一种无锁相环的高精度数字视频彩色解码方案的研究  

A digital video decoder with high precision without using a phase locked loop

作　　者：肖波[1] 沈庆宏[1] 丁银亮 

机构地区：[1]南京大学电子科学与工程系,南京210093

出　　处：《南京大学学报（自然科学版）》2009年第1期11-17,共7页Journal of Nanjing University（Natural Science）

基　　金：江苏省科技攻关计划(工业)(BE2005006);江苏省教育厅高校高新技术产业发展计划(JHB05002)

摘　　要：在模拟解码系统中,为了配合不同制式的信号,采用了多个不同频率的晶振,根据制式来进行切换,在模拟锁相环路的控制下跟踪色载波;在数字解码系统中,使用不同累加步长的数控振荡器来代替晶振,在数字锁相环的控制下跟踪色载波.全数字解码系统具有结构简单、精度高等优点,但设计较为复杂,并且对采样频率有要求,需要采样频率接近色载波频率的整数倍,否则数字鉴相会出错,故对于非整数倍采样的信号,需要先重采样至整数倍采样的信号才能使用数字色载波锁相环.各种视频制式的色载波频率都有严格的规定,故在一些数字视频解码系统中,为了简化设计便将色载波频率设为标称值,直接估计每行色载波的起始相位来进行解码.对于广播电视信号,由于其色载波频率精度极高,当使用这种方案时效果较好;对于非标准电视信号,如录像机等,其色载波频率会发生小幅漂移,而高精度的色载波对保证色调的严格不失真很重要,此时如果仍然采用无锁相环的解码方案,就会影响画面质量.本文提出了一种在50MHz采样率下,不采用数字锁相环同时又能跟踪非标准电视信号色载波频率漂移的数字视频解码方案.PAL制电视信号的色同步在50MHz采样率下有120个采样点.通过搜索机制获得合适的64个采样点,作乘累加运算以获得所需的两个DFT变换系数,根据频谱校正的原理得到了每行色载波的起始相位;在PAL制中,每场起始处有20行左右的黑电平信号,对其中8行的色载波相位利用相位展开算法展开,用最小二乘法建立线性回归模型进行拟合,获得展开相位的梯度,经过计算得到色载波频率的估计值,接着使用该帧的频率估计值和每行的相位估计值恢复出本地高精度的色载波进行色差信号的解调.文章最后给出了使用从电视信号发生器中采集的数据进行彩色解码的仿真结果.At present, whether in the analog or digital video decoder system, the phase locked loop method is used to get the local color subcarrier signal strictly with the same color subcarrier frequency and phase. In the analog video decoder system, to cope with a variety of standard television signals, several crystals with different frequencies, which are switched according to the input signal standard, are controlled by the analog PI.I. （phase lodced loop） to track the color burst signal. In the digital video decoder system, the crystals are replaced with a NCO（numericaUy controlled oscillator） with different accumulative steps, which is controlled by the digital PLI. to track the color burst signal. Full digital decoder system has a simple structure and the advantage of high precision. But the design of digital PI.I. is relatively complex, and the sampling frequency should be close to the integral multiple of the color subcarrier frequency, otherwise the phase detector will output a wrong result. The signal that isn＇t sampled by integral multiple of the color subcarrier frequency should be resampled to get a desired signal before using the digital PI.I.. For all standard TV signals, the color subcarrier frequency has the relevant specifications. So in some digital video decoder systems, the color subcarrier frequency is sel＇m a fixed value for simplifying the design, and the initial phase of color burst of every line is estimated for decoding the signal. For the broadcast TV signal whose color subcarrier frequency is very accurate, tile method has a good resuh~ for the non-standard TV signal, such as VCR（video cassette recorder） signal, whose color subcarrier frequency drifts slightly, this method will affect the picture quality, because the high accurate local color subcarrler is important in guaranteeing the precision of hue of color. This paper presents an all digital video decoder system under a sample frequency of 50 MHz, which can eliminate the need for a digital PLL, while tracking the color su

关 键 词：数字视频解码 DFT插值 线性回归 相位展开 色载波恢复 

分 类 号：TN911[电子电信—通信与信息系统]