The digital tuner boxes are designed basically for over-the-air broadcasts. The QAM (quadrature amplitude modulation) boxes that the cable companys use generally support a lot more features, such as encryption. Digital TV channels are usually sent as groups called epochs. That's why on over the air digital TV, you can get several channels, all with one base number (like old analog channel 3 could have channel 3.1, 3.2, and 3.3). With the cable company, their digital signal is composed of epochs with different frequencies and various encryption and protection, so that you can only get the channels that you pay for.
With the more advanced box you get from them, you usually also get features like pay per view, video on demand, a TV guide, and interactive TV.
So in short, yes, you almost always need the cable company's box. Sorry :(
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