. For this reason the chrominance sweep must be blanked out or filtered out at half the sampling frequency. This can be done with two additional bits which are supplied out of the fifth ROM 11 to the fourth ROM 8. With the help of these two bits the blanking out of 100% of the sinusoidal amplitude down to 75%, for example, and from 25% down to 0 is accomplished.

In order to be able to evaluate quantitatively the measurements with the zone plate signals, the frequency markers are a great help. For this purpose in the fifth ROM 11, frequency markers are stored for the horizontal sampling frequency, each of which is a duration of 4 pixels of a line. There are similarly frequency markers for the vertical scanned frequency are stored in a sixth ROM 12 which each have a duration of two lines per picture field. The outputs of these ROMs 11 and 12 are first supplied to respective first inputs of three AND gates 13, 14 and 15 have second inputs respectively connected to switches 16, two of which have second inputs connected to a switch 16 switching the horizontal frequency markers ON and OFF with a line 18 of the third, AND gate 15 has its second input connected through the line 19 to the switch 17 for turning ON and OFF the vertical frequency markers. These AND gates followed by two OR gates 21 and 22, both of which have one input connected to the output AND gate 15 while the respective other inputs of the OR gates 21 and 22 are respectively connected to the outputs of the AND gates 13 and 14. The outputs of the AND gates 21 and 22 each furnish one bit to the ROM 7. By means of the OR gates 21 and 22 correlation of the H and V frequency marker signals is provided. The horizontal frequency markers are made visible by a reduction of the luminance amplitude controlled by two bits, from 100% down to 96% then to 93% and then down to a level of 90%. The vertical frequency markers switch the luminance amplitude directly by one bit from 100% to 90%. Because of the differing numbers of lines of the two standards (625 lines PAL and 525 lines NTSC) the ROMs 4 and 12 is received from the terminal 23 a switch-over signal for switching between the two above named line rates, in order to obtain compatability the vertial wobble signal with the horizontal wobble signal.

Although the invention has been described with reference to a particular illustrative example, it will be understood that modifications and variations are possible within the inventive concept.