Are You Being Cheated by Digital Cable?

Lauren Weinstein writes "Even though your cable company may claim that a channel is in a digital tier that you're paying for, they may be sending it to you in analog form, with associated negative effects. Surprise! Are You Being Cheated by Digital Cable? 'You're paying for digital, you should get digital. Outside of the lower video and audio quality that can be present on many analog feeds, third-party devices (like cableCARD TiVos) which could otherwise record a digital signal directly, will be forced to re-digitize an analog signal, with inevitable quality loss in the process. But how to know for sure if a channel is digital or analog as received?'"

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Composite video has a maximum bandwidth

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Analog stations don't have a specific "horizontal resolution" In a way, they do; it's called the Nyquist rate. Terrestrial broadcast television (M system) and standard cable television consist of amplitude-modulated NTSC composite video with a vestigial sideband, combined with frequency-modulated audio. In NTSC, 0 to roughly 3.0 MHz of a composite video signal is luma (Y), while 3.0 to 4.2 MHz is chroma (Pb and Pr, multiplexed with QAM). Nyquist's theorem states that this Y signal can hold only 6 million samples per second. There are 15734 scanlines per second, and about 82 percent of a scanline is video, the rest being horizontal blanking. This gives a total of (6000000/15734)*0.82 = 312 distinct luma samples at the Nyquist rate. Many video systems slightly oversample this to 320, which provides (desirable) square luma elements at the field rate.