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Up front, most cable companies nowadays don't require you to subscribe to their video services to use their internet service. Most will offer you sometimes substantial discounts if you do, but it's not a requirement.
To simplify things, consider the cable line a huge antenna that transmits several "radio" frequencies (RF). Each "channel" on an analog cable line is carried by one frequency. When you change the channel, you change the frequency. On digital cable, several channels are compressed into one frequency, which is why a digital receiver programmed to do something with the data it receives via those frequencies is required. When it descrambles the information on one frequency, it has data for several cable channels, and the box sorts out which channels appear when the receiver is tuned to a certain channel. Typically you can put 12 channels per frequency, but it depends on the kind of data being transmitted. High definition channels consume more space, i.e. can't be compressed as much, and digital music can be compressed much smaller due to the lack of a standard video signal.
What this has to do with cable internet service is simply this. The cable line can carry more frequencies than are typically used by the video service. These other frequencies are used to transmit the internet service up and down the line. A cable modem will be needed to decode this information and sort out what is proper to send to your network interface in your computer. In this sense, it functions much like a digital receiver.
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