System Diagram
For example, consider the extended digital television system of Figure 3.We’ve taken some minor liberties in the figure, such as that not all compression is MPEG, and that some digital video (such as from games) will be uncompressed. Don’t let those simplifications make you miss the point, however, which is that you really can’t pick out the box labeled TV from the figure.
FIGURE 3: Extended digital television system
Depending on what components you’re working with, Figure 3 could have many different realizations. Use a combination satellite receiver and digital video recorder (such as a DirecTV TiVo) with a television, and you get the setup in Figure 4, in which we’ve grouped functions into blocks representing available consumer electronic devices.
The most interesting aspects of Figure 4, though, are the disk storage and networked video functions, because there aren’t common consumer electronics devices — other than PCs — for them. If we redraw Figure 4 to use a PC, and assign all the functions reasonably done by a PC that way, we get Figure 5. Yes, there’s another simplification, in that you can use either amplified speakers or an external stereo or home theater amplifier, but again, that’s not important. What’s important in Figure 5 is that, with the exception of a tuner and demodulator, a PC can do everything the system requires. Build the tuner and demodulator into the PC in the form of an add-in card, and the PC does everything.
Not that this plan is without flaws. The most significant one is that you can’t get add-in cards that let your PC decode digital satellite or cable TV signals, because those systems use proprietary formats with closed encryption standards. Instead, you have to use one of their set top boxes, but because few set top boxes accept channel change commands from your PC in a convenient way, you’ll have a harder problem to solve.We’ve put some hints in the “Extending the TV Server” section at the end of this chapter to help you get started. If you’re using broadcast TV or analog cable, though, you’re set.
FIGURE 4: Digital television with consumer electronics
FIGURE 5: Digital television with a PC
Returning to Figure 5, you can either put all the functions in a single PC or network multiple PCs together across a LAN and distribute the functions. The simplest realization of that idea is to split out the disk storage to a file server with huge drives, letting you share storage and recordings across several PCs and, therefore, several TVs. That approach requires running the coaxial cable to every PC serving as a television, though, which means you’re installing single-purpose cabling and have to deal with the noise and losses radio frequency (RF) transmission over coax creates.
There’s a better way. You can partition the system as in Figure 6, separating the reception and recording functions from the playback and decode ones. Your LAN now becomes the video distribution network, sending error-free streams from one PC to the next; the only PC needing the RF signal is the one with the tuner.
FIGURE 6: Networked TV distribution
You can connect as many clients to the server as your LAN can support. You’re only looking at 3–10 Mbps for MPEG-2, so in principle you should be able to handle 5 to 25 clients on a 100 Mpbs LAN (assuming no other network traffic and depending on video compression rates) without any collisions or pauses in the video. You can distribute video on a wireless LAN too so long as you scale the number of active clients to the capacity of the system.
The most serious limitation you’ll have to resolve with this system design is that you need a tuner for every independent viewer you want to be able to support. If there are three active televisions that all need to broadcast a different channel, you’ll need three tuners. You’ll see ways to meet that requirement in the next section.