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Originally Posted by some slashdot guy
Yeah, the whole thing is rather confusing until you understand exactly what all is going on.
Regular analog TV, whether broadcast NTSC/PAL/SECAM, older analog cable (for me, channels under 100), or regular VHS tapes, is a signal that is essentially fed directly to the electron beam in your traditional TV. There is some funky electronics magic going on, but essentially, each dot in each frame corresponds to a specific fraction of a millisecond of the signal. (Some analog cable channels are scrambled, and decoder boxes will correct the mangled signal. There was a project, fscktv, to do this with a video card, but I never saw it actually work.)
To record an analog signal, you have two important steps. First, you need a TV tuner card that digitizes the signal. Second, you need to compress the video into something managable, such as MPEG-2. The PVR-250 cards are popular because they do both steps.
Digital broadcast TV is simpler. You just need to extract the digital information from the broadcast, much like a modem gets the digital information encoded on a voice phone line. With digital broadcasts, the process of compressing the data is done by the broadcaster, so you don't need any extra work to get a MPEG-2 stream.
Satellite and digital cable, whether HDTV or regular resolution, are sent as MPEG-2 streams, but the problem is that they are selling access to the channels, so they usually encrypt the streams. They assume you'll use their decoder boxes that do two things. First, they unencrypt the stream. Then they decode the stream. By "decode," I mean play the MPEG stream. That means you're back to an analog signal, whether you're using composite, s-video, or component outputs. Even if you use DVI or HDMI, which are digital, it's a decoded signal, not the MPEG stream.
So to record digital cable or satellite broadcasts, you have to either re-encode the signal, which simply isn't feasible right now for HDTV, or you have to somehow get the signal before it's decoded. If your PVR is integrated with your cable or satellite box, it can get the original MPEG stream (which is how DirectTiVo works). The only other option is that some digital cable and satellite boxes have a firewire port that you can connect to your computer, where they send the MPEG stream. MythTV doesn't support that yet, but someone was working on it a while ago.
In theory, you can get a smart card from your cable company that plugs into devices with digital tuners, which allows them to unencrypt channels that you are subscribed to. Some of the more expensive HDTVs accept them so you can use them without a cable box. There's nothing really stopping someone from building a card for your computer that uses the same card.
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03-15-2005, 02:13 AM
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