SNR level necessary for ATSC reception? - Page 2 - Canadian TV, Computing and Home Theatre Forums

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post #16 of 58 (permalink) Old 2010-03-09, 07:09 PM
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Holl_ands,

never a dull moment reading your posts.
Gonna make myself an entire folder of
"Holl_ands" Bookmarks. Hehe
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post #17 of 58 (permalink) Old 2010-03-15, 04:25 PM
 
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DTV Noise

Quote:
I truely believe ther are factores out there that are causing signal flucktuations that are not yet known or corrected.
Digital TV reception is all about the signal-to-noise ratio. Some stations are broadcasting with a higher noise floor than others. Every transmission system adds noise to the signal, and some add more noise than others, the more noise a signal has coming off the transmitter, the less external interference will be tolerated by the receiver. That's why you can sometimes get better reception of a station that has a lower ERP than you can from one with a higher ERP -- eventhough they're both broadcasting from the same tower site.

You may recall a few weeks back that CIVI (/A\ Channel on analog 17) had a bunch of strange noise on it's analog signal -- and from what people were saying here it appears that noise was coming from the transmitter itself because everybody was seeing it regardless of where they lived. If that had been a digital signal, some people would have continued to get reception as usual, but others would have experienced break-ups or totally lost reception -- and we'd all be here chasing our tails trying to figure out what was going on.

One of my pet peaves is that there is no standardization in digital tuners. The "signal" or "quality" indicators on the different tuners don't measure the same things. Some receivers measure raw signal strength, others measure signal-to-noise, and some measure error correction. So what would happen here if a bunch of noise was introduced into a digital signal at the transmitter is that we'd all be totally confused.

Some people would report that they still had a 90% lock on the signal, and the ones who lost signal would be up on the roof inspecting their rig for nothing. You know, not every station around here has the test equipment needed to measure signal-to-noise coming off their transmitter. They can measure modulation, power output, and they can analyse their transport stream -- but they don't always know where their noise floor is at.
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post #18 of 58 (permalink) Old 2011-09-06, 05:02 PM
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Hi everyone, I've enjoyed reading and taking in the wealth of knowledge posted in this thread.

My HDTV (ATSC) experience is almost nil, so the learning curve is high I am however very comfortable with FM transmission)

On this topic of SNR, here is a scenario I'd love to get some feedback on....

Our station just went to air on RF ch 8 with a 6 KW Harris VAX Platinum transmitter.

We are in the middle of the Canadian Prairie, so flat is normal. Lat/long:

Our antennae is 700 Ft up, omnidirectional.

I've just finished a Harris TX course where they talked about the 15 dB cut off for ATSC. It sound kinda high to me, but hey, if that's what the man says, it's the way it is!

Here's where I try to impress the boss... and sales dept

I want to devise a field strength test scenario which takes this magical 15 dB cut off into consideration.

So the question are:

1) What are ideal atmospheric conditions for HDTV transmission? (*remember that flat terrain is the norm here)

2) Under ideal atmospheric conditions, how far can I expect a usable signal to reach before I hit the 15 dB threshold? (remember FLAT terrain here)

3) Antennae type. Here we could go into the hdtv antennae vs standard dipole, so your best advice as to which type of antennae to use is greatly appreciated.

4) I'll be working out of the tek truck. What height from the roof should the antennae be mounted? Want to minimize any reflections/multi-path which could adversely affect test results.

Well, I hope I've covered the basics but if you think of anything I've missed, please let me know.

Thanks ahead of time. This should be good!

G
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post #19 of 58 (permalink) Old 2011-09-06, 07:43 PM
 
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Based on your screen name and "6 kW transmitter" I'm guessing you're with CKCK. Here's a coverage map from my website: http://www.rabbitears.info/contour.p...=1285569&map=Y

1) Depends on what you mean by "ideal." In the right conditions, signals can be received over paths of several hundred of miles, whether analog or digital, but that's once every so often. Atmospheric conditions can provide varying amounts of enhancement at various times, with shorter ranges certainly being most common.

2) In the above-linked coverage map, a good outdoor antenna should easily include all of the colored areas, and probably somewhat outside that as well, depending on local terrain.

3) For upper-VHF, either the YA1713 or the Y10-7-13 would be best. I know some people who have done side-by-side tests and I can't remember which one was better.

4) Typically, coverage predictions are made assuming a height of 9 meters or so. My map, above, assumes a height of 4 meters to better account for indoor antennas. I imagine anything in that range is probably acceptable, though in areas with buildings, you will want to be higher than the buildings.

Others more knowledgeable than I should be able to chime in.

- Trip

RabbitEars.Info
Comments are my own and not that of my employer (the FCC) or anyone else.
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post #20 of 58 (permalink) Old 2011-09-06, 08:04 PM
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Your not likely gonna see acceptable results if your tek truck is mobile.
i.e. your gonna wanna do your testing while stationary due to doppler effects...
Grab samples at as many locations as ya can, but test while stationary.

Mobile ATSC has improvements to overcome all that, but suspecting your using a standard ATSC receiver and not M-ATSC?

Also maybe take a look at Hepburn's site to determine what the "weather" is gonna be like while your testing ahead of time, so ya can
plan accordingly. i.e. - ya may or may not wanna test on days predicted to have excessive propagation.
or maybe ya do it twice, once on a "normal" day, and again when medium to extreme tropo is predicted.
That way you get a feel for coverage under both extremes.

http://www.dxinfocentre.com/tropo.html

As for antennas, just my opinion, I don't think it's reasonable for a broadcaster to assume the end user has a yagi mounted at 9 or 10 m height.
Maybe use two antennas, to grab 2 sets of data. A set of telescoping rabbit ears extended to be optimal at your transmitter frequency.
And a VHF Hi yagi such as Trip suggests. Regardless what assumptions were used planning the coverage, that way you might gain a sense of the
real world coverage for both types of users using low gain indoor antennas, or a higher gain more directional antenna.

DB4E/VHF Yagi rotor FM Bandstop ap-8700 preamp 4way split LG lcd.
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post #21 of 58 (permalink) Old 2011-09-07, 06:22 AM
 
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Example of measurement campaign

This is an example of an engineering report filed by a VHF station to justify higher transmitter power to the FCC. Descriptions of the measurement equipment, comparisons to predictions, and displays of signal captures are included.

OTA, it's a beauty way to go!
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post #22 of 58 (permalink) Old 2011-09-07, 09:05 AM
 
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Quote:
Holl_ands,

never a dull moment reading your posts.
Gonna make myself an entire folder of
"Holl_ands" Bookmarks. Hehe
I thought I was the only one that did this, boy was I wrong.

roof mounted 35ft UHF Antennas Direct XG91, VHF Delhi VIP 302SR, RC 6292, Tii Coaxial Lightning Surge Protection
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post #23 of 58 (permalink) Old 2011-09-07, 12:34 PM
 
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Agreed! I was able to find the paper cited by Holl_ands, and it makes me wonder whether I will be able to receive WNYF-LD, being only 3.3 miles from the HC site.

I thought that I was in an enviable position, with WNYF rf18 being ONLY -83 dBm and LOS, but I have CITY-TV sitting on rf17 at -33.7 dBm, roughly a 50 dB difference.

If I'm reading the graph correctly, the CRC paper suggests that the best tuners measured started displaying artifacts at ~40 dB Desired-to-Undesired ratio (N+1). This figure degrades when other nearby strong signals are thrown into the mix.

By offpointing my antenna, I may be able to reduce the D/U ratio to 40 dB, but there's only 17 degrees of separation between the HC tower and WNYF. Perhaps I can use some shielding to make further reductions.
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post #24 of 58 (permalink) Old 2011-09-07, 05:45 PM
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old sparks, you could try ganging antennas pointed in different directions to create a deep 30 dB null at 17 degrees. Most 8-bays have a deep null at 30 degrees. You could try a single 8-bay to start.
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post #25 of 58 (permalink) Old 2011-09-07, 05:52 PM
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CTV_TX_TEK:
I see that this is your FIRST post on this forum....congrats, you'll see many other similiar posts.

www.tvfool.com or www.fmfool.com is the first stop: enter location as accurately as possible
(I use GoogleEarth) into "Online TV Maps" and enable "Show lines pointing to each transmitter"
or enable individual transmitters to see colorized medium-rez RX signal level map.
[High-rez (*.kmz) maps can be (torrent) downloaded for installation into GoogleEarth.]
Enter desired antenna height above ground. Click on RX Icon to reposition.
[Which means you'll want a laptop with some sort of Wireless I-N connection.]

If you don't want to rely on making a Wireless I-N connection in the field, there are some
general purpose Propagation Prediction Programs, such as Radio Mobile and SPLAT!,
that you can readily find and download for FREE...but requires quite a bit of set-up time....

At top right of chart, you can click on "Radar Plot" to go to the Results page (all text).
You can copy/paste that URL (http://thingy at top of browser) into a post on this forum.

Net-Margin (NM) indicates how much EXCESS signal strength is predicted if using a 0 dBd
Gain Antenna with zero loss to the Tuner. Adjust NM upwards with the Antenna Gain and
downwards by what we call the "System Noise Factor". You'll see that andy.s.lee colorized
the reception likelihoods, based on a "typical" outdoor antenna implementation....YMMV...

System Noise Factor (using a Preamp) consists of Balun & Antenna-To-Preamp Loss plus
the Noise Figure (NF) of Preamp plus post-Preamp Coax & Tuner NF contribution to
System Noise Figure (which is reduced by Scalar Gain of the Preamp)....
Or if NO Preamp: SystemNoiseFigure = Balun + Coax/Splitter Downlead Loss + Tuner NF:
https://docs.google.com/spreadsheet/...Hc&hl=en#gid=0
BTW: "Cascaded Noise Figure" label is incorrect....that would be a DIFFERENT spread sheet,
using a Preamp AND a Distribution Amplifier....NOT recommended unless you really DO HAVE
and USE the correct spread sheet.

Fol. chart is also useful, presuming a (very good) 6 dB NF Tuner (add Balun Loss):
http://www.digitalhome.ca/forum/81-over-air-ota-digital-television/42426-signal-amplifiers-amps-preamps-distro-amps-post888368.html

Raw & Net Gain vs Frequency curves for many antennas can be found here (also follow links):
http://imageevent.com/holl_ands

Don't overlook "Files by holl_ands"....lots of useful info....esp. Intermod Distortion, aka "Overload".

NM "should" be positive by 10-20 dB to ensure year-round performance in the presence of
multipath, co-channel & adjacent interference, man-made interference (esp Lo-VHF), etc.
This is frequently called the "Fade Margin" in communication systems....

You also might need to guesstimate additional losses due to trees, surrounding buildings
blocking the signal (good luck!!!!), clutter loss if the signal grazes the rooftops and indoor loss
if located indoors (est 13 dB +/- 7 dB)....or more if Low-E glass & aluminized moisture barrier.
Yes, some people get intermittent reception with slightly negative NM...but troposcatter
propagation (esp across water) can't be relied on.
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post #26 of 58 (permalink) Old 2011-09-07, 06:30 PM
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Quote:
Originally Posted by majortom View Post
Holl_ands,

never a dull moment reading your posts.
Gonna make myself an entire folder of
"Holl_ands" Bookmarks. Hehe
YUP, me too.....which makes it so much easier to spit out the desired answer to a question...
including my own....

BTW: I pity the folk still stuck in the MS IE world....where searching and rearranging
Favorites is next to impossible....with FireFox I'm a happy Wookie....err "Bookie".....
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post #27 of 58 (permalink) Old 2011-09-07, 06:54 PM
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holl_ands, good summary as always!

Note that if you use TVFool's Online Maps feature, you do not need to start with Google Earth. Just use an Google-API identifiable address, and use online maps to fine-tune the exact location.

I would recommend against relying too much on FMFool at this time, as, unlike TVFool, it does not use the Industry Canada Spectrum Management Broadcast database as a source for FM stations, and the FCC data for Canadian FM seems to be much more out of date than TV. (Actually, sometimes the FCC TV data is more uptodate -- for example, CIVP-Dt chanel 23 Chapeau does not appear in the public version of the IC database, but it (correctly, I'm told) shows "OP" in the FCC database. I suspect this is because the FCC may have access to the total IC database, including records not "OK-TO-DUMP")

However, FMFool is at least a good first approximation for long-standing stations, especially if you're trying to predict harmonic interference into VHF-high. While the regs require 80 dB down on transmitter-generated harmonics, this can still cause problems to distant VHF-High stations already near the cliff.
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post #28 of 58 (permalink) Old 2011-09-08, 02:46 AM
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GoogleEarth Enterprise Edition (NOT FREE) has the big advantage of NOT relying on constant
I-N connection to display (Torrent Downloaded) HIGHER-REZ signal strength maps:
http://www.google.com/enterprise/ear...n-biz-gee-port
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post #29 of 58 (permalink) Old 2011-09-08, 09:50 AM
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At the other end of the spectrum, I have 14 stations with a signal > 40dB. What good filters should I use (and which ones should I avoid)?

-gmd

SC: 8-Feb-02>21-Jul-09 (TFO) | EVu: 23-Jan to 03-Feb-07, back 20-Jun-09
Bell, French packages, pls! Long live OTA!
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post #30 of 58 (permalink) Old 2011-09-09, 11:30 AM
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Smile How does someone adequately say thank you?

Hello everyone,

How do I thank everyone who has responded to my query? I am to say the least humbled by the depth and quantity or responses.

There is so much info to digest here and I'm confident that I'll be able to devise an adequate field test given all your help!

Thank you, thank you, thank you!

G
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