Broadcast Technology

In the past, digital transmission of video and audio has required a broader frequency bandwidth than analog transmission to transmit the same information. Recent progress in source coding technology for video and audio, however, has made it possible to reduce the bit rate while keeping the quality deterioration to a minimum. The frequency bandwidth required for digital transmission has consequently become about the same as or several fractions that of analog transmission, making digital transmission feasible for broadcasting. Until recently, digital transmission systems for broadcasting have consisted mainly of multiplex broadcasting systems transmitting only audio and text data. Today, digital transmission including video services is possible in the form of satellite and terrestrial digital broadcasting. In this issue, we describe the basics of digital modulation systems. Figure 1 shows a digital transmission system. The system quantizes and samples analog information such as video or audio data and performs source coding on that data for the purpose of band compression. It then performs channel coding, which consists of adding error correction code to correct the errors generated during transmission, as well as data interleaving and digital modulation. The system converts the digitally modulated signal into one in the transmission frequency band, amplifies that signal at the transmitter, and transmits it by either radio wave or cable. At the receiver, the system performs inverse processing with respect to the transmission side to restore the original information; that is, it digitally demodulates the received signal, performs de-interleaving and error correction, etc.