Monday, October 30, 2006


蟹状星云的编录是M1,也就是梅西叶着名的「非彗星型录」的第一个天体。现在我们知道蟹状星云是一个超新星遗骸--大质量恒星死亡爆炸所产生的扩张碎片云。这幅有趣的假色影像,组合了来自钱卓拉、哈伯和史匹哲等太空天文望远镜的数据,在X射线 (蓝-紫)、可见光 (绿)和红外光 (红)波段探索这团宇宙碎片云。


Thursday, October 19, 2006






from :

Friday, October 13, 2006

Narrowband (narrow bandwidth) refers to a signal which occupies only a small amount of space on the radio spectrum — the opposite of broadband or wideband.
This is entirely relative to what is being described; for example, an FM broadcast station takes up 150–200 kHz on the FM band, whereas a TV station's audio is narrowband, taking up only 25 kHz, and weatheradio broadcasts are even narrower than that. It is also very often used to describe radio antennas, called narrowband when they are designed specifically for one frequency or channel only instead of a wide range.
Narrowband can also be used with the audio spectrum to describe sounds which occupy a narrow range of frequencies. In telephony narrowband is usually considered to cover frequencies 300–3400 Hz.
In the study of wireless channels, narrowband implies that the channel under consideration is sufficiently narrow that the fading across it is flat (i.e. constant). It is usually used as an idealizing assumption; no channel has perfectly flat fading, but the analysis of many aspects of wireless systems is greatly simplified if flat fading can be assumed.