Yesterday I told a friend about the pile of DTV converter boxes on my dining room table, and his face lit up with a look of eager excitement.  “High def boxes, you mean?”  No, I told him, DTV converters.  “But DTV is high definition, isn’t it?”  Not necessarily.  “Oh.  So... what is it really?”

That’s a very good question, and a good place to begin – before we start looking at the equipment, it makes sense to understand what digital TV is all about, so we know what the equipment is supposed to do.  So first off, let’s clear up what we’re talking about:

Standard Definition, or SD, is the traditional television format we’ve been watching for the last 60+ years.  All analog television is SD.

High Definition, or HD, has much more picture information than SD, so it shows far greater detail.  It is also widescreen, which is what most people notice first.

“DTV” just stands for Digital TV.  A DTV channel can carry HD, SD... or both at the same time.  Over-the-air DTV is sometimes called ATSC, after the name of the committee that decided how the system should work.

“Okay, so DTV can be HD... but more?  How does that work?”  I’ve invited Claire, a regrettably hairless expert on such things, to help me illustrate:

All television channels use the same amount of space:  you can fit one analog picture with sound into a channel,  or the same channel can hold more than 19 million bits of digital data every second.  Let’s build ourselves a new digital channel and see how it works...

All the open space between the white blocks is free for us to plug in whatever we want... so long as we can make it fit.  But before adding any programming, we need to set aside a small amount of room for housekeeping information that tells your digital television what programming is in the channel, and how to play it... this information is called PSIP, and that’s the little pink block on the right.  Now that the PSIP is there, your set’s electronic program guide will start working.

Nobody’s going to watch the program guide all day... we hope... so let’s put a real program in there.  Let’s really splurge, and make it a high def program:  the blue block network, perhaps:

Hmmm... that took up quite a bit of the channel, didn’t it?  That’s one of the limitations we have to work around: one decently encoded HD program takes up more than half of a DTV channel.

See, there just isn’t enough room for another HD program.  Sure, you could squeeze one of them way down to make it fit... but you’d just wind up with a rotten looking HD picture, which doesn’t make sense.  So... instead, let’s fill the space with a SD program:

SD programs take up much less space, and we can fit an excellent HD program and a very good SD program together easily.  In fact, we can add still another good quality SD program and completely fill the available space.

Remember at the beginning, when we said that DTV wasn’t necessarily high definition?  We could use our single channel to broadcast four very good quality SD programs... and have a little space left over for other data (that’s the little yellow block next to the pink PSIP).

How do we keep all of these programs straight, and how do we identify them?  Each program is called a “sub channel” – if the last picture is, say, channel 12 (I’m making one up, here), the red program would be channel 12.1, the yellow program would be channel 12.2, the white program would be channel 12.3, and the blue program would be channel 12.4.

So what does WSYR-DT look like right now?

Channel 9-1 is our main high definition ABC programming; channel 9-2 is standard definition VTV network, plus time-shifted news and other local programming.

It’s time for Claire to go back up to her box in the attic, so let’s wrap this up with the main points:

• DTV is just how digital programs are delivered to your TV set;
• DTV can be high definition, but doesn’t have to be;
• One DTV channel often includes several programs.

It’s worth pointing out that even a standard definition program, transmitted digitally, looks far better than the same program transmitted with the old analog system.  So what do we get with the switch to DTV?  More programming choices free over the air... some in high definition... and far better pictures and sound for standard definition programs.  Will it take some getting used to?  Sure... but not a lot, and once you see it you won’t want to go back!

Now that we’ve gotten past all of that... on to the converter boxes!

-- Jeff

p.s.:  For the curious, some of those Lego pieces are about 45 years old.  It seemed fitting to use a prop dating back to the start of color television in Syracuse, to illustrate the transition to digital television now.  Claire, the regrettably hairless expert, is my daughter’s far more recent addition...