Television -- today's the day!

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Television -- today's the day! Empty Television -- today's the day!

Post by Gary Tayman on Tue Feb 17, 2009 8:42 pm

Okay, technically the day has been pushed off till June, but the vast majority of TV stations have already either turned off their analog signals, or intend to do it at midnight tonight. So an era has passed, and digital TV is reality.

I'm all for progress. I love my LCD TV and my LCD computer screens, and in the case of the latter, couldn't wait to get the old CRT bombs out of the house. I have something like 100 hi-def channels, and hate to watch programs in standard-def if I can help it. Congratulations to the industry for making things better.

BUT -- this is quite intimidating for this old CET, who used to be able to tell you everything you wanted to know about the television system. As of today, for me it's officially a black box. What's inside? Magic. Either that or a cast of little people who love to sing, dance, and act. Ask me how a television works, and I have to shrug my shoulders; I have no idea.

So -- can anybody tell me, what's in the new plan? Okay, so it's digital. Okay, so the new system allows four standard-def channels or one hi-def, in the same space as the former analog. But how's it done? How does it work?

I can assume there's a carrier, and I can assume it's a single carrier that contains both the picture and sound information. Are the new channel allocations the same as the old? Are the channel numbers still in the same places, with channel 1 missing? Did they finally manage to stick the carrier in the middle of the channel, or is it offset in some way?

I can assume there's a digital stream modulated onto the single carrier. Is it AM, FM, or some sort of phase shift? With a dual sideband signal, the channel is 6 mHz wide, so is there a 3 mHz clock for the digital stream? Seems kinda small; I'm sure it's done another way.

Once this stream makes it to the receiver, how is it decoded? Here I can also assume the signal is done in "blocks", and these blocks are rearranged somewhat, as with CD's, so that if there's a scratch or interruption, the lost information can be reconstructed.

How many lines top to bottom? How many pixels left to right? What's the refresh rate? Is the scanning interlaced, or full scan? Is each scan done twice so as to reduce flicker, as with a projector? Are horizontal and vertical blanking/sync done in a similar manner as with analog, or another way?

Any information you can give me, to satisfy my curiosity, would be appreciated.
Gary Tayman
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Post by exray on Wed Feb 18, 2009 9:12 am

Gary, here is a pretty good write up about 8VSB transmission.
http://www.broadcast.net/~sbe1/8vsb/8vsb.htm

It gets pretty deep but there's not much easier way to explain the components and there's not always an analog analogy.b Worth noting is that 8VSB is the rf transmission format and mpeg-II is the actual video format.

From the RF side, yes, all the channels are the same frequency wise and they are 6 Mhz wide just as before. There is no 'carrier' per se. The data packets are scattered thru the 6 Mhz space somewhat randomly so that the bandpass is utilized most efficiently.

The ability to have say 4 video streams going simultaneously is analogous to loading 4 web pages on your computer at the same time in that as long as you have adequate "bandwidth" then all of the information can be passed without the packets stepping all over themselves and causing slowdowns.

The 8VSB scheme is unique to broadcast televislon. Cable and satellite use QAM which adds phase modulation to the equation and adds somewhat to its capacity.

HTH,
Bill
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Post by Don Cavey on Thu Feb 19, 2009 8:46 pm

In a word, "Yup!"

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