欢迎访问ic37.com |
会员登录 免费注册
发布采购

LM1881N 参数 Datasheet PDF下载

LM1881N图片预览
型号: LM1881N
PDF下载: 下载PDF文件 查看货源
内容描述: LM1881视频同步分离 [LM1881 Video Sync Separator]
分类和应用:
文件页数/大小: 12 页 / 302 K
品牌: NSC [ National Semiconductor ]
 浏览型号LM1881N的Datasheet PDF文件第3页浏览型号LM1881N的Datasheet PDF文件第4页浏览型号LM1881N的Datasheet PDF文件第5页浏览型号LM1881N的Datasheet PDF文件第6页浏览型号LM1881N的Datasheet PDF文件第8页浏览型号LM1881N的Datasheet PDF文件第9页浏览型号LM1881N的Datasheet PDF文件第10页浏览型号LM1881N的Datasheet PDF文件第11页  
Application Notes (Continued)  
00915004  
*Components Optional, See Text  
FIGURE 2.  
ODD/EVEN FIELD PULSE  
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.  
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 scani.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.  
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  
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  
7
www.national.com