【正文】 LM1881, LM1881X Video Sync Separator General Description The LM1881 Video sync separator extracts timing information including posite 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 to 2V pp. The integrated circuit is also capable of providing sync separation for nonstandard,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 if the rising edge mentioned above does not occur within the externally set delay period, such as might be the case for a nonstandard video signal. Features ? AC coupled posite input signal ? 10 k? input resistance ? 10 mA power supply drain current ? Composite sync and vertical outputs ? Odd/even field output ? Burst gate/back porch output ? Horizontal scan rates to 150 kHz ? Edge triggered vertical output ? Default triggered vertical output for nonstandard video signal (video gameshome puters) ? 40?C to +85?C operation (LM1881X) Application Notes The LM1881 is designed to strip the synchronization signals from posite video sources that are in, or similar to, the . format. Input signals with positive polarity video (increasing signal voltage signifies increasing scene brightness) from (pp) to 2V (pp) can be acmodated. The LM1881 operates from a single supply voltage between 5V DC and 12V DC. The only required external ponents besides a power supply decoupling capacitor at pin 8 and a set current decoupling capacitor at pin 6, are the posite input coupling capacitor at pin 2 and one resistor at pin 6 that sets internal current levels. The resistor on pin 6 (. Rset) allows the LM1881 to be adjusted for source signals with line scan frequencies differing from kHz. Four major sync signals are available from the I/C。 posite sync including both horizontal and vertical scan timing information。 a vertical sync pulse。 a burst gate or back porch clamp 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 genlock video camera/VTR signals with graphics sources,provide identification of video fields for memory storage,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(ae) which shows a portion of the posite video signal from the end of one field through the beginning of the next field. COMPOSITE SYNC OUTPUT The posite sync output, Figure 1(b), is simply a reproduction of the signal waveform below the posite video black level, with the video pletely removed. This is obtained by clamping the video signal sync tips to DC at Pin 2 and using a parator 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 (pp), the 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 (pp) input the clipping level occurs at 11% of the sync pulse amplitude. The charging current for the input coupling capacitor is mA, Normally the signal source for the LM1881 is assumed to be clean and relatively noisefree, but some sources may have excessive video peaking, causing high frequency video and chroma ponents to extend below the black level reference. Some video discs keep 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 posite sync signal can be generated from these sources by filtering the input signal. When the source impedance is 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 attenuated by almost 18 dB, effectively taking it below the parator threshold. Filtering will also help if the source is contaminated with thermal noise. The output waveforms will bee 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 contribute to the sync delay produced by any additional signal processing. Since the original video may also undergo processing, the need for time delay correction will depend on the total system, not just the sync stripper. VERTICAL SYNC OUTPUT A vertical sync output is derived by internally integrating the posite sync waveform (Figure 2). To understand the generation of the vertical sync pulse, refer to the lower left hand sectio