Method of correcting errors in bytes of teletext signals
BACKGROUND OF THE INVENTION 
     The present invention pertains to an arrangement and method for correcting errors in teletext signals. 
     More specifically, the present invention relates to a method of correcting errors in the individual bytes of teletext signals which are received successively and periodically page by page. After the teletext signals are decoded, they are stored page by page into first segments of a page memory of a television receiver. Each segment is designed to hold one page. A teletext page to be displayed on the television screen is selected from the page memory in response to a page selection signal. Erroneous characters are detected by parity checks of the bytes and indicated as blank characters which are replaced by correct characters contained in the same page at the next instant reception. 
     Such a correcting method is described in the journal &#34;Funkschau&#34;, 1977, pages 820 to 824 and 882 to 887, especially page 885, right column at the top. As shown in FIG. 18 on page 885, it can be seen that the bytes are fed to the page memory without the parity bit but are fed to the parity checker with the parity bit. The output signal of the parity checker is applied through an AND gate to the write enable input of the page memory, i.e., only if a correct parity is detected will the bytes be stored into the page memory. In case of incorrect parity, the respective location will remain disabled, so that a blank character will appear on the television screen. 
     By means of the parity check, only a single error in each byte can be detected, while double errors go unnoticed. 
     SUMMARY OF THE INVENTION 
     One object of the invention is to provide a correcting method of the above kind which also allows at least double errors to be detected and, in a preferred embodiment, to be corrected. 
     A principal advantage of the invention thus follows directly from the way in which the object is attained, since the known correcting method is improved considerably. 
     With this correcting method, errors in bytes are corrected successively. To this end, the signal resulting from the parity check is stored into a page memory, along with the character bits of each byte. The parity check signal replaces the parity bit of each byte. Two successively received identical pages are stored together with their parity check bits into first and second segments of the page memory. The two pages are compared for equality of the character bits and parity check bits. The result of the comparison is evaluated as follows. In case of equality and correct parity, the byte stored in the first segment will remain unchanged; in case of inequality of the two bytes and correct parity of one of the two bytes, the byte having correct parity will be stored into or remain stored in the first segment; in case of inequality and incorrect parity of the two bytes, no change will be made in the first segment; in case inequality and correct parity of the two bytes, the byte in the first segment will be replaced by a blank byte. The next received identical page but one is stored into the second segment by overwriting, and the correction is repeated with this byte. By this method, single errors are detected and corrected and double errors are detected and replaced by a blank character. In a preferred embodiment with a third segment of the page memory, storage of the third received identical page, and corresponding comparisons of the bytes in the three segments, the method is extended so that double errors are corrected as well. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described in more detail with reference to the single FIGURE of the accompanying drawing, which is a highly schematic block diagram of an embodiment of an arrangement for carrying out the correcting method. 
    
    
     DETAILED DESCRIPTION 
     The FIGURE shows the teletext decoder td, which can be thought of as uniting all those stages of FIG. 18 on page 885 of the above-mentioned journal which process the teletext signals so that they can be written into the page memory sp. The output of the teletext decoder td is, therefore, connected to the data inputs of the individual segments 11, 12, 13 of the page memory sp, such that only the first seven bits (reference numeral 7), i.e., not the parity bit, are connected to the data inputs. On the other hand, the output of the teletext decoder td, including the parity bit (reference numeral 8), is connected to the input of the parity checker p, whose output line carrying the signal resulting from the parity check is connected as the eighth data input line to the data inputs of the segments 11, 12, 13 of the page memory sp. 
     The first segment 11 of the page memory sp holds a given received page which contains detected erroneous characters. In the second segment 12 of the page memory sp, the next received page which is identical with the page stored in the first segment is stored; here, too, the signal resulting from the parity check is stored as the parity bit of the respective byte. 
     The two segments 11, 12 are connected by an eight-wire data bus to the memory control ss, which, in turn, exchanges data with the comparator k. Thus, corresponding bytes of the two identical pages stored in the segments 11, 12 are checked for equality of the character bits by means of the comparator k and evaluated according to the two parity bits as follows. In case of equality and correct parity of the two bytes, the byte stored in the first segment 11 will remain unchanged. In case of inequality of the two bytes and correct parity of one of the two bytes, the byte having correct parity will be stored into the first segment 11 if no byte with correct parity is present there yet, or the byte with correct parity stored in the first segment 11 will remain stored there. In case of inequality and incorrect parity of the two bytes, no change will be made in the first segment 11, while in case of inequality and correct parity of the two bytes, the byte in the first segment 11 will be replaced by a blank byte. 
     This correction process continuous without interruption since the next received page but one which is identical with the page stored in the first segment 11 is stored by overwriting the bytes previously stored in the second segment 12. Thus, the character errors resulting in the display of a blank character are continuously reduced by the continuous correction process. 
     Besides the subcircuits mentioned above, the accompanying drawing shows the sequence controller ag, which, controlled by the output pulses of the clock generator tg, supplies all other subcircuits with correct timing signals. In the teletext standard as described in the above-mentioned journal, the frequency of the clock generator tg is 6.9375 MHz. 
     The third segment 13 of the page memory sp and its control make it possible to develop and improve the error correction and, thus, the correcting method of the invention. In the third segment 13, the next received page but one which is identical with the page stored in the first segment 11 is stored in the same manner as the page stored in the first segment 11. Corresponding bytes of the three like pages are checked for equality of the character bits and evaluated in accordance with the three parity bits as follows. In case of equality and correct parity of the first two bytes, the byte stored in the first segment 11 will remain unchanged. In case of inequality of the first two bytes and correct parity of one of these two bytes, the byte having correct parity will be stored into the first segment 11 or, if the byte with correct parity is already present there, remain unchanged. In case of inequality and incorrect parity of the first two bytes but correct parity of the third byte, the latter will be stored into the first segment 11. In case of inequality and correct parity of the first two bytes and equality to the third byte, the latter will be stored into the first segment 11. The two following received identical pages are stored into the second and third segments 12 and 13, respectively, by overwriting the bytes previously stored there, and the correction process is repeated with these bytes. 
     By the extended correcting method using the third segment 13, not only single bit errors in each byte but also double errors are detected and corrected. 
     The correcting method according to the invention becomes especially efficient if, in the case of a continuous text extending over several pages, subsequent pages already transmitted during the display of, e.g., page 1 which are very likely to be displayed, too, are constructed in free segments of the page memory and corrected in accordance with the invention. When called up for display on the screen, such following pages are then immediately error-free even under disturbed receiving conditions.