【正文】 ption data for the hearing impaired. The remaining lines are used in a number of ways. Lines 17 and 18 are frequently used during studio processing to add and delete vertical interval test signals(VITS) while lines 14 through 18 and line 20 can be used forVideotex/Teletext data. Several institutions are proposing to transmit financial data on line 17 and cable systems use the available lines in the vertical interval to send decoding data for descrambler terminals. Since the vertical output pulse from the LM1881 coincides with the leading edge of the first vertical serration, sixteen positive or negative transitions later will be the start of line 14 in either field. At this point simple counters can be used to select the desired line(s) for insertion or deletion of data. VIDEO LINE SELECTOR The circuit in Figure 3 puts out a singe video line according to the binary coded information applied to line select bits b0– b7. A line is selected by adding two to the desired line number, converting to a binary equivalent and applying the result to the line select inputs. The falling edge of the LM1881’s vertical pulse is used to load the appropriate number into the counters (MM74C193N) and to set a start count latch using two NAND gates. Composite sync transitions are counted using the borrow out of the desired number of counters. The final borrow out pulse is used to turn on the analog switch (CD4066BC) during the desired line. The falling edge of this signal also resets the start count latch,thereby terminating the counting. The circuit, as shown, will provide a single line output for each field in an interlaced video system (television) or a single line output in each frame for a noninterlaced video system (puter monitor). When a particular line in only one field of an interlaced video signal is desired, the odd/even field index output must be used instead of the vertical output pulse (invert the field index output to select the odd field). A single counter is needed for selecting lines 3 to 14。s. ODD/EVEN FIELD PULSE An unusual feature of LM1881 is an output level from Pin 7 that identifies the video field present at the input to the LM1881. This can be useful in frame memory storage applications or in extracting test signals that occur in alternate fields. For a posite video signal that is interlaced, one of the two fields that make up each video frame or picture must have a half horizontal scan line period at the end of the vertical scan— ., at the bottom of the picture. This is called the ―odd field‖ or ―even field‖. The ―even field‖ or ―field 2‖ has a plete horizontal scan line at the end of the field. An odd field starts on the leading edge of the first equalizing pulse, whereas the even field starts on the leading edge of the second equalizing pulse of the vertical retrace interval. Figure 1(a) shows the end of the even field and the start of the odd field. To detect the odd/even fields the LM1881 again integrates the posite sync waveform (Figure 2). A capacitor is charged during the period between sync pulses and discharged when the sync pulse is present. The period between normal horizontal sync pulses is enough to allow the capacitor voltage to reach a threshold level of a parator that clears a flipflop which is also being clocked by the sync waveform. When the vertical interval is reached, the shorter integration time between equalizing pulses prevents this threshold from being reached and the Q output of the flipflop is toggled with each equalizing pulse. Since the half line period at the end of the odd field will have the same effect as an equalizing pulse period, the Q output will have a different polarity on successive fields. Thus by paring the Q polarity with the vertical output pulse, an odd/even field index is generated. Pin 7 remains low during the even field and high during the odd field. BURST/BACKPORCH OUTPUT PULSE In a posite video signal, the chroma burst is located on the backporch of the horizontal blanking period. This period, approximately 181。s in this graph is linear, meaning that a value as large as M? can be used for RSET (twice the value as the maximum at 30 181。s. The vertical default sync delay time must be longer than the vertical sync period of 64 181。s serration pulse spacing. A mon question is how can one calculate the required RSET with a video timing standard that has no serration pulses during the vertical blanking. If the default vertical sync is to be used this is a very easy task. Use the ―Vertical Default Sync Delay Time vsRSET‖ graph to select the necessary RSET to give the desired delay time for the vertical sync output signal. If a second pulse is undesirable, then check the ―Vertical Pulse Width vs RSET‖ graph to make sure the vertical output pulse will extend beyond the end of the input vertical sync period. In most systems the end of the vertical sync period may be very accurate. In this case the preferred design may be to start the vertical sync pulse at the end of the vertical sync period, similar to starting the vertical sync pulse after the first serration pulse. A VGA standard is to be used as an example to show how this is done. In this standard a horizontal line is 32 181。s, just long enough to prevent a double vertical sync pulse at the vertical sync output of the LM1881. The LM1881