Advances in digital technology have greatly benefitted cable operators, both large and small. Services such as voice and data can be put in place of standard analog video channels on the network. Alternatively, six to 10 standard definition and one to three high definition programs can be substituted for a single analog channel, providing services such as Pay Per View (PPV), Video on Demand (VOD) or just high definition viewing of a program.

These digital services are implemented on the operator’s network using a technology called Quadrature Amplitude Modulation or QAM (pronounced “kwam”), which is a technique for putting digital information in an analog format and limiting the amount of bandwidth or network capacity required. The technology allows new services to be introduced by simply substituting a digital QAM channel for an analog video channel or adding it to the existing channel line up.

So, what is QAM? A QAM signal is actually comprised of two analog signals (carriers) whose amplitude (level) and phase (position) create symbols that represent a unique string of 1s and 0s (bits), much like the position of the hands on a clock indicate the time. The more symbols used in the QAM signal, the greater the number of bits each symbol can represent. A 256 QAM signal has 256 symbols with each symbol representing 8 bits. On the network this translates into a single channel data rate of greater than 40 Mb/s. A QAM signal is generated by a device called a modulator which converts bits to symbols, and changes the level and phase of the carriers to represent the particular symbol. At the customer side, symbols are collected and converted back to bits and then to the signal that feeds the television.

In addition to new services, QAM signals also assist operators in making their headends more efficient. New digital modulators enable very compact equipment, supporting up to 160 QAM channels in a single card. They also have a lower power consumption per channel which together translates into lower power bills and smaller office space requirements for the operator.

As a result, QAM technology is very much a part of the modern CATV network. While it is not a very sexy acronym, and with the lack of a “U” in the acronym after the letter “Q” is mispronounced by definition, it has nonetheless been a tremendous benefit to cable operators and in turn to us as subscribers.

After all, can you live without the Internet or ESPN in HD?

About the Author

Mark Vogel

Mark Vogel is the Broadband director of technology development responsible for the research and development of new optical and RF technologies for use in the CATV industry. He has been with CommScope for 10 years and holds over 20 patents. Prior to his current role, he was manager of technology development for seven years and held similar positions at Telecordia, Ameritech, and 3Com. Mark earned his Bachelor of Arts in Physics and a Master's degree in Electrical Engineering from Washington University.

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