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Quote:
Originally Posted by Chuck Tribolet  /t/369015/san-francisco-ca-ota/10260#post_24476896


I thought that in the analog days, VHF low-band was where the stations really wanted to be.  What changed?


Chuck

Elevated noise floor on low-VHF, ATSC susceptibility to impulse noise interference, lower transmitter powers for digital. etc.
 

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Minimum Decodable Signal


After Ron was questioning the sensitivity of my tuner I decided to take a closer look at that. Using KTXL RF 40 as my signal source since that's one of the flattest signals I have, I added attenuation before the TV and used the SNR reading to get as close to 15.2 dB as I could with my 1 dB step attenuator. The SNR was running about 15.1 to 15.4 dB, very close to the digital cliff.


Attached is an image of the signal with the resolution bandwidth of the spectrum analyzer set to 1 MHz. I used video averaging to get a smoother trace. The TV is using a 6 MHz bandwidth so the spectrum analyzer is showing a signal 7.8 dB lower than the TV is seeing. The analyzer shows a signal of -92.3 dBm. Adding 7.8 dB to that means the TV is able to decode a signal of -84.5 dBm. This is very close to the -84 dBm predicted by the ATSC spec assuming a TV tuner noise figure of 6 dB. The ATSC spec assumes 1 dB of inefficiencies in the tuner. Depending on what that really is I think I can infer a noise figure of 5.5 to 6.5 dB for the tuner in my Sony TV. This assumes that the spectrum analyzer accurately measures the signal and I have no way to verify that.


Everything I'm seeing indicates the TV is performing as it should.


Chuck


 

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Quote:
Originally Posted by Chuck Tribolet  /t/369015/san-francisco-ca-ota/10260#post_24476896


I thought that in the analog days, VHF low-band was where the stations really wanted to be.  What changed?


Chuck


I wanted to make a comment about the low power that low VHF stations run. I mentioned this somewhere else recently and I hope I'm not repeating it here.


There's something called the dipole factor or antenna factor. As the frequency goes down the length of the dipole goes up. The longer the dipole the more energy it intercepts from a given field strength and it generates a higher voltage at the terminals. A dipole on channel 2 generates 20 dB more signal than a dipole on channel 35. This means that the transmitter needs 20 dB less power to generate the same received signal on channel 2 as it does on channel 35. A 10 KW transmitter on channel 2 is equal to a 1000 KW transmitter on channel 35. A 15 KW LP UHF station is equal to a 150 watt LP low VHF station. A 3KW low VHF station like KFTY or KCSO isn't really such low power.


Also low VHF is much better at refracting over hills and buildings and is much less affected by trees.


A 5 dB gain low VHF antenna is the same as a 25 dB gain UHF antenna. Nobody has a UHF antenna like that.


Unfortunately noise is a terrible issue on low VHF. Those sparkles in the picture on low VHF stations in the analog days are the kiss of death for ATSC. Also the problems with indoor antennas experienced on high VHF are even worse on low VHF.


Chuck
 

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For KFTY 2 here... This is what played out.

My antenna was 15+ feet away from the house. Then about 15 feet up on a ladder at that point. I would expect the roof of the house to have more noise than this location.

I understand the noise issue, ... and this was mostly experimental....... and I don't care for Daystar, Hope & Dream type network channels.

And nobody is going to put a tower in the middle of the yard for this one channel.


Is this noise issue an "American" thing ?.... aka.... "ATSC"

I don't hear much about other countries digital tv systems having this type of problem.

Is our DTV system defective ?
 

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Quote:
Originally Posted by 888CALLFCC  /t/369015/san-francisco-ca-ota/10260#post_24479085


Is this noise issue an "American" thing ?.... aka.... "ATSC"

I don't hear much about other countries digital tv systems having this type of problem.

Is our DTV system defective ?

I'm not sure that low VHF is being used in other countries who have DTV. For the countries I was able to track down, their DTV looks to start at channel 7. The major source of noise on low VHF is power line noise. It's not completely dissipated at high VHF but it's much less. It's largely non-existent on UHF.


Relatively few stations are using low VHF. It's mostly being used by LP stations that have no other channel options.


Chuck
 

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Quote:
Originally Posted by Calaveras  /t/369015/san-francisco-ca-ota/10260#post_24479932


I'm not sure that low VHF is being used in other countries who have DTV. For the countries I was able to track down, their DTV looks to start at channel 7. The major source of noise on low VHF is power line noise. It's not completely dissipated at high VHF but it's much less. It's largely non-existent on UHF.


Relatively few stations are using low VHF. It's mostly being used by LP stations that have no other channel options.


Chuck

VHF Low was also used in areas where it was cost effective to stay on their old analog spot. Examples are

KXGN CBS 5 Glendive, MT

KNOP NBC 2 North Platte, NE

KXLF CBS 4 Butte, MT (for some reason they are using RF5)

WLBZ NBC 2 Bangor, ME

KOTA ABC 3 Rapid City, SD (using RF2)

KYUS NBC 3 Miles City, MT


Some bigger markets that are on VHF low on a Big 4 or PBS

Vegas NBC (RF3 PSIP 2)

Philly ABC (RF/PSIP 6)

Memphis NBC (RF/PSIP 5)

Eureka, CA NBC (RF/PSIP 3)

Medford, OR NBC (RF/PSIP 5)

Des Moines, IA ABC (RF5).....although they have a fill in translator on UHF

Quad CIties, IA/IL CBS (RF4)....but same thing as Des Moines....fill in translator on UHF
 

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Quote:
Originally Posted by Calaveras  /t/369015/san-francisco-ca-ota/10260#post_24477501

Minimum Decodable Signal


After Ron was questioning the sensitivity of my tuner I decided to take a closer look at that.


Chuck
I definitely misinterpreted your KFTY spectrum analyzer trace the other day. I didn't properly take into account the rather high noise figure of the Rigol DSA-815 (even with the built-in preamp, it's estimated to be at least 15 dB).


The high analyzer noise figure makes estimating actual signal to noise ratios difficult. I was comparing your trace with a 23 dB preamp to my local trace without a preamp. After you pointed that out in a subsequent post, I realized it was a totally apples and oranges comparison, even though the traces look very similar.


A 23 dB preamp would mostly overcome the analyzer noise figure, so the KFTY trace was pretty much showing the actual signal to noise ratio (around 16 dB, which matched the reported TV signal to noise ratio).


Still learning.



Ron
 

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I experienced some interesting conditions here this afternoon at about 5 pm. Signals from the South Bay were lower than usual and KAXT 1 was below the cliff edge at 13.5 dB SNR. Signals from up north and Walnut Grove, though, were really good. I was surprised to see KEMO 50 coming in at 29 dB SNR! It's usually about 20 dB. KMAX 21 and KQCA 58 were both in the mid-20's, KXTV 10 was at 22 dB and KOVR 13 was at 18-19 dB. Since the South Bay signals were down and Walnut Grove signals were up, KGO's translator wasn't bothering KCRA and it was coming in at 19 dB, too. The one station that surprised me was KVIE 6 (RF9). It's usually about the same as KXTV, but today it was way low just making it over the cliff edge with lots of break up.


You never know what you'll find when you tune across the channels.


Larry

SF
 

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Some notes:
Quote:
Originally Posted by unclehonkey  /t/369015/san-francisco-ca-ota/10200_100#post_24480119


KXLF CBS 4 Butte, MT (for some reason they are using RF5)

They were analog 4, digital 5. So they just kept the channel they were already using, which was cheaper than rebuilding for 4.
Quote:
KOTA ABC 3 Rapid City, SD (using RF2)

Has a UHF fill-in translator.
Quote:
Philly ABC (RF/PSIP 6)

Would probably be on UHF had there been anywhere to go. From what I hear, WPVI still wants to be on UHF, but there is still nowhere to go.
Quote:
Memphis NBC (RF/PSIP 5)

Has petitioned the FCC to move to 17.
Quote:
Medford, OR NBC (RF/PSIP 5)

Multitude of UHF translators, like every other station in the market.


- Trip
 

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Quote:
Originally Posted by dr1394  /t/369015/san-francisco-ca-ota/10260#post_24480494


I didn't properly take into account the rather high noise figure of the Rigol DSA-815 (even with the built-in preamp, it's estimated to be at least 15 dB).

The noise figure is pretty bad, isn't it? If you can infer the noise figure from the level of the trace with no signal input then with the preamp it looks to be about 18 dB and with no preamp it's about 42 dB. How do you get a noise figure that bad? Put a big attenuator on the input?


Chuck
 

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This afternoon I was getting a lock for 19-1 with an antenna aimed for Sutro. There was nothing on 19-3. Has a new translator started like the one for 33-1 on UHF from Walnut Grove?
 

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Quote:
Originally Posted by Calaveras  /t/369015/san-francisco-ca-ota/10200_100#post_24481620

Quote:
Originally Posted by dr1394  /t/369015/san-francisco-ca-ota/10260#post_24480494


I didn't properly take into account the rather high noise figure of the Rigol DSA-815 (even with the built-in preamp, it's estimated to be at least 15 dB).

The noise figure is pretty bad, isn't it? If you can infer the noise figure from the level of the trace with no signal input then with the preamp it looks to be about 18 dB and with no preamp it's about 42 dB. How do you get a noise figure that bad? Put a big attenuator on the input?


Chuck
It depends on how they built their front end. Some variable gain amplifiers work by having a variable attenuator in front of a high gain amplifier. Always noisy, but best noise performance is when it is operating at maximum gain. This comes as a really convenient single IC.
 

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Quote:
Originally Posted by DAP  /t/369015/san-francisco-ca-ota/10290#post_24484744


It depends on how they built their front end. Some variable gain amplifiers work by having a variable attenuator in front of a high gain amplifier. Always noisy, but best noise performance is when it is operating at maximum gain. This comes as a really convenient single IC.

I admit to not being current in what's available these days in RF ICs. This sounds like what they must be doing as you can adjust the input attention in 1 dB steps and there are no mechanical step attenuators in the analyzer.


It would be nice if the analyzer had better noise figure and a larger dynamic range but I guess that's what $20K+ analyzers are for.



Chuck
 

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Quote:
Originally Posted by Calaveras  /t/369015/san-francisco-ca-ota/10290#post_24485463

...
It would be nice if the analyzer had better noise figure and a larger dynamic range but I guess that's what $20K+ analyzers are for.



Chuck
Hi,

 

http://www.halted.com/ccp28290-temic-4707-ph5-50-860-mhz-vhf-uhf-tuner-great-for-4707-ph5-3x7827-23731.htm

 

Temic 4707-PH5 50-860 Mhz VHF/UHF Tuner Great for DIY Spectrum Analyer Project!

 

$4.95

 

Technical data:

 

http://www.halted.com/objects/catalog/product/extras/28290_23731_4707-PH5-3x7612.pdf

 

SHF
 

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Is it much better? When taking RBW in account, I only see a 3 dB decrease in the DANL. Am I missing something?


In a 10 Hz RBW I'd like to see
 

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Quote:
Originally Posted by SFischer1  /t/369015/san-francisco-ca-ota/10260#post_24474583


Hi,

The "TSID" being unique is not on a transmitter basis as KGO RF 7 and RF 35 both contain the same TSID.

I think the RF 35 system is just repeating the analog signal, I don't think it's a digital system that could replace the TSID (or do other nice things, like processing the error correction and re-encoding to send out a cleaner signal)


[edit: sorry for crazy theory]
 

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Quote:
Originally Posted by Calaveras  /t/369015/san-francisco-ca-ota/10290#post_24487882


Is it much better? When taking RBW in account, I only see a 3 dB decrease in the DANL. Am I missing something?


In a 10 Hz RBW I'd like to see
 

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Finally finished up a little project on my antenna system today. Had thought it would be nice to eliminate the two "wall wart" power supplies that I had feeding my antenna pre-amp and distribution amp in the attic. And instead power those two amps off of my solar battery setup. A small box on the right houses a DC-DC step-up converter to output 15VDC to the two amps. The DA is to the left and the power inserter for the pre-amp is above that:



I measured a continuous draw of about 12 watts combined between the two wall warts, and on DC, the draw is about 0.25amps (3.5 watts), so should be no problem to power 24/7 off my battery bank. Conversion should pay back in about a year with the power savings.
 

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Quote:
Originally Posted by toast0  /t/369015/san-francisco-ca-ota/10290#post_24490620


I think the RF 35 system is just repeating the analog signal, ...

Analog signal? The only analog signal around these parts is the channel 6 station in San Jose.


Regarding the channel 35 KGO signal, there's a receiver at Mt. Allison that receives the KGO 7 digital signal over the air and that feeds the translator transmitter.


Larry

SF
 
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