Originally Posted by mrvideo
Off hand, do you know what the bitrate is for the NBC HD transponder? There are two program streams?
From there I can kinda figure out what the average video bitrate is, unless you know that as well.
WARNING! Skip this post if you don't like tech-speak...
Sorry, I don't know the NBC HD transponder bitrate. I think the symbol rate is in the 26 megasymbol/sec range. At least on the east coast feed, they also multiplex in the NBC Weather Plus feed to those affiliates who carry that on their digital multicast channels.
Analog vs digital power? The 8VSB digital modulation scheme is receiveable down at a lower signal level than analog. So the FCC set lower power levels for stations' digital transmitters to achieve comparable coverage with analog. As far as I know all Madison stations are operating at their authorized power level or applied for more. The Commish authorized WMTV-DT for 47 kW effective radiated power. For 3 years we were on at 21 kW until the new tower went up with the new antenna. We applied for 56 kW and got it. By comparison WMTV's analog power is 890 kW. Even back when we were at 21 kW on digital, we were getting reception reports from Orangeville and Rockford IL.
I think the other question was about 8VSB and noise. On standard analog TV, any noise or interference is in the same "domain" as the amplitude modulated video...which means you'd see it in the picture. One of my instructors in college used the analogy of a little guy with a flashlight inside your TV. He scans across the screen and turns the brightness of the flashlight up and down to paint the picture. A noise burst fakes out the receiver and messes up that portion of the scan line by making it lighter or darker than the intended video. A lightning bolt, vacuum cleaner, noisy fluorescent light ballast, or the spark plugs on the Harley blasting down your street corresponds to disruptions in that scan line of video.
Audio is FM, frequency modulated (different "domain"), so that that kind of noise normally does not affect the audio.
With digital TV, the little guy with the flashlight scanning across the screen has been replaced by 1080 rows of 1920 flashlights operators in each row. (Or whatever your TV's pixel specs are...) They're waiting for an instruction list of how bright to make each pixel (think Excel spreadsheet). So now we have to transmit bytes instead of just a varying analog brightness voltage. (See MPEG-2 discussion earlier in this thread.)
8VSB describes a digital symbol by crossing one of 8 voltage amplitude levels at a particular moment in time. So it IS still AM, sort of. With enough noise or interference, the receiver can't find those 8 distinct voltage points. But 8VSB also uses several methods of coding and data spreading to make the bit stream more robust in the presence of noise. A REALLY simplified example:
If the original video data is:
8VSB adds some duplication and spreads the data out so you get:
Neighbor's air conditioning kicks in and you actually receive:
Your receiver unscrambles the stream:
And can reconstruct the missing D from the duplicate data...and it doesn't care about the other duplicate byte that was missed.
Usually there's enough error correction that the receiver can reconstruct the missing pieces of the "spreadsheet." But if the noise or interference lasts long enough that the receiver can't correct the errors, the TV just blanks out with no explanation to you.
At least with analog you usually get some kind of picture...
I hope that was on-topic and not too geeky.