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Discussion in 'Over the Air TV By RabbitEars.Info' started by comfortably_numb, Feb 4, 2019.
Too bad nobody has an ATSC 3.0 tuner that can pick it up.
Because it is SFN, it seems likely that there will be some locale restrictions regarding who is going to be able to see a signal.
Wouldn't SFN require some sort of directable receive antenna to make sure you get exactly one signal?
I don't know for sure. It seems almost like they believe in magic with ATSC 3.0, when we all know that ATSC 1.0 reception is Voodoo as it is! lol
Basically an SFN will consist of several transmitters that are 'locked' together to prevent them interfering with each other. If anyone had service from MediaFLO a few years ago it worked on this principle. In the case of FLO TV, the engineering part of the concept worked well but the business model was flawed. Had the broadcast industry been allowed to adopt the more efficent, more robust and generally better OFDM modulation instead of 8VSB being forced by the FCC back when the standards were being decided we would be that much farther down the road now. ATSC 3.0 using OFDM modulation is the way of the future if broadcast TV is to maintain relevance in the long term.
Are ODFM transmissions easier to tune at greater distances than 8VSB?
In my experience with FLO TV, having been the contract engineer for the service in Little Rock, AR for a few years, the issue was improved portability and building penetration as much as anything. An apples to apples comparison is difficult to say due to the fact that I was using a cell phone to receive the OTA signal (it was on UHF channel 55) with no outboard antenna. I can say that I had no problem viewing it sitting in my living room (30 or so miles from the transmitter) or really anywhere in the buildings where I tried it. ATSC 3.0 is really data casting where multiple streams of TV signals are included in the data.
Here are a few of my own observations of ATSC 3.0.
ATSC 3.0 will be easier to receive than 8VSB. This is primarily because multi path hurts reception of 8VSB but multipath can actually help 3.0. This is because 3.0 is comprised of multiple narrowband signals and 8VSB is one wideband signal.
The original 8VSB standard was known as the “Grand Alliance” because it contained parts of each of the original proponents patents. None of the original proponents proposed COFDM, so there was no way to include OFDM without protracted litigation.
Today’s COFDM is a vast improvement over the one proposed in the 1990’s. The new 3.0 signal to noise ratio is lower due to LDPC (low density parity checks). The video compression (HEVC) is about 4 times more efficient than the original MPEG2.
One of the features of ATSC 3.0 is that each station decides their own data rate and modulation parameters. For instance, one station may opt for slower data rates and robust modulation. This would easily outperform the current digital standard. Another station may opt for higher data rates and less robust modulation for 4K resolution. This would offer coverage similar to today’s 8VSB signal. Station three could choose an SFN and then offer better coverage than 8VSB at data rates that offer multiple 1080 streams. All of these stations could add a lower resolution stream that is ideal for mobile or handheld reception at extremely low signal levels. It is even possible to tie the robust mobile/ handheld audio stream to the HD or 4K video stream in order to avoid audio stutter when the signal is weak enough to cause the picture to freeze.
Stations may opt to start out with a low data rate robust stream using a single transmitter and speed up the data rates as they add SFN transmitters.
For a SFN to work, all transmitters must transmit the same bits in unison. Any signal picked up from a second or third transmitter will add to the signal received. There is no need for the consumer to aim at a particular transmitter.
One thing to consider; MediaFLO used many 50 KW transmitters with some lower power fill-ins. Broadcasters will be running up to 1000 KW on a main signal with SFN transmitters as high as 150 KW.
The transmitted signal is organized as IP packets instead of an ASI stream. This allows use of the signal in multiple ways, many of which are yet to be considered.
Great discussion and it feels like the specifics of 3.0 are coming into view
Just be aware Texas is well known for Tropo ducting! Great for TV DXing!
May mess up SFN sometime from time to time.
Interference patterns happen and to suggest that any technology is only helped by them is a tough story to buy into. A node (the physical location where two identical waves precisely cancel each other out) is a node no matter the modulation scheme. I reason that if a node happens to be at the antenna location, no amount of technology downstream is going to turn a nulled signal into a usable signal. I sense that some laws of Physics may be being ignored for the greater joy-joy.
The wavelength of channel 18 is around .6 meters. That's a lot of nodes per square mile if there are only two transmitters seen in an area. Spacial diversity is a double-edged sword if the receiving antenna configuration isn't also diverse.
Start buying into OFDM SFNs. Here’s one explanation:
3.4.4. Guard interval
In an SFN each transmitter is required to radiate the same OFDM symbol at the same time. This comes from the fact that echoes (natural or artificially generated by co-channel transmitters) shall be confined in the guard interval period. The OFDM receiver has to setup a time-window during which it samples the on-air OFDM signal. The objective is to synchronize this time-window with the useful period of the OFDM symbol. Accordingly, it will ignore the signal during the guard interval period where the receiver signal is made of a mixture of two or more OFDM symbols. If the transmitters deliver the same OFDM symbol at the same instant, or with a sufficiently small time delay, the differential propagation path delay to the OFDM receiver will remain inside the guard interval period. Accordingly, the sum of the received signals will be constructive because they constitute the same OFDM symbol (no inter-symbol interference), see also section 2.
(From EBU BPN 066)
I'm not sure how this answers the question of an antenna located at a node where there is a cancelled signal.
Remember that there are 2000 or more individual carriers in a OFDM signal. Your fear of nodes may exist on single frequencies with no modulation, but never all carriers. Remember that modulation changes frequencies and amplitudes of each carrier. The node disappears due to time differences between transmitters. Even then, the transmitted data is time interleaved and frequency multiplexed. Any missing data can be reconstructed.
Remember that it is nearly impossible to remember things that haven't been offered in the first place. OFDM modulation technology isn't something that is easy to get documentation for and we Americans aren't real up on the technology that they've been using elsewhere on the planet for the last 12 years (DVB-T2).
Won't something that is close enough get mostly swallowed up in a conventional (horizontally oriented) TV antenna? The wavelength difference is something like 7.3mm from the bottom of channel 18 to the top. I can see where a vertical antenna might look like a point, but an antenna oriented any other way can't be treated that way, can it?
Is this something like sending 2000 different signals on <3KHz wide bands or are these carriers riding on a larger frequency slice?
If it is just too hard to boil it down, you can say that too.