Method and Apparatus for Converting an Image Signal

The present invention relates to method and apparatus for converting an image signal of a hivision TV to an image signal of a computer terminal, and more particularly to method and apparatus for converting an image signal effective in a superimpose apparatus which combines an image generated by a character terminal or a graphic terminal with an image generated by a standard hivision TV.

Combination of the image of the computer terminal and the image of the standard TV system such as NTSC system has been expected in various fields. Image resolutions of personal computers and work stations has been recently more and more increasing, and a non-interlace system having 700-800 dots horizontally and approximately 520 dots vertically has been becoming common. When the character image or graphic image (called terminal image hereafter) of such a terminal is to be combined with an image of the NTSC system TV, the following problems arise. In the NTSC system, (1) a scan line (dot) density per field is approximately one half of that of the terminal image, (2) signals between two adjacent fields have one-half scan line shift because of interlace scan (scan lines of first and second fields are shifted by one-half scan line interval), and (3) if a video signal is converted to a pixel density of the terminal image, an unnatural border line appears on an animation image (normally received image).

A difference between the dot densities of the NTSC system and the terminal image may be resolved by using a hivision signal which is a future TV signal system. The hivision TV signals has a specification of 1125 scan lines (interlaced scan), 20 MHz video band and 5/3 aspect ratio. The number of scan lines per field and the field frequency are essentially same as those of the terminal image.

In the combination of the hivision signal and the terminal image signal, the amounts of information are balanced as described above but the shift of one-half scan line interval between the adjacent field signals due to the interlaced scan system remains, and a difference between occupation times per dot is created due to the difference between aspect ratios. The difference between the occupation times per dot is due to the fact that the hivision signal and the terminal signal are of the same scan time in spite of the fact that a horizontal scan distance of the hivision signal is 5/4 of that of the terminal signal (assuming that an image height for the hivision signal is equal to that for the terminal signal). If the hivision signal is outputted to a display having and aspect ratio of 3/4, the image is vertically expanded. Accordingly, in the prior art as shown in JP-A-60-42993, where a clock time for reading and writing a field memory is changed, it is accomplished by using one field of memory of the hivision TV signal. As a result, a memory capacity is as large as approximately 2M bytes. In the prior art, the shift of information by one-half scan line interval between adjacent scan lines of the interlaced signal, which is one of the problems mentioned above, is not resolved. When the interlace system signal is converted to a non-interlace system signal, signals shifted by one-half scan line interval are displayed on the same scan position and hence the image vibrates (at a high contrast area) or becomes dim.

An example of conversion of the hivision TV to the NTSC system is disclosed in JP-A-59-104866.

It is an object of the present invention to resolve the problems encountered in the prior art, eliminate the difference between aspect ratios and the shift of information by one-half scan line interval and convert a hivision TV image signal to a computer terminal image signal without losing a high image quality of the hivision TV.

In order to achieve the above object, in accordance with the present invention, three memories for three continuous scan lines in one field are provided, and one line signal of the hivision TV image signal is written into one of the memories (write mode) and two line signals stored in other two memories are read (read mode). A ratio of a write clock to a read clock is set to 5/4 so that the difference between the aspect ratios is resolved. On the other hand, the two line signals read from the memories are interpolated to produce an image signal corresponding to a position of a first field line from the image signal of a second field of a hivision TV image (interlace) to resolve the shift of information by one-half scan line interval.

FIG. 1 shows a block-diagram of a main portion of a first embodiment of the present invention,

FIG. 2 shows a waveform of a synchronization signal of a hivision signal,

FIG. 3 shows a scan line structure,

FIG. 4. shows a block diagram of a dot clock generator,

FIG. 5 shows a block diagram of an address generator,

FIG. 6 shows a time chart for illustrating read/write timing of a line memory,

FIG. 7 shows a block diagram of a read/write controller,

FIG. 8 shows a circuit diagram of SW5 and SW6,

FIG. 9 shows a raster structure for illustrating signal processing in the first embodiment,

FIG. 10 shows a block diagram of a synchronization signal generator,

FIG. 11 shows a raster structure for illustrating signal processing of a second embodiment of the present invention,

FIG. 12 shows a block diagram of a main portion of a second embodiment of the present invention, and

FIGS. 13 and 14 show embodiments of reduced line memory capacities.

FIG. 1 shows a block diagram of a configuration of a first embodiment of the present invention. In FIG. 1, f.sub.Hh denotes a horizontal synchronization signal of a hivision TV signal, f.sub.Vh denotes a vertical sunchronization signal of the hivision TV signal, V.sub.ih 1 denotes a video signal of the hivision TV signal, V.sub.ic 2 denotes a video signal of a terminal image signal, Sync denotes a signal for synchronizing with a video generator such as personal computer or work station terminal, and V.sub.io denotes a combined signal of the above image signals.

An O/E 1 generates a signal for discriminating a first field and a second field of the hivision TV signal. A D.sub.ck1 generator 2 generates a dot clock D.sub.ckl which is an original signal of an address signal for writing into a line memory, based on the hivision horizontal synchronization signal f.sub.Hh. A D.sub.ck2 generator 3 generates a dot clock D.sub.ck2 which is an original signal of an address signal for reading from the line memory, based on the signal f.sub.Hh. W address generator 5 and R address generator 6 generate a write address and a read address, respectively, of the line memories 7-1, 7-2 and 7-3 in accordance with the dot clocks generated by the D.sub.ck1 generator 2 and the D.sub.ck2 generator 3, respectively. An ADC 15 is an AD converter which converts an analog video signal of the hivision TV signal to a digital signal, and DAC 10 and 11 are DA converters which convert digital signals to analog signals. Signals processing for the AD converted bits is common to all bits and hence signal processing for one bit is explained. SW1, SW2 and SW3 select read addresses and write addresses of the line memories 7-1, 7-2 and 7-3 in accordance with a signal from a R/W controller 4. SW4 supplies a signal ADC 15 to the line memory selected to the read mode by the R/W controller 4. SW5 and SW6 supply the outputs of the line memories selected to the write mode by the R/W controller 4 to the DAC 10 and the DAC 11, respectively. An adder 12 adds the signals of the DAC 10 and the DAC 11. A level adjuster 13 attenuates a signal of the adder 12 by 6 dB. SW7 selects an output signal by the signal of the O/E 1. A video adder 14 signal by the signal of the O/E 1. A video adder 14 combines the signal selected by the SW7 with the video signal V.sub.ic2 of the terminal image signal. A synchronization signal generator 16 generates a signal which synchronizes a video circuit (not shown) of the personal computer which generates the terminal image signal.

Prior to the explanation of the operation of each unit, the one-half line interval shift which causes a problem in converting the hivision TV image signal (interlaced scan) to the terminal image signal (non-interlaced scan) is explained in further detail with reference to FIGS. 2 and 3. In superimposing the hivision TV image signal and the terminal image, the interlaced scan may be used, but flicker may be created in a highly fine display system if the terminal image signal is interlaced-scanned. Accordingly, it is advantageous to use the non-interlaced scan in superimposing those signals.

FIG. 2 shows waveforms of horizontal and vertical synchronization signals of the hivision TV signal, in which f.sub.Vh denotes the vertical synchronization signal and f.sub.Hh denotes the horizontal synchronization signal. In the hivision TV, the synchronization signals have a relation of T.sub.V= 562.5 T.sub.h (where T.sub.V is a vertical period and T.sub.h is a horizontal period). For the non-interlaced-scan it is necessary that n (number of scan lines) in T.sub.V= nT.sub.h is an integer. It is selected to an integer N (562, 563,...) close to 562.5. A new horizontal period T.sub.hn is T.sub.V /N. Thus, the horizontal period is different for T.sub.h. If the processing for the aspect ratio and one-half line shift is carried out under this condition, the field memory or frame memory as disclosed in JP-A-60-42993 is required. In the present invention, as shown by f.sub.Hn in FIG. 2, a horizontal synchronization signal is generated at the next vertical synchronization signal (time t.sub.1) without changing the length of T.sub.h, and the waveform maintains the period T.sub.h until the next vertical synchronization signal appears.

A raster under this condition is shown in FIG. 3. FIG. 3(a) shows a raster of the hivision TV signal, and FIG. 3(b) shows a raster structure is the above system. A scan line 563 in a second field overlaps with a scan line 1 in a first field. (It may overlap with a scan line 2). The information on the scan line 563 is image information which is shifted by one-half scan line interval from true information. The shifted information are displayed on the same scan position for each field. As a result, a high intensity outline area flicks or becomes dim. Such a problem is resolved in the present invention by a simple construction.

Configuration and operation of the first embodiment are explained in detail. FIG. 4 shows a circuit diagram of the D.sub.ck1 generator 2. It shows a known crystal oscillation type PLL circuit. A VXCO 23 frequency-divides by a counter 25 an output of a crystal oscillator 27 whose oscillation frequency may be slightly variable by a control input V.sub.c, and compares the divided frequency with f.sub.Hh by a phase comparator 21. The output of the phase comparator is supplied to a LPF 22 which produces V.sub.c to control the VXCO 23 so that f.sub.Hh is divided by the count of the counter to D.sub.ck1. If the count of the counter is 2048and one horizontal period has a frequency of 33.75 kHz, the crystal oscillator 27 oscillates at 69.12 MHz. (According to the sampling theory, it is necessary that the video signal band 20 MHz of the hivision TV is no larger than D.sub.ck1.) The D.sub.ck1 thus generated is used as a sampling clock of the ADC 15 and an input signal to the write address generator 5, which may be a 2048-bit counter which is reset by the horizontal synchronization signal f.sub.Hh. Each bit output of the counter is used as write addresses of the line memories 7-1, 7-2 and 7-3. The write address counter may be shared by the counter 25 for D.sub.ck1. The D.sub.ck2 generator 3 may be constructed in the same manner as the D.sub.ck1 generator but the division frequency of the counter and the frequency of the crystal oscillator are different. The division frequency k of the counter is determined by an integer which is close to 2048.times.4/5. In the present embodiment, k=1632(preferably 8's multiple) and D.sub.ck2 =55.08 MHz.

FIG. 5 shows a configuration of the read address generator 6. A counter 61 is reset by the horizontal synchronization signal f.sub.Hh and counts up to 1632. (It may be shared by the counter of the D.sub.ck2 generator). A sum of the output of the counter 61 and an offset digital value i (i.ltoreq.2048-1632) is the read address. This means that 1632 words starting from any i-th word are selected from one line of hivision signal to adapt the aspect ratio.

The operation of the R/W controller 4 is explained with reference to FIGS. 6 and 7. FIG. 6 is a time chart for showing operation waveforms. FIG. 7 shows a circuit configuration. DFF 401, 402 and 403 and AND 404 of FIG. 7 receive W/R signals of line memories 7-1 to 7-2 shown in FIG. 6. The W/R signals to the line memories are used as switching signals for multiplexers 405, 406 and 407 so that read addresses and write addresses are selectively supplied to the line memories. The W/R signals of the line memories are used to control memory write signals and pixel input signals so that they are supplied to the line memories selected to write mode. Only one line of memory is selected to the write mode at one time.

FIG. 8 shows a specific circuit of SW5 and SW6. The SW5 and SW6 selectively supply the outputs of the line memories 7-1 to 7-3 to the DAC 10 and the DAC 11. A relation between the outputs of the line memories supplied to the DAC 10 and the DAC 11 and the line memories in the write mode is shown in Table 1.

The outputs of the DAC 10 and the DAC 11 are added by the adder 12 and reduced to one half by the level adjuster 13. Thus, the output signal of the level adjuster 13 is a linear interpolation signal of adjacent scan line signals in the field. That is, it is a scan line signal at a mid-point of two adjacent scan lines. This is because of high correlation to the signals between the adjacent scan lines.

FIG. 9 shows raster structures for illustrating signal processing in the present embodiment.

In the first field, the output of the DAC 10 is outputted as it is by the SW7, and in the second field, the output of the level adjuster 13 is selected so that the signals of the scan line positions of the first and second field in the interlaced scan are aligned. SW7 is switched by the signal of the O/E 1.

The hivision TV signal processed as described above and the terminal video signal V.sub.ic2 are combined by the video adder 14 so that they are superimposed.

The synchronization signal generator 16 produces a non-interlaced synchronization signal f.sub.Hn shown in FIG. 2 from the interlaced horizontal synchronization signal. A specific circuit of a main portion thereof is shown in FIG. 10. A phase detector 161, a low-pass filter LPF 162, a VCO 163 and a DEF 165 form a double PLL circuit. A mono-multivibrator 164 determines a pulse width of the horizontal synchronization signal. Outputs of AND circuits 166 and 168 provide synchronization signals having a period T.sub.h shifted by T.sub.h/2 (where T.sub.h is a horizontal period of the hivision TV signal). They are switched by the signal O/E for each field to produce a new horizontal synchronization signal f.sub.Hn. The new horizontal synchronization signal and the hivision synchronization signal are used as the synchronization signal Sync to externally synchronize the video circuit such as terminal.

A second embodiment of the present invention is explained with reference to FIGS. 11 and 12. As shown in FIG. 11, the second embodiment is characterized by that the position of the scan line after processing is at any position between scan lines. In FIG. 11, it is displaced by distance 3 from the upper one of the adjacent scan lines of the first field and by distance 1 from the lower one. The signal processing circuit excluding DAC 10, DAC 11 to SW7 is similar in configuration to that of FIG. 1.

As shown in FIG. 11(b), a new signal of the first field is a combined signal with inversely weighted distance by signal weights, and a new signal of the second field is also a combined signal with inversely weighted distances from the adjacent scan lines of the field. Those signals are selected by SW7 as shown in FIG. 12 at the field period to produce a new video signal. Assuming that the position of the first new video signal is between 563 and 2 in FIG. 11, and a distance from the scan line 1 is l and a distance from the scan line 2 is m, weight A, B, C and D of weighters 171, 172, 173 and 174 of FIG. 12 are m/(l+m), l(l+m), (l+m)/2(l+m) and (l-m)/2(l+m), respectively. Assuming that the first new video signal is between 1 and 563 in FIG. 11, the above relation is met if it is spaced by lfrom the scan line 562 and m from the scan line 563.

In the first and second embodiments, a full line of hivision signal is written into the line memory, and only a necessary portion thereof is read from the line memory. If the read timing can be properly controlled, the line memory may have only a memory capacitance necessary for readout.

Such reduction of memory capacity of the line memory is explained with reference to FIGS. 13 and 14. FIG. 13 shows a relation between memory allocation in one horizontal period of the hivision TV signal and an effective display period. A period m shown in FIG. 13 is an effective display period of the hivision TV signals (m.congruent.0.75 T.sub.h). On the other hand, the conversion of the image aspect ratio from 5/3 to 4/3 means to expand a period k in the period m and accomodate it in the original period m (see FIGS. 13 line E and F). Thus, a minimum line memory capacity necessary for such processing is k=m.multidot.4/5.congruent.0.75T.sub.h.multidot. 4/5.congruent.0.6 T.sub.h. Accordingly, the line memory need store only approximately 60% of 2048 words shown in FIG. 13. (See line D).

FIG. 14 shows a configuration therefor. As seen from the above, the effective memory capacity is of k words. A start timing of the effective image is determined by a j-counter 171 which counts j outputs of D.sub.ck1. It is reset by the output of the j-counter 171, and then counts k outputs of D.sub.ck1 by an output of a k-counter 172. Accordingly, data is written into the line memory from the start of the output of the j-counter 171 to the end of k counts by the k-counter 172.

On the other hand, the start timing of display is determined by an output of an l-counter 173 which counts l outputs of D.sub.ck2. It is reset by an output of the l-counter 173 and then counts k outputs of D.sub.ck2 by a k-counter 174 to produce a read address. Since the counters 172 and 174 are k-counters, the same minimum information, that is, k words, are processed in the write mode and the read mode, although timings from the horizontal synchronization signal f.sub.Hh and the write and read periods are different.

The write addresses and the read addresses are switched by the multiplexors 405, 406 and 407 of FIG. 7 to produce address signals of the line memories 7-1, 7-2 and 7-3. (In FIG. 14, only SW1 405 is shown.) In this manner, the capacity of the line memory is reduced by approximately 40%.

In the first and second embodiments, the signal processing such as weighted addition of the scan line signals is effected with the DA-converted signals, although they may be processed in the form of digital signal and thereafter converted to analog signals.

In accodcance with the present invention, the one-half scan line position shift between the fields of the hivision signal is compensated by a small memory capacity, that is, three line memories per pixel bit so that non-interlaced scan is attained without loosing image quality. At the same time, the hivision signal image having an aspect ratio of 3/4is attained so that it may be superimposed to the terminal signal with high quality.