引用
最初由 shinjico 发布
惊....锅可以收到HDTV?? 那种数据流量微波通讯怎么也不够的吧....
土了吧嘿嘿，锅可比CABLE牛B啊
On a satellite, we have a number of so-called transponders. Those are circuits that receive the signal from the earth, modulate, amplify and re-transmit it back to the earth.
There are about 20 - 30 transponders on a single satellite, and each has a bandwidth between 27 and 72 MHz (most transponders use 36 MHz, which offers about 38 Mbit/s). A TV channel occupies only a fraction of this bandwidth (how much?) and bitrate (many DVB channels use a lower bitrate than DVD, so the average is below 5 MBit/s), which allows for a number of channels on a single transponder. Most satellites also carry digital radio, which of course uses a lot less bandwidth. The signal sent back to earth using QPSK modulation. This means, a single transmitted symbol contains two bits, and every symbol has the same amplitued and is 90° apart from the next.
On a cable network, the available channel bandwidth is smaller (8 MHz), and the modulation scheme used is 64QAM. As one symbol can have 64 values rather than only 4 as in DVB-S, more information can be transported per symbol, which leads to the same available bitrate per channel (38MBit/s). 64QAM is more complex to decode and more error-prone, but since the signal quality is better than when receiving from a satellite (keep in mind that the distance from the satellite to your dish is huge), you're still likely to get a less distorted signal.
