Ask Engadget HD: What is switched digital video (SDV)?

With scads of cable providers slowly but surely converting markets and delivering channels via switched digital video, we reckoned now was an excellent time to dust off the terminology dictionary and dig in deeper. Fittingly, Jarad agreed:

"I've seen that lots of markets are being switched over to SDV lately, and I've noticed CableCARD users are generally upset whenever this happens. Why is that? I thought SDV was supposed to free up space so that more high-definition channels could be distributed on the current network?"

If you've suddenly found yourself wondering the same, head on past the break for the low-down.

Here's the scoop. A quick Google search will surround you in all the technobabble you can handle on this issue, but we're going to do our best to lay this out in a way the layperson can understand. Essentially, cable providers have traditionally delivered every single channel that it offers to every single home in a service group at all times. In other words, hundreds of channels could be sent to one TV, although just a single channel is watched at a time. It doesn't take an electrical engineer to understand how wasteful that is.

In the past, this practice has worked a-okay since each additional analog or digital SD channel only required a small amount of extra bandwidth; now, however, customers are demanding more and more HD programming, and as you can likely glean, these newfangled HD channels eat up spare bandwidth in a hurry. Given the alternative of ripping up existing networks and replacing them with higher-speed alternatives (read: no way that's happening), cable providers got smart and conjured up the idea of using switched digital video.

In essence, SDV frees up more bandwidth on existing systems by only delivering a single channel to your television at a time. We'll admit, that's a very simplified version of what's going on, but for all intents and purposes, that's the skinny. In order for the provider to provide you with the correct channel, however, some form of two-way communication has to take place. Your TV has to instruct your carrier what channel to send. If that last line sounds sort of familiar, you're not losing it just yet -- video-on-demand (VOD) works in a similar manner in that your TV (or set-top-box) commands the cable system to send it a certain program.

So, now that you understand that SDV simply delivers the single program you want to watch at one given time -- and that, in doing so, it frees up bandwidth for more content to be added -- what's the big fuss all about? As with most new technologies, getting everyone to adapt is, at least on some level, a nightmare. In theory, consumers should adore SDV in that it doesn't place an undue burden of cost on the cable carrier to free up bandwidth and add more high-definition channels that it otherwise simply would not have room for. Unfortunately, it's not that simple.

If you'll recall, we mentioned that your TV or set-top-box / receiver / HD DVR had to talk to the cable company in order for the correct signal to be sent. Well, what if your equipment is mute? As in, it can't talk. Believe it or not, first-generation (read: the ones that everyone has right now) CableCARDs do not posses the ability to talk back to the cable system. What it can do is listen, but it cannot actually request that anything be sent. Thus, older CableCARDs are addressed as "one-way" devices. However, even the new multi-stream CableCARDs have to be loaded within a two-way-capable device (unlike the TiVo HD, sadly) for a two-way conversation to occur. For those unaware, quite a few individuals who have purchased TiVos (you know, those DVRs with contracts and monthly fees attached) rely on CableCARDs in order to utilize TiVo's DVR technology with basically any cable carrier. Now, those very individuals are being forced to live without any new channels delivered via switched digital video because their equipment lacks the "two-way" capabilities that are required for SDV signals to be received.

And it's not just TiVo users who are in a pickle. CableCARD-equipped home theater PCs (HTPCs) as well as HDTVs with built-in CableCARD slots are now left to wonder how on Earth they'll ever receive any programming when the entire lineup eventually phases over to SDV. Thankfully, this mythical device now known as a Tuning Adapter (it has survived several name changes) promises to provide SDV compatibility for existing CableCARD-equipped TiVos, but those who own CableCARD-equipped HDTVs may not be nearly as fortunate. For those folks, it's simply a game of wait-and-see.

At this point, you may be thinking the obvious. Why doesn't everyone stuck with a first-gen CableCARD simply pick up a two-way capable set-top-box / HD DVR from their cable company and get on with life? Believe us, this is exactly what the cable companies want you to do. After all, they certainly appreciate that $5 - $15 receiver rental fee each month. However, the entire premise of the CableCARD was to give consumers more choice and remove the necessity of having to use the cable company issued equipment; by rendering CableCARDs essentially useless in the world of SDV, we're reverting right back to the times of zero consumer choice. Sure, if you're happy with the set-top-box offered by your cable provider, by all means, pick one up and start enjoying your SDV-delivered channels. But if you're dead set on using a TiVo / HTPC, you'll find yourself hunting for a solution in the coming months. That, however, is another Ask Engadget HD entirely.

Got a burning question that you'd love to toss out for Engadget HD (or its readers) to take a look at? Tired of Google's blank stares when you ask for real-world experiences? Hit us up at ask at engadgethd dawt com and keep an eye on this space -- your inquiry could be next.