DVR Technology: Like Magic

by Charles Bloodworth
June 2000

What’s that Bruce Springsteen song, "500 channels and nothing on?" That’s a pretty fair description of broadcast television most nights. But wouldn’t it be great to be able to turn on your television and have your favorite shows magically appear? Well, with the new digital video recorders (DVRs), like ReplayTV and TiVo, you can. These innovative devices are becoming more and popular as they rapidly redefine the way we view television. In this article, we’re going to lift the veil and take a peek at the technology behind the personal DVR. Once you realize what’s inside a DVR and understand how it performs its tricks, as amazing as it is, you’ll see that its no more magical than a VCR or a desktop computer. So let’s take a look and see how this new technology performs its magic.

Instead of being stuck with those crusty old reruns, a DVR will automatically search out and record all of the shows that you want so that by the time you’re ready to watch, they’re lined up and ready to roll. If you decide to watch what is on a channel at the moment, a DVR will give you added control over the show you’re watching. If the phone rings during a program, for example, you can pause the live broadcast, answer the phone and when you hang up, press play to resume watching right where you left off. Putting the networks on hold certainly seems a bit like magic, but the actual technology is quite simple.

A Quick Review

If you’ve been following the progress of these new products in the pages of

Smart TV, then you might already have a fair idea of all the features these DVRs offer. Though some of the features and options vary between the various DVRs, here’s a list of some of them to introduce you or help refresh your memory.

Electronic Program Guide (EPG)DVRs continually provide updated program information to help you plan your viewing schedule a week or more in advance.

Smart RecordingSome DVRs can record your viewing habits in a database, where they can learn what you like. They will then recommend or record programs that will likely interest you.

VCR-like Functions Hit Pause and Rewind while viewing live broadcasts, you no longer have to wait for a commercial to leave the room. You can pause a program when you leave the room, then fast-forward through the commercials to catch up to the live broadcast when you get back.

VCR-like OperationOnly simpler and faster. The EPG makes programming simple enough for anyone to master. You can highlight a program on the EPG and press one button to record it. No tapes to label and catalog. No waiting for tapes to fast-forward or rewind to the desired position. One important point to remember, however, the recordings are only temporary, as disk space is limited.

So, How’s it Done?

One way to understand the inner workings of the DVR is to think of it as a VCR that uses a hard drive instead of a videotape to store the recorded program. From the functional standpoint, the most important difference between storing data on a disk drive and storing it on a tape, is the speed at which you can access the data later. With tape, you must fast-forward or rewind until you get to the segment of the tape you want to watch. With a hard drive, as with an audio CD, you can access all of the data randomly and quickly.

If you’re wondering why you can’t simply replace the tape recorder in your VCR with computer hard drive, you’ve got a good idea but it’s more complex than that. For one thing, video must be digitized and compressed before it can be stored on a hard drive. You need the proper hardware to control the digitization/compression process, and to control playback. In the end, a digital video recorder is more like a computer than a VCR, but this brings the user a good deal of advantages.

A DVR needs to compress one stream of video, while it decompresses another stream of video. This is how it can "pause" live television, or record one program while it plays another program from its hard drive.

Are you beginning to see how some of this magic works? The DVR doesn’t really pause the live broadcast at all. In fact, when using your DVR to view live broadcasts, you’re not really watching the live broadcast (no matter how much it may feel like you are). It’s an illusion. Inside the DVR, there’s a sleight of hand trick going on. The DVR instantly digitizes incoming video into the MPEG-2 format using a hardware MPEG encoder. Two processes are happening simultaneously. One is recording the video you think you’re watching. The other is decoding and playing back a recording that has already been temporarily stored on the hard drive. In the case of watching a live broadcast, however, the recording that you’re watching hasn’t been on the hard drive for very long, usually for a time period of less than a second.

Exactly how long the "live" broadcast is on the hard drive before you see it depends on the particular DVR implementation, but the video the DVR records can be read for playback almost instantaneously. No matter how much it looks like you’re watching the live broadcast through the DVR, you’re really watching a recording. You can pause, rewind and review without affecting the recording of the incoming broadcast. So when you hit the pause button, the basketball game doesn’t stop and wait for you, after all. Shaquille O’Neal goes right on dunking, and your DVR dutifully continues to record the game. When you’re ready to go back to watching our program, the DVR is ready to resume playback right where you left off. And through it all, the recording process keeps on doing its job without interruption.

One might wonder why it’s taken so long for this technology to appear on the market. Well, processing video with a computer is not a trivial undertaking. You need lots of memory, lots of hard-drive space, lots of speed and affordable hardware to digitally compress the video. The ever-increasing capacity and falling prices of hard drives and computer memory, combined with the availability of low-cost video digitizers have contributed greatly to making this type of product a reality.

A phone modem is the final piece of the DVR puzzle. With its modem, a DVR can call a service provider and download the Electronic Program Guide and anything else the provider chooses, including DVR software updates. All this takes place transparently, behind the scenes, and provides an interactive, two-way link between the service provider and your DVR.

Can You Say EPG?

The other magic trick that the personal DVRs perform is intended to help you plan your viewing schedule. Current DVRs use your phone line to download program information, which you can access and use in your planning. By simply selecting the programs you’re interested in, you tell the DVR software all it needs to know to record your favorite programs. No more setting start and stop times or entering code numbers the way you had to with a VCR. Just click on the name of the show.

Some of the services go even further. By taking advantage of sophisticated software algorithms to identify programming that might be of interest to you, the DVR can notify you of these programs and if you choose, even record them for you automatically.

Now that we’ve exposed the magician’s secrets, you know how a DVR creates the illusion of controlling live television. The good thing about learning these secrets, is that when you actually see it, it still looks like magic. Illusion or not, pausing, fast forwarding and watching an on-demand slow-motion, instant replay is cool and will forever change the way you view television. With a DVR you’ll be humming a new tune, "500 channels and everything’s on when I want it."