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1、LM1881, LM1881-X Video Sync SeparatorGeneral Description The LM1881 Video sync separator extracts timing information including composite and vertical sync, burst/back porch timing, and odd/even field information from standard negative going sync NTSC, PAL* and SECAM video signals with amplitude from
2、 0.5V to 2V p-p. The integrated circuit is also capable of providing sync separation for non-standard,faster horizontal rate video signals. The vertical output is produced on the rising edge of the first serration in the vertical sync period. A default vertical output is produced after a time delay
3、if the rising edge mentioned above does not occur within the externally set delay period, such as might be the case for a non-standard video signal.Featuresl AC coupled composite input signall 10 k input resistancel 10 mA power supply drain currentl Composite sync and vertical outputsl Odd/even fiel
4、d outputl Burst gate/back porch outputl Horizontal scan rates to 150 kHzl Edge triggered vertical outputl Default triggered vertical output for non-standard video signal (video games-home computers) l -40C to +85C operation (LM1881-X)Application NotesThe LM1881 is designed to strip the synchronizati
5、on signals from composite video sources that are in, or similar to, the N.T.S.C. format. Input signals with positive polarity video (increasing signal voltage signifies increasing scene brightness) from 0.5V (p-p) to 2V (p-p) can be accommodated. The LM1881 operates from a single supply voltage betw
6、een 5V DC and 12V DC. The only required external components besides a power supply decoupling capacitor at pin 8 and a set current decoupling capacitor at pin 6, are the composite input coupling capacitor at pin 2 and one resistor at pin 6 that sets internal current levels. The resistor on pin 6 (i.
7、e. Rset) allows the LM1881 to be adjusted for source signals with line scan frequencies differing from 15.734 kHz. Four major sync signals are available from the I/C; composite sync including both horizontal and vertical scan timing information; a vertical sync pulse; a burst gate or back porch clam
8、p pulse; and an odd/even output. The odd/even output level identifies which video field of an interlaced video source is present at the input. The outputs from the LM1881 can be used to gen-lock video camera/VTR signals with graphics sources,provide identification of video fields for memory storage,
9、recover suppressed or contaminated sync signals, and provide timing references for the extraction of coded or uncoded data on specific video scan lines. To better understand the LM1881 timing information and the type of signals that are used, refer to Figure 1(a-e) which shows a portion of the compo
10、site video signal from the end of one field through the beginning of the next field.COMPOSITE SYNC OUTPUT The composite sync output, Figure 1(b), is simply a reproduction of the signal waveform below the composite video black level, with the video completely removed. This is obtained by clamping the
11、 video signal sync tips to 1.5V DC at Pin 2 and using a comparator threshold set just above this voltage to strip the sync signal, which is then buffered out to Pin 1. The threshold separation from the clamped sync tip is nominally 70 mV which means that for the minimum input level of 0.5V (p-p), th
12、e clipping level is close to the halfway point on the sync pulse amplitude (shown by the dashed line on Figure 1(a). This threshold separation is independent of the signal amplitude, therefore, for a 2V (p-p) input the clipping level occurs at 11% of the sync pulse amplitude. The charging current fo
13、r the input coupling capacitor is 0.8 mA, Normally the signal source for the LM1881 is assumed to be clean and relatively noise-free, but some sources may have excessive video peaking, causing high frequency video and chroma components to extend below the black level reference. Some video discs keep
14、 the chroma burst pulse present throughout the vertical blanking period so that the burst actually appears on the sync tips for three line periods instead of at black level. A clean composite sync signal can be generated from these sources by filtering the input signal. When the source impedance is
15、low, typically 75, a 620 resistor in series with the source and a 510 pF capacitor to ground will form a low pass filter with a corner frequency of 500 kHz. This bandwidth is more than sufficient to pass the sync pulse portion of the waveform; however, any subcarrier content in the signal will be at
16、tenuated by almost 18 dB,effectively taking it below the comparator threshold. Filtering will also help if the source is contaminated with thermal noise. The output waveforms will become delayed from between 40 ns to as much as 200 ns due to this filter. This much delay will not usually be significant but it does con
