江湖风云录太玄经:英语翻译

At the present time，the most common way of transmitting television signals by satellite is to use wideband frequency modulation（WBFM，宽带频率调制).With a typical C-band North America domestic satellite,this FM signal occupies the entire transponder bandwidth of 36 MHz,which allows for a 4 MHz guard band between transponders.The baseband signal used to modulate the main carrier includes the composite video(合成视频信号),complete with both luma and chroma signals(亮度和色度信号),and one or more audio channels transmitted on subcarriers. The most common frequency for the program audio subcarrier is 6.8 MHz for North America domestic satellites.The audio signal is used to frequency-modulated a 6.8 MHz carrier,and the resulting signal is added to the baseband video signal,which contains frequencies from about 30 Hz to 4.2 MHz.The combined video and multiplexed audio signal is then used to frequency-modulate the main carrier.Figure 20.16 shows the signal and a simplified block diagram of the way in which it is created.
Stereo television audio is quite common and can be transmitted via satellite in any of several ways.The simplest method is to use two separate subcarriers,one each for the left and right channel signals.Usually these are at 5.58and 5.76 MHz,with a combined left-plus-right(mono,单声道) signal also transmitted at 6.8 MHz for the benefit of users without stereo equipment. Another method is to transmit the left-plus-right signal on 6.8 MHz and a left-minus-right difference signal(差值信号) on another subcarrier,typically at 5.8 MHz.The stereo audio signal can be recovered(恢复) by simply adding and subtracting the left-plus-right and left-minus-right signals.Figure 20.17 shows these possibilities.