Personal Video Recorder Basics
All our favorite TV shows are recorded digitally and edited digitally. But, we receive them using some analog technology, such as cable, plain air or satellite. Now, that's just wrong. Luckily, an alternative is available that offers better picture quality, Dolby Digital sound and EPG, the electronic program guide. Ladies and gentlemen, please welcome DVB, digital video broadcasting.
DVB works with an MPEG-2 compressed stream and has a theoretical maximum bit rate of 15Mbit/s. Because DVB never is used for video alone, extra audio tracks and other information are added to create a richer user experience. All of this data is stored in small packages. Of course, if some packets don't arrive or are corrupted, they leave artifacts in the picture and possibly in the sound stream. The program, on the other hand, continues as if nothing has happened once correct packages arrive again, so there is no need to worry about bad sync. The signal also is forgiving, because the antennas usually are dimensioned with enough reserve to cope with rain, snow or small animals.
Three different models of DVB are available, DVB-S for satellite, DVB-C for cable and DVB-T for terrestrial reception. All three basically are the same; the differences lie in the tuners. DVB-T is rather new and not yet used widely; the other two, especially the satellite variant, are quite common and popular all over the world.
Modern set-top boxes available from major manufacturers have some nice features, such as hard disk recording and MP3 playback. Nevertheless, they lack more advanced options, including archival of recordings to SVCD or some MPEG-4 sibling. With the advent of affordable DVD burners and media, backing up your favorite TV shows to DVD is well within reach. In Europe, digital personal video recorders (PVRs) carry a hefty price tag of around 500 EUR, and most come without a hard disk. A full-featured DVB-S card, on the other hand, can be found at on-line retailers for as low as 165 EUR.
But, what good is cheap hardware without good software? The program VDR, video disc recorder, enables you to build a powerful set-top box on your own using your favorite flavor of Linux and a DVB card. The machine I built incorporates basic features, such as watching TV, recording and time-shifting, plus advanced features, including MP3/Ogg playback, playback of all video formats supported by MPlayer and backup of the recorded material to MPEG-4, video CD or DVD. A commercial set-top box hardly stands a chance against this feature list.
To build our box, we need some hardware. Bear in mind that the recordings need a lot of space. A 120GB hard drive typically holds some 60 hours of video, which should be plenty of space. You can get away with less if you back up your movies more often to get them off the drive, but I recommend at least a 20GB drive, which hold about three or four movies.
We also need a processor. If you want to encode the videos, you need a faster one; if not, an old 200MHz machine should do. I wasn't able to find anything slower than a Celeron 1,700MHz, which is more than enough power, even for the encoding process. Playback using MPlayer also requires a fast processor and at least 1GHz, though it's rumored to work with less. I've tested MPlayer on a slower machine, and the image quality does suffer quite a bit. The reason for this is the way MPlayer uses the MPEG-2 decoder on the DVB board. Non-MPEG-1/2 material is converted to MPEG on the fly, which eats up quite a few processor cycles.
This leads us to the most important piece of hardware; the DVB card. You need a full-featured card with a hardware MPEG decoder. These are more expensive, but they have several connections for sound and TV. Cards like the WinTV Nova work best as secondary cards to record several programs at once. If you can, go for the satellite option. It is by far the most flexible solution because you are not dependent on some cable provider. Apart from that you can link up several satellite dishes to watch even more channels. I also discuss the DVB-S variant in this article, but deploying a different solution is not really a different process. I opted for a Hauppauge Nexus-s. It's probably the most expensive card, but it doesn't suffer from the overheating problems older models experienced, plus it has a good tuner and comes with an infrared remote and receiver.
For the base software load, I used Red Hat Linux 9, but any distribution should do. A small installation with GCC and development packages for libjpeg should be enough. X isn't needed because the full-featured DVB cards have video-out capabilities. Don't forget to install all the kernel development packages; we need those to compile the DVB driver.
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