International projectionist (Jan-Dec 1948)

between C-C (Section B) shown in the detail view in Section C. Actually only about half the total blanking time is used, and the front or leading edge of the sync signal is placed as close as possible to the beginning of the blanking pulse. The small allowance which is made takes care of some variation in timing and insures that the sync pulse will not run into the video portion of the signal and thus upset the line timing. Vertical Blanking, Synchronization The vertical blanking interval follows the last line of each field and consists of the following four parts, considered separately : (1) Six equalizing pulses one-half line apart precede the sync pulse and accomplish (a) the maintenance of horizontal or line synchronization, and (6) the "equalization" of the interval preceding the vertical sync pulse so that conditions preceding the vertical sync pulse are identical for alternate fields. The need for these equalizing pulses arises because of the interlacing of alternate fields. Figure 19 shows that the lines in the second field (B) are interlaced with those of the preceding field (A). If the equalizing pulses were eliminated and the vertical sync pulses were inserted in (A) at the end of the last line, then the vertical sync pulse would have to appear in the next field (B) at the middle of the line: the reason for this is that 1/60 second later the beam is in the middle of the line because of the interlacing. Thus, without the equalizing pulses, the conditions preceding the vertical sync pulse would be different for each of the two fields. This would tend to produce a different type of vertical sync pulse for alternate fields, upset the synchronization and give rise to the distortion known as "pairing of the interlace." In a paired interlace the even-scanned lines do not lie midway between the oddscanned lines, because of the difference in timing on alternate fields. In connection with the maintenance of line synchronization during the vertical blanking interval, note that the leading edges of the equalizing pulses function to maintain synchronization. Not all the pulses are used for each field, however. Thus, note that because of the interlacing the first, third, and fifth equalizing pulses are used on the first field (A), and the second, fourth, and sixth pulses are used on the succeeding field (B). This explains why six pulses are used, each spaced one-half line apart, rather than three pulses spaced one line apart. It would be possible, of course, to use three different pulses in each field, but if this were done the signal preceding the vertical sync pulse would be different for succeeding fields and there would be a resultant absence of equalization. Vertical Sync Pulse (2) The vertical sync pulse (or field synchronizing pulse) follows directly after the equalizing pulse interval and consists of six broad pulses in which the edges are serrated or cut at one-half line intervals. The function of the vertical sync pulse is to provide the control signal which tells the vertical oscillator that it is time to begin the retrace and thus to return the beam to the top for the beginning of the next field. The pulses in the vertical sync-pulse interval are considerably broader than SYSTEM SHOWN HERE REFLECTS THE AMERICAN STANDARD OF 525 LINES AND 30 FRAMES-60 FIELDS PER SECOND INTERLACED FIG. 19. The standard television signal. Part (A) shows the signal at the end of any one field; part (B) shows the signal at the end of the next field, 1/60 second later. The difference between the two fields is caused by interlacing. bottom or picture: L yL lilUi £aUAU2ING '" PUL5C ~" INTERVAL VCHTiOL -SYNC — PULSE INTERVAL u EQUALIZING ~ PULSC »*TC*W*L I >» — top or PICTURE TjnrinlijjuulLiL^^ ! , n „ \;« i 'i / >i ' i 'i VERTICAL U.ANK.NC Wl?^;^/" W°TeM' h t.+o«e I v . «l l 1 1 'i > > TIMC » 'JJJULJl_Jl_fLJ A t E). SHOW BLANKING t, SYNCHRONIZING SIGNALS IN REGIONS OF SUCCESSIVE VERTICAL BLANKING PULSES. (HORIZONTAL DIUlNSONS not to scale. all DIMENSIONS ARC rBOM black level unless otherwise specifically INDICATED! UMOMUU CARRIER VOLTAC* FICruRC * ILAMUHC -BEACH LCVtL 0) W HOR BLANKING REPETITION RATE 5,750 PER SEC DETAIL BETWEEN C-C DIMENSIONS TO SCALE 1. Diagram C shows enlarged detail view of signal in view B between lines C-C. 2. Diagram D shows enlarged detail view of the sync signal in view A between lines D-D. M 3. H-time from start of one line to start of next line = 1/15,750 second. M.OMHMAX.— jH| EOUAUZINC -I nn.sc C\ (-M.04N d) \ZA VERTICAL SYNC PULSES DETAIL BETWEEN D-D SUCNSONS TO SCALE OOSHUUH —(•(ATM 1/60 4. V-time from start of one field to start of next field second = 262.5H. 5. Leading and trailing edges of both horizontal and vertical blanking pulses have slopes (not indicated in A & B) which should be kept as steep as possible. 6. Receiver vertical retrace shall be complete at end of .07 V. 14 INTERNATIONAL PROJECTIONIST • October 1948