>>We've come full circle here in reinventing the use of analog modem techniques to deliver higher speeds of "digital payloads" over what can only be termed, by any definition,analog technologies. It's going to remain that way for quite some time, unless they un-do the HFC work that has already been done, and unless the price points for digital conversion processes in the outside plant can come down significantly to justify a true digital architecture.<<
Yes, Frank, a hybrid fiber coax (HFC) cable system essentially functions like a souped up version of a data modem (modulator / demodulator) in order to send digital signals over a amplitude modulated radio frequency cable system. Baseband digital signals are usually modulated with a version of a QAM (quadrature amplitide modulation) modem for video services, or a QPSK (quatranary phase shift keying) modem for voice or data services. After all the digital signals from dozens or hundreds of sources are modulated into different radio frequencies, they are electrically combined at the cable tv headend so they can be transmitted onto a single piece of coaxial cable. With all the signals combined, they are then able to be routed to a distributed feedback type of optical transmitter, specially designed for amplitude modulated signals over fiber. All of the amplitude modulated signals are transmitted over a radio frequency based HFC network until they reach a set top box at the customer location, where they are demodulated and restored to baseband digital signals.
HFC is a wonderful and fairly straightforward way to transmit a big load of information onto a radio frequency network, and I like HFC for many reasons. Its just that the coaxial distribution part of the HFC network better be built very, very tight and designed very well, or the transmission quality of the lower and upper band frequencies (i.e., where the voice and data services usually ride up and downstream!) will be lousy. In summary, HFC differs significantly from a time division multiplexed digital transmission system, where the baseband digital signals are not modulated and demodulated. HFC will be a big part of the cable world for several decades at least...
Ray. |
