A Frame Line Memory Control Circuit and Method for Controlling Storage of Video Data of a Video Signal

The present invention relates generally to controlling a memory in a video printer and in particular, to a method and circuitry for controlling the memory by adopting a frame line reading and writing system upon supplying printing data to a thermal print head of the video printer.

One of known arts for controlling a line memory providing a thermal print head (TPH) with printing data in a video printer is a field line system, a schematic block diagram of which system is disclosed in FIG. 2. Referring to this system, during a writing operation of printing data into a line memory 4, a controller 14 generates a given write address by controlling a write address counter 8, and a switch 10 transfers the write address from the write address counter to the line memory 14 under control of the controller 14. Hence, once analog video data from a video processor 1 are converted to digital data in an analog-to- digital converter 2 under control of the controller 14, the digital data are then supplied to the corresponding write address of the line memory 4 and stored therein as printing data for use in a subsequent printing operation of the video printer.

When reading the stored printing data out of the line memory 4 upon a printing operation, the switch 10 is connected to a read address counter 9, which under control of the controller 14 generates a set of read address to therefrom retrieve a corresponding one of the printing data previously stored in designated memory 4 allocations within the line memory, thereby transferring the read address to the line memory 4. Thus, a thermal print head (TPH) data processor 5 operably connected with the controller 14 and the line memory 4 receives the printing data read out of the line memory and, under control of the controller 14, provides the received data to a thermal print head (TPH) driver (not shown in the drawing) after processing a kind of conversion of printing data for better application in the TPH driver.

FIG. 1 illustrates the correlation between scanning of video data on a video screen and sampling of the video data to be stored into the line memory, by way of example, in view of the state of the art. Referring to this correlation, the video data on a single video screen are sampled and stored line by line into the line memory, and the stored video data are printed line by line, as follows: ##EQU1## wherein, n and m are respectively integer designations giving the maximum location of the video data scanning and sampling, and L.sub.1 through L.sub.m each designate a corresponding memory line in the line memory.

FIG. 3 is a schematic diagram for illustrating the correlation of consecutive read and write operations onto the line memory in the field line system, as shown in FIG. 2, provided with a repetition of one odd field and one even field of an input video signal. In this context of such a field line printing system every field of the input video signal is sequentially taken into the line memory by an interlacing scanning and sampling method. A common phenomenon of data collision occurs between odd fields such as a third field, a fifth field, a seventh field, etc., since the line memory is provided with only one input/output port in the usual cases. This data collision inevitably leads to poor picture quality in printing. Furthermore, while the line memory is usually adapted to store only video data corresponding to a single line to print out, it often fails to provide economical memory usage since nearly all commercially supplied memory devices do not fit such small sized line memory.

A U.S. Pat. No. 4,819,008 to Nagato et al. discloses a thermal print head driver circuit for a thermal printer permitting multiple graduation printing.

It is therefore an object of the invention to provide a circuit and method capable of controlling the write and read operations of video data onto a line memory without occurrence of any data collision between odd fields, the line memory providing a thermal print head of a video printer with printing data, in a frame line printing system.

To achieve the above object and other advantages of the present invention, a preferred embodiment of the present invention having a line memory storing video data, includes:

a controller generating a multiplicity of control signals;

an analog/digital conversion means for converting an analog video signal from a video processor to corresponding digital video data under control of a first control signal from the controller;

latch means for temporarily storing the video data from the conversion means during a printing operation;

memory means for writing said video data and reading out same upon request from the controller, the memory means comprising the line memory;

a first switching means connectable to the conversion means and the latch means, for selectively transferring the video data from either one of the conversion means and the latch means to the memory means, under control of the controller;

a write address counter means for providing a given write address to store the video data to the memory means under control of the controller;

a first logic gate means for producing a first gate control signal to enable the operation of the latch means and the write address counter means;

a read address counter means for providing a given read address to retrieve the video data out of the memory means under control of the controller;

a second logic gate means for producing a second gate control signal;

a second switching means connectable to the read address counter means and the write address counter means, for selectively providing either one of the read address and the write address to the memory means in dependency of the second gate control signal from the second logic gate means; and

a third switching means connectable to the write address counter means, for transferring a selected one of the write addresses from the write address counter means to the second switching means under control of the controller.

A more complete appreciation of the invention, and many of the attendant advantages thereof, will be readily enjoyed as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like members indicate the same or similar components, wherein:

FIG. 1 illustrates the correlation between scanning of video data on a video screen and sampling of the video data to be stored into the line memory, by way of example, in view of the state of the art;

FIG. 2 is a schematic block diagram of the one of known arts for controlling a line memory providing a thermal print head with printing data in a video printer, in a field line system;

FIG. 3 is a schematic diagram for illustrating the correlation of consecutive read and write operations onto the line memory in the field line system, as shown in FIG. 2;

FIG. 4 is a block diagram showing a preferred embodiment of a line memory control circuit according to the present invention;

FIGS. 5A to 5E, and 6A to 6E each are waveform diagrams showing every operation sequence in parts of the embodiment circuitry disclosed in FIG. 4; and

FIG. 7 is a schematic diagram similar to that of FIG. 3, for describing the correlation of consecutive read and write operations onto the line memory in the preferred embodiment circuit of the present invention as shown in FIG. 4.

Referring to FIG. 4, the preferred embodiment of a line memory control circuit of the invention, a line memory 4 stores video data to print line by line in a video printer, the line memory provided with a frame-by-frame reading and writing system upon storing said video data therein and upon providing printing data to a thermal print head of the video printer. A thermal print head (TPH) controller 14 generate a multiplicity of control signals. An analog/digital converter 2 converts an analog video signal from a video processor 1 to corresponding digital video data under control of a first control signal from the controller 14. A latch 6 temporarily stores the video data from the converter 2 during a printing operation. Line memory 4 writes the video data and reads out the same upon request from the controller 14, where the video data corresponds to each line printing out in the video printer. A first switching circuit 3 is connected to the converter 2 and the latch 6, for selectively transferring the video data from either one of the converter and the latch to the line memory 4, under control of the controller 14. A write address counter stage includes a first write address counter 8 providing a write address during non-printing sequence and a second write address counter 13 providing a write address during a printing sequence. The write address counter means provides a given write address to store the video data to the line memory control of the controller. An AND gate 7 receives a 2.5 MHz clock signal, a write enable signal (WE) and a second control signal C2 from the controller 14, for produces a first gate control signal to enable the operation of the latch 6 and the second write address counter 13. A read address counter 9 is connected to the controller for providing a given read address to retrieve the video data out of the line memory under control of the controller. An OR gate 12 receives the write enable signal and a control signal C6 for producing a second gate control signal. A second switch 11 is connected to the read address counter 9 and a third switch 15, for selectively providing either one of the read address and the write address to the line memory in dependency of the second gate control signal from the OR gate 12. The third switch 15 is connected to the first and second Write address counters 8 and 13, for transferring a selected one of the write addresses from one of the write address counters to the second switch 11 under control of the controller. A thermal print head data processor 5 is connected to the line memory, for performing a conversion process to enable a better matching of the video data in the thermal print head (not shown), under control of said controller.

FIGS. 5A to 5E, and 6A to 6E are waveform diagrams showing every operation sequence in parts of the embodiment circuitry disclosed in FIG. 4. In the diagrams, a waveform 5A shows a print enable signal, a waveform 5B shows a vertical synchronizing signal, a waveform 5C shows a control signal C7 from TPH controller 14, a waveform 5D shows another control signal C3 or C4, and a waveform 5E shows an odd/even decision signal which is at logic high level for an odd field and at logic low level for an even field. Further in the waveform diagrams, a waveform 6A shows a horizontal synchronizing signal, a waveform 6B shows an exemplar output in every gradation read out of the line memory 4 upon printing, a waveform 6C shows a timing pulse generated at every fourth horizontal synchronizing signal, a waveform 6D shows a write enable signal applied into the line memory upon printing and is enlarged to correspond to one pulse duration of the waveform 6C, and a waveform 6E shows an address timing signal for the line memory during the write enable duration.

FIG. 7 is a schematic diagram similar to that of FIG. 3, for describing the correlation of consecutive read and write operations onto the line memory and the printing operation to the input video signal according to the preferred embodiment as shown in FIG. 4. Each symbol T1, T2, T3, etc. designates one vertical period, namely each field as even or odd.

Upon a data write operation into the line memory 4, the first write address counter 8 is operated under control of the TPH controller 14 in order a series a write address, in particular, a series of odd numbered addresses to the line memory, when the controller directs the first switch 3 to select the upper contact. Hence, the analog video input signal from the video processor 1 is converted into a digital video signal in the A/D converter 2 under control of the controller, and through the first switch 3, is delivered to the line memory 4 so that the video signal is finally written into the corresponding address location of the line memory.

The video signals supplied to the line memory are taken in a repeating sequence of an odd field and thereafter an even field. Based upon the above-mentioned write address, video data of an odd field corresponding to a scanning line L1 of FIG. 1 are first sampled and stored into the line memory as printing data, which is carried out during a period T1 in FIG. 7. Then, during a period T2, the first address counter 8 provides the line memory with another write address, that is, a series of even addresses, while since the first switch maintains its state, video data of an even field corresponding to the scanning line L1 are secondly sampled and stored into the line memory.

During a period T3, both the printing and writing operations are performed simultaneously. In case the video data stored in the line memory are read out to be printed in the TPH, gradation of printing is first checked. For example, in the preferred embodiment circuit, a number of horizontal synchronizing pulses within a single vertical period is 262.5 and the pulse width thereof is 4.45 .mu.-sec (in 9 NTSC system), as seen by FIGS. 6A and 6B. Here, assuming that the gradation of printing is 64 steps and a frequency of an input clock signal to the write address counter is 2.5 MHz, a time needed for one gradation may be 0.4 .mu.-sec * 512 =204.8 .mu.-sec, wherein the number 512 correspond to a given even address in the line memory. Hence, the read address counter 9 provides the addresses 0-512 to the line memory at 64 times, thereby enabling the printing of the video data corresponding to the line L1. Concomitant with this printing operation, the video data of an odd field corresponding to a Line L2 are to be sampled and stored into the line memory. At this time however, occurrence of data collision should be prevented. To achieve this, a write address is to be provided to the line memory within a time duration of a horizontal synchronizing pulse signal just following the performance of one gradation, which time duration substantially means a "write enable period upon printing". The second write address counter 13 operates during a time duration of logic high of a "write enable signal upon printing", shown in FIG. 6D, designating the write enable period upon printing, thereby providing the corresponding addresses 515, 517, 519, . . ., 1021, 1023 to the line memory.

In the meanwhile, the latch 6 temporarily stores four sets of video data of six bits attendant in four time intervals of the horizontal synchronizing signal, which video data is transferred from the latch to the line memory in synchronism with a 2.5 MHz clock signal and logic high durations of the second control signal, i.e., a clock signal of FIG. 5C. This sequence of operation enables the storage of four sets of video data per gradation into the line memory, resulting in 9 total of 256 lines of video data. At this time, the first switch 3 is switched over to the other contact position, thereby enabling the transfer of digital video data from the latch to the line memory. The operation of both the second write address counter 13 and the latch 6 are dependent upon an output of the AND gate 7 receiving a 2.5 MHz clock signal, the write enable signal upon printing and the C2 control signal from the controller, wherein all three high inputs to the AND gate will enable the foregoing operation of the latch.

During the period T4 of FIG. 7, the video data of an even field corresponding to line L2 are sampled and then stored into the line memory. Thus the first write address counter 8 is enabled to provide the corresponding write addresses 516, 518, 520, . . ., 1022, 1024 to the line memory. At this time the first switch 3 is switched to transfer the video data from the A/D converter to the line memory 4 under control of the controller 14. This operation is substantially carried out during the time duration shown by a reference numeral 4 of the FIG. 5B waveform.

Then, during a period T5, the video data corresponding to the line L2 stored in the line memory are printed out and simultaneous to this printing operation, the video data of the odd field corresponding to a line L3 are further sampled and stored into the address locations 1, 3, 5, . . ., 511, 513 of the line memory.

Subsequent to the period T5 operation, the reading, writing and printing operations for remaining periods, which are similar to the foregoing ones, will be repeated until reaching the last period for line Lm.

As is understood from the foregoing description with reference to the attached drawings, the inventive circuit enables reading and writing of data without data collision upon printing, owing to effective use of the write enable periods in a line memory of a video printer. This greatly improves the video resolution of the video printer since it enables the frame line printing system in the video printer, as well as providing economical memory usage for the line memory.

While the foregoing provides a full and complete disclosure of the preferred embodiments of the present invention, various modifications, alternate constructions and equivalents thereof may be employed without departing from the true spirit and scope of the invention. Therefore, the above description and illustration should not be construed as limiting the scope of the invention, which is defined by the appended claims.