Windows XP Media Center Edition: Exposed

How a PVR Works

Despite the fact that you can buy a set-top PVR for well under $400, the process of simply “pausing live TV” is quite complicated and very hardware intensive. So before we delve into Microsoft’s hardware requirements for MCE let’s go through how a PVR works.

Let’s say you get all of your TV channels through a standard coaxial cable; the process is very similar with satellite dishes and cable boxes but for simplicity sake we’ll just assume we’re talking about a regular cable feed.

The cable feed is plugged into a coaxial input on the PVR that is controlled by a TV tuner chip; on a PC this would be found on a TV tuner card. After selecting the channel to watch, the TV tuner then hands the signal off to the PVR’s MPEG-2 encode engine. The MPEG-2 encode can be done via a dedicated processor or in software and handled entirely by the host CPU in a PC based PVR. As the stream is recorded it is buffered and written to the PVR’s hard drive, which means that as long as the PVR is on it is always writing streams of data to the hard drive.

If you’re doing more than just recording a show, as in you are actually watching it while it is being recorded, then as the data is being written to the hard drive it is also being read and fed into a MPEG-2 decode engine. Since a good amount of the MPEG-2 decode pipeline is already done on today’s GPUs, the process is usually not very CPU intensive and is split between the host CPU and the graphics hardware – at least in a PC; a set-top PVR may have a chip dedicated to MPEG-2 decode.

After the MPEG-2 stream is decoded from the hard drive it is then sent off to a TV encoder before it exits the PVR through a S-Video or other video output cable to your TV. On a PC based PVR the MPEG-2 stream could even bypass the TV encoder and be sent directly to an application for display on a monitor.

With this in mind, realize that when you’re “pausing live TV” the process doesn’t actually stop. The TV encoder may only be displaying a single frame from the hard drive, but the rest of the PVR process must continue so that when you hit pause again you haven’t lost any data. The hard drive is always being hit, the memory is constantly being read from and the CPU is forever encoding – you can already begin to see that something as simple as a digital VCR would take some serious hardware to implement properly.

A PC based PVR would require a very fast CPU, high bandwidth (and low latency) memory subsystem and very fast I/O. Slowdowns would be unacceptable; if the hardware isn’t able to keep up with the demands of the entire recording/playback process then you end up with recordings that stutter, have garbled audio or worse.

Then we have the problem of maintaining stability; while today’s PC hardware has matured considerably even compared to what was around just a couple of years ago, stability is a very serious concern. With such great stress being placed on most of the major subsystems in a PC (CPU, memory, I/O), a single failure in any one of them could interrupt your viewing or recording. Who wants to explain to a room of people why they need to “reboot the TV” because it crashed? Drivers become more important than ever as managing the interaction of all of this hardware must be done flawlessly, not to mention that the OS must be robust as well.

We can say with reasonable confidence that with a Windows XP Professional base, the OS side of things is taken care of. But what about the hardware itself? What about drivers?

Microsoft’s solution to the problem is actually twofold…