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LM1881 Arkusz danych(PDF) 7 Page - National Semiconductor (TI) |
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LM1881 Arkusz danych(HTML) 7 Page - National Semiconductor (TI) |
7 / 12 page Application Notes (Continued) 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 appli- cations or in extracting test signals that occur in alternate fields. For a composite 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 — i.e., at the bottom of the picture. This is called the “odd field” or “even field”. The “even field” or “field 2” has a complete 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. Fig- ure 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 composite sync waveform (Figure 2). A capacitor is charged during the period between sync pulses and dis- charged when the sync pulse is present. The period between normal horizontal sync pulses is enough to allow the capaci- tor voltage to reach a threshold level of a comparator that clears a flip-flop 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 flip-flop 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 comparing 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 composite video signal, the chroma burst is located on the backporch of the horizontal blanking period. This period, approximately 4.8 µs long, is also the black level reference for the subsequent video scan line. The LM1881 generates a pulse at Pin 5 that can be used either to retrieve the chroma burst from the composite video signal (thus providing a subcarrier synchronizing signal) or as a clamp for the DC restoration of the video waveform. This output is obtained simply by charging an internal capacitor starting on the trailing edge of the horizontal sync pulses. Simultaneously the output of Pin 5 is pulled low and held until the capacitor charge circuit times out — 4 µs later. A shorter output burst gate pulse can be derived by differentiating the burst output 00915004 *Components Optional, See Text FIGURE 2. www.national.com 7 |
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