Is amplitude modulation irrelevant in (modern) digital TV antenna systems?

Thread Starter

#12

Joined Nov 30, 2010
18,210
This is kind of a, "Confirm or deny my theory" situation. I have a TV antenna on a pole. I have an antenna amplifier to send the signals down through a couple of splitters and about 70 feet of "RG-6 Quadshield" (co-ax). I am going to change the configuration of the antenna heads because one of the local TV transmitters moved and now all local transmitters are between 91 degrees and 94 degrees (east of me).

Now that all over-the-air signals are digitized, I suspect that there is no information carried in the amplitude, therefore turning the amplifier up to where the envelope of the signals hits the DC rails should not be detrimental. Am I correct?

Under one theory, I would adjust the amplifier until only the strongest channel signal will hit the rails of the amplifier and almost all the other channels will be of lesser amplitude. Under a second theory, I could just turn the amplifier to eleven and the channels which are "pegged" and then some, will still be entirely useful. Am I correct?
 

nsaspook

Joined Aug 27, 2009
7,298
No, it's not irrelevant in 8VSB modulation. 8vsb is a ASK modulation method that uses trellis coded modulation for a raw 32 Mbit/s before error correction down to 19.39 Mbit/s of usable data on a 6MHz channel.


What you want to a clean signal (not amplitude limited by antenna amplifier gain) with the proper S/N ratio to reduce bit error rates below the forward error correction limit. I use a Winegard CM7777 preamp at the masthead with a RF attenuator to provide a quality signal to the house distribution network via a network of splitters. The receiver signal 'quality' on most units is a digital quality instead of RF but they are related.
https://www.channelmaster.com/TV_Antenna_Preamplifier_p/cm-7777.htm



https://www.solidsignal.com/c.asp?c=3839&d=TV-Antennas-and-Supplies&utm_campaign=google_adwords_sitelink_tv_antennas_supplies&utm_source=google&utm_medium=cpc&gclid=EAIaIQobChMImPzJr4an2QIVCoV-Ch1ypgzdEAAYASABEgKbd_D_BwE
 
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Thread Starter

#12

Joined Nov 30, 2010
18,210
That seems like a hard, "no" but you kind of muddied up the water.
I have clean signals and good s/n ratio. I'm only using the amplifier to drive the losses in the splitters and the cable length.
It seems to me that I should turn the amplifier up until I find the envelope defined by the DC rails, then turn it down a tad.
If I get AGC symptoms at the TV, I would turn the gain down some more.
Right?
 

nsaspook

Joined Aug 27, 2009
7,298
That seems like a hard, "no" but you kind of muddied up the water.
I have clean signals and good s/n ratio. I'm only using the amplifier to drive the losses in the splitters and the cable length.
It seems to me that I should turn the amplifier up until I find the envelope defined by the DC rails, then turn it down a tad.
If I get AGC symptoms at the TV, I would turn the gain down some more.
Right?
That's basically right but 8vsb is also prone to phase errors from multi-path. A too strong linear signal could also have a strong multi-path element that would completely destroy the digital signal even with good RF levels. IF you see signal loss in high winds or when airplanes pass near you might want to reduce the gain a little if you have plenty to spare. The latest TV receivers (5 gen+) are much better than the older 2nd gen cards I started with before the complete change over to DTV.

https://www.creativeplanetnetwork.com/news-features/distribution-digital-television-signals-367557
 
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Thread Starter

#12

Joined Nov 30, 2010
18,210
I'm good on the multi-path thing. No significant buildings (reflectors) in the way. (Geologically speaking, Florida resembles a big sand bar.) This isn't a problem installation, it's just a theory investigation, an educational opportunity. Antenna aiming websites tell me I just barely need an antenna, so that tells you about s/n ratio. The closest thing to a, "problem" is that I used to install TV antennas for my day job so I have what it takes laying around. I'm just checking to see if I can cause a problem with my ignorance.;)

Thank you.
 

crutschow

Joined Mar 14, 2008
24,988
So even though the modulating signal is digital, 8-VSB is a form of amplitude modulating the carrier.
What's interesting is that the purpose of these complex modulation schemes is to uniformly use as much of the carrier bandwidth as possible, i.e. the signal looks rather like white noise.
That's how they can get 32Mbit/s into a 6MHz bandwidth.

I remember following the early days of HDTV development where they were stymied by now to transmit 7 times the analog picture information in the standard US 6MHz analog bandwidth, which the FCC wanted for compatibility. The minimum seemed to be using some form of compression with two 6MHz channels.
Then the development of digital picture compression techniques such as MPEG-2, and advanced digital modulation schemes showed that all that info could be transmitted digitally in the 6MHz bandwidth, killing the analog schemes.
 

nsaspook

Joined Aug 27, 2009
7,298
The amplitude component is rarely irrelevant in modern modulation schemes. 8-VSB is an odd duck in the world of digital data transmissions because IMO US TV stations wanted to maintain most of their investment in transmitter gear but the old gear was really not suited for full range I/Q modulation without expensive modifications (increased power, phase linearity and channel interference) so they concentrated on the one side of the I/Q modulation equation leaving an 'Achilles heel' for 8-VSB that resulted in a huge battle with COFDM-based DVB-T systems most of the world uses. On the positive side for 8-VSB, receiver technology has made things better but it can't overcome the basic problem of the total inability to receive 8-VBS when in motion (like in a moving car).
http://www.tvtechnology.com/expertise/0003/comparing-8vsb-and-cofdm-for-dtv-terrestrial-broadcasting/183711
https://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#COFDM_vs_VSB

 
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Thread Starter

#12

Joined Nov 30, 2010
18,210
I picked up a tiny bit of education while employed as a military radio repairman (at the manufacturing site) but there were only two functions to my job: 1) Find little mistakes like solder bridges or a wrong part installed in a circuit board, and 2) black box replacement. That level of functionality was achievable with zero understanding of the modulation methods.:(

What I want you to understand is that I'm paying attention to this conversation and it is very interesting to me.:)
Please continue, in spite of the fact that most of what I can contribute is ignorance.
 

nsaspook

Joined Aug 27, 2009
7,298
You ask the questions and we will try to find the answers.
crystal_ball_fortune_teller_new.gif

https://www.anandtech.com/show/12308/atsc-releases-atsc-30-digital-tv-standards-suite-internet-tv
And going hand-in-hand is finally embracing the OFDM modulation method, which is generally better in non-line-of-sight scenarios, over ATSC 1.0’s 8VSB. Where 8VSB has a 19.3Mb/s bit rate, ATSC 3.0’s OFDM-based method, with more efficient LDPC forward error checking codes, has several modes from the most-robust 1Mb/s to the highest-capacity 57Mb/s. But putting aside capacity, the different modes allow simultaneous support of fixed and mobile devices.
 
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Thread Starter

#12

Joined Nov 30, 2010
18,210
Speaking as a pragmatist, I got the information I needed to rebuild and adjust my TV antenna system in post #2. Everything after that is general education. (I am too ignorant to get specific with my questions.) For instance, the "question" by crutschow (post #6) contained more answers than questions to my POV.
 

nsaspook

Joined Aug 27, 2009
7,298
I get just about perfect HDTV reception with an old Radio Shack antenna but the poor guy has been beat up over the years from wind and ice storms so I'm just about ready to rebuild my antenna system too.

Its lost a few VHF elements since this picture was taken.
 
I'm almost doing the same thing, but I'm using taps. Something did go wrong in my distribution, but I haven;t had the time to track it down.

I do have the ability to measure the 8VSB signal level and BER (Bit error rate). I also haven;t had the right circumstances to actually set the compass heading of the rotator.

I definitely don't have enough signal and the distribution amp has lost the ability to provide some amount of variable gain.

Besides, Samsung hasn't a clue how to design ATSC tuners. The System 6 TV hasn't worked properly since day1 and the first firmware update broke it even more. Closed-captioning routinely disappears and the joystick doesn't work. If you check to see if CC is enabled, it fixes itself for a while. If you channel bump, it fixes it for a while. If Samsung connect to the TV, it fixes both problems for a while. You can't roll back the firmware either.

Serious flaws are:
1) You can't add channels. Only delete all and add is available.
2) You can't tune non-scanned channels by the physical channel number.

To make things really interesting, the remote uses IR to turn the TV on and OFF and Bluetooth for everything else.
 

Thread Starter

#12

Joined Nov 30, 2010
18,210
I'm glad I don't live in (post#11). I can do a gigantic head with a rotor and an amplifier, but all I need is something on a pole, pointed at about 93 degrees magnetic. After that, I cause my own problems by trying to supply 3 rooms in a long house (with a good signal).

I have a box which howls a tone based on signal strength of any one channel and I can use that to aim the antenna. Aiming used to be a slam-dunk. Now, it's more like, "Keep a pipe wrench by the TV. You're going to need it on a foggy night." That's the definition of, "something is wrong". So I checked my stock of RG-6 crimp connectors and bought some all-thread to make new pipe clamps for the mast, and maybe the head, depending on the rust quality.

I'll probably do it Monday. Right now, the neighbor is having a garage sale and there are too many spectators to suit me. I don't need any distractions while I'm on the roof.
 

crutschow

Joined Mar 14, 2008
24,988
They used to use 300Ω twin-lead from the antenna since it was a better match to the antenna impedance and had a much lower loss at UHF than coax.
Would that be still a viable consideration for fringe reception or does it have too many other negatives, such as being more susceptible to EMI, to be considered?
 

nsaspook

Joined Aug 27, 2009
7,298
I'm glad I don't live in (post#11). I can do a gigantic head with a rotor and an amplifier, but all I need is something on a pole, pointed at about 93 degrees magnetic. After that, I cause my own problems by trying to supply 3 rooms in a long house (with a good signal).
My house looks like a snoop shop with all the antennas and arrays.

I don't need the huge antenna for raw gain, I need it for the directional pattern because I live in a really bad spot for multi-path from aircraft. We're talking several second dropouts with planes in the PDX glideslope when it's down the river. My antenna is placed off center from the towers so I have a deep null in the antenna pattern facing most of the reflected RF energy from flyovers.
 

Thread Starter

#12

Joined Nov 30, 2010
18,210
My house looks like a snoop shop with all the antennas and arrays.
I think you spelled that wrong. It looks like a spook shop with all those antennas.:D
I don't need the huge antenna for raw gain, I need it for the directional pattern
Reminder of Orange County, with Saddleback Mountain reflecting the L.A. transmitter information into the back-side of the Yagis with a time delay. So I used Log-Periodic antennas (with about a 5 degree aperture), aimed at the L.A. antenna farm, about 20 degrees west of north.

Same for the Pasedena neighborhood in Pinellas County Florida. One big, honking, 6 story apartment building scattering TV signals all over the place.:mad:

Central Indiana? No such problem. A couple of bow tie antennas to catch the two nearest cities across miles and miles of corn fields.
Central Kentucky? :mad::mad::mad: Mountains in every direction. Fifty feet up wasn't enough half the time, and a rotor was mandatory.
The results were still random.:(

And so I reveal another aspect of my life as a roving tron-tech.:p
 

Thread Starter

#12

Joined Nov 30, 2010
18,210
Its SNOOPIE. Those guys are normally up high near the antennas during a possible contact.
Do you have any idea what this looks like to a "civilian"? Let me give you an idea:
THREAT LEVEL PIZZA DELIVERY! CHILD'S KITE CRISIS! LT. CMDR. PANTIES IN A TWIST!
Nuclear Powered Supercarrier, over a thousand feet long, 20 stories high, carries enough fire power to level New York City in 20 minutes...afraid of row boats.
Would you like another billion dollars with that order, sir?
Better say, "yes". Your country is almost bankrupt from delivering million dollar drone strikes all over the world...on credit.
 

nsaspook

Joined Aug 27, 2009
7,298
Do you have any idea what this looks like to a "civilian"? Let me give you an idea:
THREAT LEVEL PIZZA DELIVERY! CHILD'S KITE CRISIS! LT. CMDR. PANTIES IN A TWIST!
Nuclear Powered Supercarrier, over a thousand feet long, 20 stories high, carries enough fire power to level New York City in 20 minutes...afraid of row boats.
Would you like another billion dollars with that order, sir?
Better say, "yes". Your country is almost bankrupt from delivering million dollar drone strikes all over the world...on credit.
You can see a lot on radar but you can't see if it's, PIZZA DELIVERY!

http://www.cnn.com/2013/09/18/world/meast/uss-cole-bombing-fast-facts/

It's mainly for intelligence, they fly a helo like the one on my avatar to take pictures and we take pictures right back.
 
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