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شنبه 10 شهریور‌ماه سال 1386 :: 05:49 ق.ظ ::  نویسنده : محمدرضا گرامی

Head End System

This figure shows a diagram of a simple head-end system. This diagram shows that the head-end gathers programming sources, decodes, selects and retransmits video programming to the distribution network. The video sources to the headend typically include satellite signals, off air receivers, microwave connections and other video feed signals. The video sources are scrambled to prevent unauthorized viewing before being sent to the cable distribution system. The headend receives, decodes and decrypts these channels. This example shows that the programs that will be broadcasted are supplied to encoders and modulators to produce television channels on multiple frequencies. These channels are combined onto a single transmission line by a channel combiner.

Set Top Box Operation

This figure shows a block diagram of a video set-top box. This device converts incoming RF channels into a lower frequency that can be provided to a television. This diagram shows an analog setup box usually involves the conversion of the incoming RF channel frequency to channel 3 or channel 4. The conversion of a digital cable channel is more complex as it requires demodulation, decoding (data decompression), and RF modulation back to a form suitable for a television. Optionally, a digital decryption section and/or video descrambling section may be included.p>

Cable Modem System

This figure shows a basic cable modem system that consists of a head end (television receivers and cable modem system), distribution lines with amplifiers, and cable modems that connect to customers' computers. This diagram shows that the cable television operator's head end system contains both analog and digital television channel transmitters that are connected to customers through the distribution lines. The distribution lines (fiber and/or coaxial cable) carry over 100 television RF channels. Some of the upper television RF channels are used for digital broadcast channels that transmit data to customers and the lower frequency channels are used to transmit digital information from the customer to the cable operator. Each of the upper digital channels can transfer 30 to 40 Mbps and each of the lower digital channels can transfer data at approximately 2 Mbps. The cable operator has replaced its one-way distribution amplifiers with precision (linear) high frequency bi-directional (two-way) amplifiers. Each high-speed Internet customer has a cable modem that can communicate with the cable modem termination system (CMTS) modem at the head end of the system where the CMTS system is connected to the Internet.

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