What is a subchannel?

One of the neater things about digital television is that, well, it’s digital. It’s not just controlled by simple electronics, and that means that you can get TVs to do tricks that they couldn’t do a decade ago. One of those tricks is stacking more than one program source on the same broadcast channel.

I know, that’s a lot of fancy language, so let me take it step by step.

A broadcast channel is another way of saying “a signal that takes up 6MHz in the range that the government says they can broadcast it.” Channels are all the same size, no matter what, and that’s because before computers, there wasn’t a lot of fancy stuff you could do to a channel. They all had to look an act identical (except of course for the frequency they broadcast on) because without computers, TVs of the day had to be set up to receive things just one particular way.

In 2009, all full-power TV broadcasting in the US went 100% digital. That means that a digital signal, basically a computer file, was being broadcast in that channel. Just like your home internet, there’s a maximum speed for TV broadcasting, and it’s 19.2Mbit/sec. Compare that with your home internet which is probably between 15 and 50 Mbit/sec. The trick here is that unlike the old broadcast channels, the new ones are all digital and there are going to be computers on each end that can manipulate the signal. That means broadcasters have options. They can send out a really pristine 19.2Mbit/sec HD picture on one channel, or they can process the signal so it fits in less space. This sometimes means cutting down on quality, or sometimes it just means that they use super-fancy computers to do a better job of compressing the signal.

No matter how they do it, it’s possible to send out a full-resolution HD picture and still have some room to spare on your channel. This gives broadcasters the ability to have more than one TV source per broadcast channel. Usually, that secondary source is standard definition, because an SD picture can actually be compressed down to about 1 megabit per second before you see real quality issues. Sometimes though, you see two HD sources on the same channel, although both have reduced quality.

These secondary channels are called “subchannels” and in broadcast parlance you’ll see the main channel designated (for example) as 7.1 while the subchannels are 7.2, 7.3, or so on. If your main channel is also standard definition, you can fit 8 or more program sources on one channel, and you’ll see this done with foreign language programming a lot.

It’s been hard to get pay-TV companies to pick up subchannels, so what you’ll find on them mostly appeals to people who watch live TV. That means folks over 35, and for people like that (for the record, I’m a person like that) it’s a goldmine of old television programs from years past. Channels like Decades, This TV, MeTV, Cozi TV, Buzzr, and getTV show the kind of reruns you grew up with, complete with cheezy per-inquiry advertising just like you remember it. Because subchannels are standard-definition (usually), old TV shows are a perfect program source.

Pay TV companies are slow to pick up subchannels because the same tricks that make them possible with TV broadcasts don’t work when you’re talking about cable or satellite TV, at least not to the same extent. You have to us a lot more equipment to get a subchannel onto a pay-TV channel, and you’d probably have to pay more money, too.

However, for those of us who like to actually settle down and watch live TV in the evening, it’s a genuine pleasure to tune to one of these subchannels and enjoy TV the way it was when we grew up. Best of all, they’re free!