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七郎 / Seven Liu

科学与艺术,恰如人生中的红颜知己。 本博内容保留所有权利.

 
 
 

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3.6 Jitter parameters 抖晃参数  

2012-07-23 17:10:50|  分类: 读点英文 |  标签: |举报 |字号 订阅

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The jitter is a measure of the spread in time of the transitions in respect to their ideal position in the data stream. If there were no disturbances or variations, all lengths would be exactly 2T, 3T, etc. and well positioned to the regenerated clock. The jitter values are found by measuring several time events and processing them statistically. Jitter in Blu-ray disc could be measured with two types of equalizers, a usual (Conventional Equalizer) and a nonlinear adaptive limiting filter (Limiting Equalizer). The specification only has defined limits for the combination of conventional equalizer and limit equalizer. Furthermore there are basically two types of jitter defined for optical media, Data to Clock jitter and Data to Data Jitter. In the Blu-ray specifications the only type mentioned is Data to Clock jitter.

Jitter(抖晃)是测量数字信号流中过渡时间相对于理想位置的差异的分布状况。如果没有干扰或变动,所有的符号长度将会是精确的2T、3T等,相对于再生时钟的位置很准确。Jitter值是通过测试几个时间事件并经统计处理而获得的。BD盘片可使用两种不同类型的均衡器进行测量。一种是线性相位横截性滤波器(phase linear transversal filter,是传统型的滤波器)和非线性自适应限位滤波器(限位均衡器)。规格书中只定义了传统滤波器组合和均衡器。此外,对于光学介质来说,定义了两种类型的Jitter:"数据-时钟(DC)Jitter"和"数据-数据(DD)Jitter"。在BD规格书中只提到了DC Jitter。

Data to Clock Jitter (DC-Jitter) is the jitter between the sliced HF-signal and its regenerated phase locked clock signal (PLL Clock) normalized with the PLL Clock period. To perform a measurement of this type of jitter some signal processing is necessary. The different process steps necessary for measuring jitter is shown schematically in the BD-ROM specification (Figure 10.8-1). To perform the jitter measurement the HF-signal is sliced using a first order integrating feedback auto slicer. The slicer controls a slicer level to achieve a duty cycle of 50%. Before the slicer there is a very important low pass filter, the post-LPF. This filter affects the jitter values a lot and is of high importance.

DC Jitter是切片高频信号和再生的经锁相环时钟归一化的锁相环时钟信号之间的Jitter。进行这种Jitter测量要对信号做一些处理处理。测试Jitter所需的各种信号处理步骤可参见BD-ROM规格书内的示意图(图10.8-1)。做Jitter测试时,高频信号的切片采用了一阶集成反馈自动切片器。切片器控制切片电平使占空比达到50%。在切片器之前,有一个非常重要的低通滤波器,即下图中的post-LPF。该滤波器会影响到jitter的测试值,所以非常非常重要。

By use of a PLL (Phase Locked Loop) a clock-signal is regenerated from the HF signal. The PLL is designed to minimize the deviation of the edges of the PLL clock and the edges of the HF-signal. If this is achieved perfectly the disc has no DC-Jitter although such a situation never exists in practice. The standard deviation is normalized with the PLL Clock period to get the DC-Jitter. Furthermore the deviations can be added to a histogram to visualize the jitter, this as a complement to the standard deviation.

通过使用锁相环(PLL),从高频信号中再生出时钟信号。锁相环的设计目的是使PLL时钟信号边沿与高频信号的边沿的差异最小化。当然没有DC-jitter是最好的,但实践中从未实现过。标准偏差以PLL时钟周期进行归一化处理。另外,可以将变动性做成柱状图形式,使之形象化而作为标准偏差数值的补充。

It is important to consider the amount of data the jitter value is measured for. Jitter according to the specification shall be measured over tracks. This means that the amount of edges taking in count is different on inner radius and on outer radius.

要注意测试jitter值所需要的数据量(轨道长度)。根据规格书,测试Jitter时将跨越轨道进行测量,这意味着测试jitter时内外圈所统计的符号边沿数量不同。

Let t denote the deviation in time from the sliced signal edge to the PLL clock edge. Let n be the number of edges. Then we may write:

我们用t表示切片信号边沿到PLL时钟边沿的时间,用n表示边沿的数量,可以写出下列等式:

Now the standard deviation is expressed in seconds. In the specification the unit is percent and to achieve this t is normalized with the PLL clock period and the jitter value may be written as:

现在是以秒为单位来表示标准偏差,而规格书规定的单位是百分比,所以要用PLL时钟周期对标准偏差进行归一化,得到下列等式:

where TPClk is the time period of the PLL Clock.

等式中TPClk是PLL时钟周期。

 

 

Leading and trailing edge

前沿和后沿

Which edges are leading or trailing is somehow a bit complicated. It has to be considered what the normal state of the disc is. The leading edge is the edge starting a mark or a pit. The trailing edge is the edge finishing a mark or a pit.

前沿和后沿的概念有些难懂,所以要考虑到盘片的正常状态是什么。前沿是指一个"刻录标记"或"坑"的开始,而后沿是指一个"刻录标记"或"坑"的结束。

 

 

In digital communications, the purpose of equalizer is to reduce intersymbol interference to allow recovery of the transmit symbols. It may be a simple linear filter or a complex algorithm.

在数据通讯中,使用均衡器的目的在于减少码间干扰以便于传输信号的恢复。它即可以是一个简单的线性滤波器,也可能是一套复杂的算法。

 

Conventional equalizer

传统均衡器

The conventional equalizer is an equalizer of the same type as in DVD. It is a 4-taps transversal filter. The filter coefficients are specified in the specification and are the same for every format type. r = 33/32. See figure A 10.8-3 in the ROM specification. Equalization is necessary to read out the signal due to very small signal levels for the shorter symbols. When using a linear equalizer like this it is difficult not to increase lSI when boosting the high frequency components.

传统均衡器是指和DVD所用的相同类型的均衡器。它是一个四路输出横截滤波器。滤波器的系数在规格书中有规定,对于所有格式类型都是一样的。r=33/32. 参阅BD-ROM规格书图A10.8-3。对于较短信号的极低信号电平的读取来说,使用均衡器是必要的。使用这种线性均衡器放大信号较高频率成分时,要让码间干扰不增加是非常困难的。

Therefore boosting is kept relatively low and the jitter level using only the conventional equalizer is rather high.

所以,放大程度保持相对较低,而只使用传统滤波器的jitter水平是相当高的。

Limiting equalizer

限制性均衡器

To understand this parameter it is interesting to know what purpose the limit equalizer has. In such high recording densities as in Blu-ray Disc, disc noise is increased and SNR is decreased. The signal levels from the optical pickup head decrease with the pit size and the lSI is increased with the recording density. In earlier technologies such as DVD a conventional equalizer has been used to gain the signal levels. It is impossible to boost the frequency components with a linear equalizer for Blu-ray without increasing the effects of ISI. The boosting is therefore kept to a relatively low level and a non-linear equalizer (Limit equalizer) is developed.

要理解这个参数,懂得限制均衡器的功能是非常有趣的。在BD盘片如此高的记录密度下,盘片噪音会增加而信噪比降低。因坑形尺寸缩小所以信号电平降低,而码间干扰由于记录密度提高而增加。诸如DVD等早期技术为进行信号增益而采用传统滤波器。但对于BD格式来说使用线性均衡器来放大各频率成分而不使码间干扰升高是不可能的。所以放大被保持在相对较低的程度,并研发了新的非线性均衡器(限制性均衡器)。

The Limit Equalizer improves SNR and reduces the influence of disc noise. The high frequency components of the signal are boosted without increased lSI.

限制性均衡器提高了信噪比并降低了盘片噪音的影响。它在放大信号高频成分的同时不会使码间干扰升高。

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