| 单词 | Figure C-1 |
| 释义 | Figure C-1 Contemporary coax-based CATV systems commonly support aggregate raw bandwidth of 500–850 MHz. In the United States, each 6 MHz video channel carries a signal in the National Television Standards Committee (NTSC) format. ( Note: In European and many other countries, the phase alternate line (PAL) format requires an 8 MHz channel.) Within each 6 MHz channel, approximately 250 kHz is transition bands, or guard bands, 4.2 MHz is required for the video signal, 300 kHz is required for the associated audio signal, and the balance is due to the vestigial side-band amplitude modulation (VSB-AM) technique utilized as the signal is placed on the radio frequency (RF) carrier. In the mid-to-late 1990s, CATV networks began to merge and were acquired by large multiple system operators (MSOs) in the United States. Many coax-based systems were upgraded to support not only downstream television, but also high-speed full-duplex (FDX) Internet access and, more recently, voice. The process began of converting many systems from analog to digital format and deploying optical fiber, at least in the trunk network.The Multimedia Cable Network Systems Partners Ltd. (MCNS) developed the Data over Cable Service Interface Specification (DOCSIS) standards for cable modems, for which CableLabs assumed administrative responsibilities. DOCSIS supports high speed data, full duplex (FDX) data communications, primarily targeted at Internet access. Contemporary digital CATV networks also support voice communications. The most common approach currently employs pulse code modulation (PCM) encoding and time division multiplexing (TDM), although some CATV networks use adaptive differential pulse code modulation (ADPCM) with silence suppression.These are precisely the same techniques traditionally employed in the public switched telephone network (PSTN). The emphasis, going forward, is on voice over Internet Protocol (VoIP). See also ADPCM, amplifier, antenna, cable modem, coaxial cable, DOCSIS, downstream, drop, FDM, FDX, headend, HFC, Internet, LOS, microwave, MSO, NTSC, optical fiber, over the air, PAL, PCM, PSTN, RF, satellite, silence suppression, tap, TDM, transition band, trunk, and VSB. |
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