You might not have noticed that HDTV consists of a lot more information than SDTV. Eventually, Nellie the Neuron is going to use that line to pull me around to ranting about the latest plans to fix DTV. But first, I've got a thing or three to say about video compression.
We didn't always live in an age of bit-rate-reduced video. I mean, I can remember when folks first wanted to transmit video digitally between cities, the phone companies were suggesting T-3 at around 45 Mbps and network transmission engineers were complaining that was too low – even for NTSC!
Now, we're delivering HDTV to homes and even to some stations at under 20 Mbps and no one complains if a network uses T-3 rates for its HD distribution. What the heck. Uncompressed HD is "only" 1188 Mbps – and that's 8-bit "4:2:2."
There used to be a big call for "mezzanine" HD compression at around 300 Mbps or so. But I ain't seen much of that lately unless it's internal to an HD D-5 machine.
It ain't so bad to use 45 Mbps for HD distribution, on account of compression technology being one whole heck of a lot better today than it was back when we started cramming NTSC into bits. And it'll get better still.
Methinks I remember Bill Glenn giving a SMPTE paper once in which he said you could probably squeeze a 200:1 compression ratio, visually. losslessly out of HD if you played every trick just right. So, let me see: 1188 Mbps divided by 200 – two goes into 11 ... is 5.94 Mbps. Call it six.
THREE FOR THE PRICE OF ONE
That's pretty neat-o! Just think of it! At that rate, you could squeeze three HD signals into a single ATSC bitstream with room left over for audio and PSIP. Way, way cool!
Maybe, someday before we're all dead, someone will get to do that. But it ain't going to happen this year. Or next. Or the next. Or ...
"But, Mario, you just said it could be done!"
I said that Bill Glenn said it could be done if you played every trick just right. But Our Beloved Commish does not allow us to play every trick just right. Our Beloved Commish issued its DTV rules in April 1997 and said rules say you have to use ATSC A/53 (minus a couple of tidbits). And A/53 says you've got to use MPEG-2.
Hey – no doubt about it – MPEG-2 encoders get better every day. But Nellie doesn't think an ATSC-legal encoder is ever going to squeeze good-looking HD down below 6 Mbps.
I wouldn't blame you a least-significant bit for thinking "So what?" right about now. ATSC encoders do a reasonable job of squeezing HD into around 18 Mbps and that's enough room to fit in an FCC channel with space for audio and PSIP, too. And, when the encoders get better, even basketball games in HD should look pretty good. But maybe we won't have a full 18 Mbps.
Nellie promised, and Nellie is delivering. You probably couldn't help but notice that NxtWave and Zenith have formed an alliance on enhanced 8-VSB. Here's a blurb from the press release:
"Specifically, the joint system combines NxtWave's error-correction coding and pre-coder solution with Zenith's dataframe mapping, interleaving and packing algorithm."
Now then, unless I'm way off base here, that stuff sounds like what NxtWave and Zenith were both pushing at this year's NAB show. The idea is to transmit some data pretty robustly and the rest sort of normally (with maybe some benefits due to enhanced training, for receivers that can deal with it).
Even at the NAB show, the NxtWave and Zenith systems looked pretty danged similar in the ways they worked. That's probably why they could join forces.
"But, Mario, what does it all mean?"
That's one heck of a good question and I ain't sure I've got the right answer, but maybe I've got the right questions:
1. What is the purpose of DTV?
2. Who's expected to receive what part of DTV via antenna?
DIGITAL TRANSMISSION
First question first. There are plenty of folks in Congress, the TV broadcasting biz and the TV set-making biz who'll say the purpose of DTV is HDTV. And there's a bunch of history to back them up. When Our Beloved Commish launched the inquiry that ended in the April 1997 DTV rules, digits had nothing to do with it. The idea was to transmit HDTV to homes. Period.
But after GI sent in its way of transmitting HDTV digitally, the HDTV inquiry became the DTV inquiry. And, when Our Beloved Commish issued the DTV rules, one of the things they specified was that no station needed to transmit any HDTV. Period.
Now then, I might not be a communications lawyer (if I said I was or wasn't, you might get too big a clue to my secret identity this month), but I've got an interpretation of Our Beloved Commish's DTV rules. If you're currently transmitting your NTSC off of a 1972-vintage U-matic, you may transmit your DTV off of that same U-matic, with the blessings of the FCC.
Of course, if all you want is to transmit U-matic-quality SDTV, you don't need the whole 19.4 Mbps of the DTV channel. So, maybe you can use the rest for more channels of ad-supported free-to-view programming or pay-per-view movies or hot Internet pages (which could be "hot" in more than one sense of the word). Our Beloved Commish said last month that even public broadcasters were allowed to make some moolah off of some of their excess DTV capacity.
Now then, Paxson says cable ops ought to carry whatever is in a DTV channel, whether it's HD or a multicast. So far, Our Beloved Commish says ixnay on the ulticastmay. Cable ops – so far, anyway – need to carry HD if an HD program is on the DTV channel; but, HD or otherwise, they just have to carry one video service – no multicasts.
So, along come NxtWave and Zenith. The fact that they've submitted "enhanced" VSB proposals means they're willing to admit that DTV without cable (or satellite) ain't guaranteed to be received. But take another gander at tables 1 and 2.
If all we want to get out of the NxtWave/Zenith system is an improved training sequence, that ain't going to use up a lot of data rate. But it's also still probably going to leave a bunch of DTV viewers without reliable reception, especially indoors.
If we want to make sure that datacasters can pretty reliably deliver an ISDN-line's worth of data – around 100 kBps, maybe – then we're still way up near the tops of tables 1 and 2. We've still got around 19 Mbps of normal data rate to use to deliver HDTV or multicast video. But, if we want to make sure that viewers can receive at least SDTV pictures – (fizzle) (crackle) (sizzle) – whoa, Nellie!
What's a reasonable amount of MPEG-2 for SDTV? 2 Mbps? 4 Mbps?
According to tables 1 and 2, if you want to deliver even just 2 Mbps of robust data, you're down to under 14.5 Mbps of normal data – 14 Mbps for HDTV if you really squeeze hard. If you want 4 Mbps of robust data, you're down to under 10 Mbps of normal data – including audio and PSIP. Is that enough for HDTV? Not in my book. Not today, anyhow.
So, if your answer to question 2 involves reasonable-quality TV programming for all, your answer to question 1 better not be HDTV. At least not with this latest DTV fix.
CATEGORIES
LATEST ARTICLES