题目内容

os模块的______方法用来返回包含指定文件夹中所有文件和子文件夹的列表。

查看答案

搜索结果不匹配？点我反馈

更多问题

对于设定提存计划，企业应当根据在资产负债表日为换取职工的会计期间提供的服务而应向单位主体缴存的提存金，确认为应付职工薪酬。（）

A. 对
B. 错

1. Why is a data stream usually represented as an aperiodic digital signal? Why is amodulated signal represented as an aperiodic analog signal?1．为什么一个数据流通常表示成一个非周期性的数字信号？为什么一个调制信号表示成一个非周期性的模拟信号？Answer:A data stream is of digital signal, but not periodic. Thus, it is represented as an aperiodic digital signal, see Figure 2.2 (b). After a digital signal modulates carriers, which is of periodic analog signals, a resulted signal forms. This resulted signal is called a modulated signal that has the same type of carriers. Therefore, a modulated signal is usually represented as an aperiodic analog signal, see Figure 2.1 (b).

2.Compare the number of required frequencies and the size of bandwidth to represent the following signals: (a) periodic analog, (b) aperiodic analog, (c) periodic digital, and (d) aperiodic digital.2．比较需要的频率数量和带宽大小来表示下列信号：（a）周期性模拟(b)非周期性模拟，（c）周期性数字，及（d）非周期性数字。Answer:Period analog signal: one discrete frequencyAperiod analog signal: bandwidth-limited continuous spectrum Period digital signal: infinite discrete frequenciesAperiod digital signal: continuous spectrum , but bandwidth is unlimited.

What is the difference between fading and attenuation?3．衰落和衰减之间的差异是什么？Answer:Fading: Fading is the fluctuation in the strength of radio signals that varied in time, frequency, and phase in the transmission medium. It is often modeled as a random process. In wireless communications, a modulated waveform traveling over a certain medium could experience fading. Fading is a time-varying deviation of attenuation since it varies with time, geographical position, or radio frequency. There are two types of fading: multipath fading if caused by multipath propagation and shadow fading if shadowed by obstacles. A channel experiencing fading is called a fading channel.Attenuation : Attenuation is the gradual loss in intensity of flux such as radio waves or electrical signals. Attenuation affects the propagation of waves and signals. Amplification is a means of countering the attenuation impairment.