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post #14941 of 16235 Old 05-25-2012, 03:14 PM
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rabbit73 writes:
> An 8VSB signal generator is not in my price class.

I don't know what your price class is, the least expensive
8VSB modulator I know of is from
http://www.sr-systems.de/
Something on the order of 1000 Euros.

Nick Sayer uses one for a HAM TV station:
http://nsayer.blogspot.com/search/label/tv

You can get a NTSC modulator for something like $10-15 at Target, K-Mart, etc.
8VSB modulators should not be so rare and expensive. *grumble*
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post #14942 of 16235 Old 05-25-2012, 06:47 PM
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Chuck:
Quote:
Originally Posted by Calaveras View Post

Actually that's not clear to me at all. When I was performing these measurements in the lab we'd just give the number as a kind of shorthand. The "/sec" part was always left out so I can't tell from above what they really mean. I just assumed what was meant was 3x10E-6/sec.

I'm probably being too picky, and it doesn't make much difference when everyone uses the same measurement expressions for comparisons, but let me try again to make the distinction between Bit Error Ratio and Bit Error Rate by using a car analogy:

RATIO: So far, I have driven 5 miles of the 100 miles to Richmond.
Notice there is no mention of how fast I'm driving, just as there is no mention of how fast the errors are happening in Bit Error Ratio.

RATE: I can drive 60 miles per hour when I travel to Richmond.
Notice that there is no mention of how many miles to Richmond, just as there is no mention of total bits in Bit Error Rate.

If I may quote authorities who are a lot smarter than I am:

Digital Communications Test and Measurement: High-Speed Physical Layer Characterization
by Dennis Derickson and Marcus Müller

Publication Date: December 20, 2007 | ISBN-10: 0132209101 | ISBN-13: 978-0132209106 | Edition: 1

Dennis Derickson is an assistant professor at California Polytechnic State University. He spent eighteen years as member of technical staff and project manager at Hewlett-Packard and Agilent Technologies before serving as applications engineering manager for Cierra Photonics. He has authored or coauthored fifty publications in high-speed communications and is the editor of Fiber Optic Test and Measurement (Prentice Hall, 1998). Dennis has a Ph.D. from the University of California, Santa Barbara.

Marcus Müller is an R&D lead engineer with Agilent Technologies' High-Speed Digital Test segment in Boeblingen, Germany. He specializes in bit error ratio and jitter analysis of high-speed links, and has contributed to new methods for total jitter measurement at low bit error ratios, and jitter tolerance test. Marcus received his M.Sc. degree from Stuttgart University, Germany, in 1999.

Chapter 4 (bit error ratio testing): A fundamental property of a digital communications link is the bit error ratio (BER), which is the number of bit errors divided by the total number of bits sent. This chapter along with Appendix A provides a detailed description of the test hardware and test methodology for BER. (Marcus Müller wrote this chapter.)

Quote:


The bit error ratio (BER) is a measure of the percentage of bits that a system does not transmit or receive correctly. It is a dimensionless number, ranging from 0.0 to 1.0: If the BER = 0.0, then all bits are transmited correctly; at the other extreme, if the BER = 1.0, every bit is received in error.

The bit error ratio is calculated by dividing the number of erroneous bits by the number of compared bits.

A second measure for the error performance of a digital transmission system is the bit error rate. It's different from the bit error ratio in that it relates the number of errors to the test time, rather than the number of bits compared during the test. Note that bit error rate is not dimensionless; its unit is errors per time.

In everyday use bit error rate is more descriptive than the bit error ratio: A PCI Express link, for example has a target ratio of 10E-12. At the 2.5 Gbit/s data rate, that's an error rate of 0.025 errors per second.

The terms bit error rate and bit error ratio are often mixed up in everyday usage, especially since the BER acronym is used for both. However, since the bit error rate is hardly used in practice these days, it is usually safe to assume that bit error ratio was intended.

In a earlier post I said that ATSC A74 used a number for Bit Error Rate that was really Bit Error Ratio:
Quote:


A DTV receiver should achieve a bit error rate in the transport stream of no worse than 3x10E-6 (i.e., the FCC Advisory Committee on Advanced Television Service, ACATS, Threshold of Visibility, TOV) for input RF signal levels directly to the tuner from –83 dBm to –5 dBm for both the VHF and UHF bands.

As Marcus Müller said, Bit Error Ratios are dimensionless, which means that there are no units, so you shouldn't add per sec. The reason Bit Error Ratio is dimensionless is because when you divide error bits by total bits for the ratio, the bits units cancel according to math rules, and you are left with a small number divided by a large number which is conveniently expressed in scientific notation like, in this case, 3x10E-6.

In an earlier post dr1394 conveniently showed us:

# of Pre RS errors/sec
<1
<10
<1000
<10000
<100000 heavily impaired

That's Bit Error Rate.

Q.E.D. (This QED is not a ham Q signal)

73,
rabbit
W4...

If you can not measure it, you can not improve it.
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post #14943 of 16235 Old 05-27-2012, 02:36 PM
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Following up about my comments about maximizing the ATSC pilot. From what I have read, the receiver needs to find the pilot signal first so that synchronous detection and subsequent processing can occur. Insufficient pilot signal strength due to multipath or fading means no display. @rabbit73, you said in a old, old post that you have a NTSC field strength meter. Would that detect and display the strength of the pilot signal only? I could use something similar to that to find the best location to install my antenna in order to receive weak signals.
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post #14944 of 16235 Old 05-27-2012, 04:28 PM
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retiredengineer:



Quote:


Would that detect and display the strength of the pilot signal only?

Not just the pilot only, but the pilot or any spot on the whole signal that you want.

When I use my old Sadelco 719E signal level meter (SLM, originally designed for analog signals but usable for digital) and manually tune through the 6 MHz 8VSB DTV signal, I notice a slight increase in signal stength at the lower end of the channel which is caused by the pilot.

I do not use the pilot for signal strength readings, I make a reading at center channel. My Sadelco DisplayMax 5000 SLM (designed for analog and digital, OTA & cable) also uses center channel for a reading of digital 8VSB & QAM.

The readings for NTSC signals are made, of course, at the picture and audio carrier frequencies.

My Sadelo DispalyMax 800 can read digital signals two ways:

1. It scans the digital channel and then gives an average power reading derived from many readings across the channel. The display looks a lot like a spectrum analyzer display, and I can see if the top of the signal is flat or not.
2. The second way is to tune the meter to center channel for a reading. This gives me a real-time reading without having to wait for a scan that can be used for immediate feedback to help aim an antenna for max signal. It only gives a relative reading in that mode, so after aiming I can switch to the scanning mode for an accurate power reading in dBmV. Besides giving immediate feedback in the single frequency mode, the meter is more sensitive then down to about -35 dBmV VS about -20 dBmV in the scanning mode. The meter refuses to give a reading below -20 dBmV in the scanning mode.

The attachments show you what the display on the 800 looks like.

If the signal has enough SNR to decode, then you will see the pilot. I don't see any point in focusing on measuring the pilot to the exclusion of the rest of the signal.
LL
LL
LL
LL


If you can not measure it, you can not improve it.
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post #14945 of 16235 Old 05-27-2012, 05:06 PM
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Thanks for your response and displays. If I am in a weak signal location that doesn't decode, then I need an alternate method to find and optimize the signal. How can I find a strong signal if I can't even auto scan the signal into the tv's memory and tune to it? I don't need the tv or tuner to help find the strong signal. I look at the field strength meter tuned to the ATSC pilot as I walk around with my antenna like dowsing for water. Once found, that is where I plunk down my antenna and hope for the best. I need the ATSC pilot to properly receive the signal so that is where I am focusing.
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post #14946 of 16235 Old 05-27-2012, 05:23 PM
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Quote:


If I am in a weak signal location that doesn't decode, then I need an alternate method to find and optimize the signal.

How weak are your signals? What do your tvfool & fmfool reports look like?

What channel are your trying to get?

What equipment are you using for a tuner and antenna?

What signal level meter do you have?

Yes, the pilot is slightly stronger than the rest of the signal, but I'm not sure that it is going to make that much difference when hunting with a SLM. You could try adding a preamp to your SLM to increase its sensitivity and listen closer to the noise floor. That might not work if you have any strong signals that would overload the preamp and mask any weak signals. My 719E almost makes it down to -90 dBm without a preamp (the conversion factor between dBm and dBmV is 48.8).

You might need an instrument with greater sensitivity, like a spectrum analyzer that reads below -100 dBm so that you could see the signal rising above the noise floor even if it can't be decoded.
http://www.tequipment.net/RigolDSA815.html
http://www.hdtvmagazine.com/columns/...ruary-2009.php

An alternate way might be to use a USB tuner, as mentioned previously by holl_ands, that can give a SNR reading of a weak signal to show you that it is there, even if it can't be decoded. You could locate your antenna in a spot that gives you the highest SNR reading even if it never makes it to 15.5 dB. Trip has also mentioned using that technique when he was at school.
http://www.hauppauge.com/site/produc...a_hvr950q.html

IIRC, some people have said that the Zenith DTT900/901 CECB can show you the presence of a signal even it it can't be decoded, but I haven't tried that yet for myself. That technique is used by DXers who enter the channel number and wait for the tropo signal to arrive at their location.

I can't think of any more ideas right now; maybe some of the other guys can.

If you can not measure it, you can not improve it.
Lord Kelvin, 1883
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post #14947 of 16235 Old 05-27-2012, 07:50 PM
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Your points are valid but nowhere do you mention signals that have multipath distortion. I receive signals as one edge with significant multipath signals that causes multiple hot and cold spots. I don't think I have a flat 6 MHz spectrum. Instead numerous peaks and notches across the spectrum. What happens if a notch just happened to occur at the pilot frequency? No reception because the pilot frequency is required to synchronize the receiver's coherent detector. Moving the antenna might move the notch away from the pilot frequency allowing reception.
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post #14948 of 16235 Old 05-27-2012, 09:02 PM
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I find it difficult to give you an intelligent answer because you are withholding the specific information about your situation that I asked for. I had to hunt for it in your posts.

In cases where you have serious multipath reflections that cause the BER to increase, then it is necessary to monitor signal quality as well as signal strength.

If it is important to you to monitor the position of the notch, then a spectrum analyzer is necessary, just as it is to see signals that are above the noise floor but are, as you said, too weak to decode.

What model is your Sony? Does it show errors to monitor signal quality?
My Sony gives signal quality as errors per unit time, not percent.

Is it possible that the nearby high voltage lines are increasing your noise level?

11-7-11:
Quote:
Originally Posted by retiredengineer View Post

Putting your antenna in the attic should not cause any problems. Any multipath present in the attic can easily be handled by the equalizer in the TV.

I have my antennas (separate VHF and UHF antennas) in the attic and use a pre-amp and receive all local stations with no problem. I ran a TVFool analysis and I can receive KCET which has a signal power of -84 dBm which is lower than your values. Note that all my reception involve one edge paths which includes multipath.

My TVFool results are:

http://www.tvfool.com/?option=com_wr...4bbabadb1bba15

10-6-11:
Quote:
Originally Posted by retiredengineer View Post

I use two attic antennas (CM4221 and Y5-7-13) and I get all the local stations that are not low power. See my TVFool results for my location. All the local channels are not LOS, yet, I can still receive them and all my signal levels are lower than yours so you have a very good chance of receiving them with an attic antenna.

If it was good then, why is it suddenly bad now?

I see the hills in the KABC profile in the attachment. The expressways don't help either! Your location is pretty bad for OTA.
LL

If you can not measure it, you can not improve it.
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post #14949 of 16235 Old 05-28-2012, 08:12 AM
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Quote:
Originally Posted by rabbit73 View Post

If the signal has enough SNR to decode, then you will see the pilot. I don't see any point in focusing on measuring the pilot to the exclusion of the rest of the signal.

When looking a DTV signal on a spectrum analyzer the pilot doesn't change amplitude with different Resolution Bandwidths like the rest of the signal does. It looks like a CW signal while the rest of the signal looks like noise. The pilot is typically used to measure the signal strength on the analyzer. Of course this doesn't mean the rest of the signal is irrelevant. A signal can be undecodeable because any portion of the signal is too weak.

The pilot is also useful when doing antenna pattern measurements when the signal is very weak because you can measure the pilot in very narrow bandwidths when you can't see the rest of the signal.

Chuck

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post #14950 of 16235 Old 05-28-2012, 08:29 AM
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Quote:
Originally Posted by retiredengineer View Post

Moving the antenna might move the notch away from the pilot frequency allowing reception.

I've performed a few tests in marginal reception areas moving the antenna around over hundreds of feet. It's frequently possible to find an exact spot where the problem station could be received but it almost always made another station go away. It's very difficult to find a spot that's optimum for all the stations when you're in that sort of situation. The best solution seems to be to mount the antenna as high as possible and not be looking through any nearby trees.

Chuck

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post #14951 of 16235 Old 05-28-2012, 08:32 AM
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rabbit73. Thank you for taking the time to answer my concerns. There are two reasons for my post. A new station has appeared (KHIZ that I want to receive but I can't) and I have a friend who is having trouble receiving the local stations as one edge even though he is close to the transmitter on Mt.Wilson. Being a retired engineer who wants to solve problems, I thought why not take a portable field strength meter, attach it to an antenna then survey his property for the best reception location. I just wanted feedback from the experts to see if it was feasible.

Here is my latest TVFool report. I still receive the local stations with the antennas in the attic despite having my cement shingles roof replaced with asphalt shingles. My latest challenge to myself now is to start receiving the low power stations. Thanks again for the replies.

http://www.tvfool.com/?option=com_wr...073b7b9469b626

Calaveras, also thanks to you for your replies.
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post #14952 of 16235 Old 05-28-2012, 09:33 AM
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Quote:
Originally Posted by Calaveras View Post

The pilot is also useful when doing antenna pattern measurements when the signal is very weak because you can measure the pilot in very narrow bandwidths when you can't see the rest of the signal.

That makes sense; thanks.

If you can not measure it, you can not improve it.
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post #14953 of 16235 Old 05-28-2012, 10:13 AM
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retiredengineer:

Thanks for the background information and new tvfool report. It gives me a better understanding of your situation.

I see KHIZ CH44 listed twice, but there is no indicator for co-channel by the callsign. Do you mean the KHIZ at 323 degrees or the one at 78 degrees?
Quote:


I have a friend who is having trouble receiving the local stations as one edge even though he is close to the transmitter on Mt.Wilson. Being a retired engineer who wants to solve problems, I thought why not take a portable field strength meter, attach it to an antenna then survey his property for the best reception location.

Ah, yes. If I were you I would want to practice my technique at home before going over there to make a good showing.
Quote:


I just wanted feedback from the experts to see if it was feasible.

I think it's feasible, but you haven't told me what signal level meter you will be using. You have a good chance of finding the signal with the meter, especially if you are able to add a preamp before the meter. With my old analog 719E meter with the audio turned on, digital signals sound like white noise that is stronger than the noise floor as I tune through them. The meter does show the signal even if it can't be decoded by the tuner.

Once you have found the signal there are two more problems to be solved.
1. Does the signal have a SNR greater than 15.5 dB?
2. Is the signal quality sufficient so that there aren't more errors than the FEC can handle?

As you saw from Trip's situation in TN, it is possible to have a strong signal with more than enough SNR but it still can't decode because of high BER. The scan from his meter didn't show a multipath problem, but that's what it was.

I think you will be able to determine if you have enough SNR, but measuring the errors is going to be a little more difficult. My Sadelco 5000 can measure BER and MER, but they are only emulated measurements using noise as a reference. If you can use the Sony diagnostics screen it will show you SNR in dB and errors, if it can decode.

I have found that my Apex DT502 CECB is useful when aiming an antenna because it has two signal bars, one for signal quality (the inverse of errors) and the other for signal strength. But, it only shows that information if the box can decode the signal, which is the same problem as the Sony.

Having your antenna in the attic gives you handicap. Your tvfool report assumes that your antenna is not in the attic. The attenuation caused by the attic can vary a lot, but must be measured by placing your antenna inside and outside to make two measurements to find the loss. Are you able to have an outside antenna, or is it mandatory that it be in the attic?

Do you think that using the Hauppauge WinTV HVR-950Q USB tuner with the TSReader Lite software, as suggested by holl_ands, would be of any help with your problem?

Can you borrow a spectrum analyzer?

If you can not measure it, you can not improve it.
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post #14954 of 16235 Old 05-28-2012, 12:06 PM
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The Sony error reading is pretty much useless because it only shows uncorrected errors and then only very close to 15.2 dB SNR.

I think what does a better job is the MyGo TV battery operated 7". It only has a signal quality meter but it gives a reading quite a bit below 15 dB SNR, I think until it can't phase lock on the pilot. It also has direct RF entry which is nice if you take it where you can receive different stations that aren't programmed in. Its built-in antenna is poor but it has an adapter for an external antenna. The tuner is nearly identical in performance to the one in my Sony except I don't think it's as good on low VHF but fine on high VHF.

Chuck

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post #14955 of 16235 Old 05-28-2012, 03:18 PM
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Calaveras,
Do you have a model number for the MyGo TV?
Thanks
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post #14956 of 16235 Old 05-28-2012, 04:44 PM
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Rabbit73, answers to your questions.

73: I see KHIZ CH44 listed twice. Do you mean the KHIZ at 323 degrees or the one at 78 degrees?

ME: The one at 323 deg. Where I live, KHIZ at 78 deg is shadowed by the nearby mountains. I think the one at 323 deg is an auxiliary transmitter that will be used to fill-in the shadow, hopefully soon.

ME: I have a friend who is having trouble receiving the local stations .... why not take a portable field strength meter, attach it to an antenna then survey his property for the best reception location.

73: Ah, yes. If I were you I would want to practice my technique at home before going over there to make a good showing.

ME: I agree, hate to fall flat on my face.

ME: I just wanted feedback from the experts to see if it was feasible.

73: I think it's feasible, but you haven't told me what signal level meter you will be using.

ME: The Display 800 you are using seems to be a good choice.

73: Once you have found the signal there are two more problems to be solved.
1. Does the signal have a SNR greater than 15.5 dB?

ME: Understand 15.5 dB is the minimum SNR for noise limited reception. A few dB has to be added if multipath is present.

2. Is the signal quality sufficient so that there aren't more errors than the FEC can handle? As you saw from Trip's situation in TN, it is possible to have a strong signal with more than enough SNR but it still can't decode because of high BER. The scan from his meter didn't show a multipath problem, but that's what it was.

ME: I am not under the delusion I can find the perfect location. Just wanted to eliminate a reception problem caused by a very weak signal. Solve one problem at a time.

73: Having your antenna in the attic gives you a handicap..... Are you able to have an outside antenna, or is it mandatory that it be in the attic?

ME: Outside, I can only mount an antenna on the chimney. At my age, I don't want to be on the roof with my fragile body. When I have the antenna in the attic, I could move the UHF antenna a few feet in different directions to see if it improves reception which it did. I fortunately didn't have to move the VHF antenna.

73: Do you think that using the Hauppauge WinTV HVR-950Q USB tuner with the TSReader Lite software, as suggested by holl_ands, would be of any help with your problem?

ME: It would if I had a powerful computer which I don't have. Just a 773 MHz computer running Windows XP SP2. Good for email and surfing.

73: Can you borrow a spectrum analyzer?

ME: No, but I wish I could.

Finally, appreciate your suggestions. Budget constraints and my wife forces me to consider the least costly solutions.
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post #14957 of 16235 Old 05-28-2012, 06:28 PM
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Quote:
Originally Posted by arxaw View Post

Calaveras,
Do you have a model number for the MyGo TV?
Thanks

This is the link:

http://www.mygotv.com/products/porta...ld-digital-tv/

Looks like they've updated it from when I bought mine. Mine is ATSC and NTSC. Now it's ATSC and Clear QAM. I see it still has direct RF entry. I hope they haven't messed up how the signal meter works.

Chuck

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post #14958 of 16235 Old 05-28-2012, 07:06 PM
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Much appreciated.
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post #14959 of 16235 Old 05-28-2012, 10:45 PM
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Quote:
Originally Posted by Calaveras View Post

I've performed a few tests in marginal reception areas moving the antenna around over hundreds of feet. It's frequently possible to find an exact spot where the problem station could be received but it almost always made another station go away. It's very difficult to find a spot that's optimum for all the stations when you're in that sort of situation. The best solution seems to be to mount the antenna as high as possible and not be looking through any nearby trees.

Chuck

Indoors can be a reception problem. While an antenna can't be moved hundreds of feet, I've seen the same results you've found. The correct aim/location for one channel is not correct for others. To get Walnut Grove stations indoors here, the minimum number of antenna aims/locations is three.

2 antennas are better than 1. 3 or 4 antennas would be even better.
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post #14960 of 16235 Old 05-29-2012, 05:45 PM
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Finding channels missed by tuner scan, Part 1 of 4

 

Quote:

Originally Posted by retiredengineer View Post

Rabbit73, answers to your questions.

73: I see KHIZ CH44 listed twice. Do you mean the KHIZ at 323 degrees or the one at 78 degrees?

ME: The one at 323 deg. Where I live, KHIZ at 78 deg is shadowed by the nearby mountains. I think the one at 323 deg is an auxiliary transmitter that will be used to fill-in the shadow, hopefully soon.

Good; that one is easier.

Quote:


ME: I just wanted feedback from the experts to see if it was feasible.

73: I think it's feasible, but you haven't told me what signal level meter you will be using.
ME: The Display 800 you are using seems to be a good choice.

Ooops! I thought you were going to use an old field strength meter.
 

Quote:
Originally Posted by retiredengineer View Post

Following up about my comments about maximizing the ATSC pilot. From what I have read, the receiver needs to find the pilot signal first so that synchronous detection and subsequent processing can occur. Insufficient pilot signal strength due to multipath or fading means no display. @rabbit73, you said in a old, old post that you have a NTSC field strength meter. Would that detect and display the strength of the pilot signal only? I could use something similar to that to find the best location to install my antenna in order to receive weak signals.

I made a few tests today using my old Sadelco 719E (less expensive version of the FS-4 that I bought when I retired in 1988) that showed me it is possible to hunt for, detect, and measure the missing weak channels. It was necessary to use my CM7777 preamp to read the weak ones, but I was able to do that without preamp overload because my strongest signals are weak enough to avoid overload. I used my 719E because it makes it possible to turn the tuning knob through the channels manually, just like a spectrum analyzer automatically would during its scan.

Here are the two meters. The one on the left is the Sadelco 719E; the upper knob is for UHF, the lower for VHF-lo, FM, and VHF-hi. The one on the right is the DisplayMax 800. I repeated the image in the attachments in case the link to my image host is broken.



If you are going to use the Sadelco DisplayMax 800, you will not be able to manually tune slowly through the freqency range, it will be necessary to enter the desired frequency on the keypad. You can, however, hop thru the range in fixed frequency increments of 125 KHz; the measurement bandwidth is 280 KHz. If you are going to use the pilot (see attachments), that frequency is just above the lower edge of the channel:

Quote:


The first “helper” signal is the ATSC pilot. Just before modulation, a small DC shift is applied to the 8-level baseband signal (which was previously centered about zero volts with no DC component). This causes a small residual carrier to appear at the zero frequency (unmodulated carrier) point of the resulting modulated spectrum. This is the ATSC pilot. This gives the RF PLL circuits in the DTV receiver something to lock onto that is independent of the transmitted data.
Although similar in nature, the ATSC pilot is much smaller than the NTSC visual carrier, consuming only 0.3 dB or 7 percent of the transmitted power.

Quote:


The 8-VSB pilot is normally 309.441 kHz above the lower edge of the Channel assigned to the DTV transmitter, except when the DTV transmitter is required by the FCC to offset its Pilot Frequency in order to minimize interference caused to a lower adjacent Channel analog TV station.

Quote:


.....the DTV pilot carrier, which is 0.31 MHz above the lower channel edge........It is 11.3 dB below average signal power.

There is another problem. With the 719E I am able to listen to the noise of the channel as I tune through it to see if it rises above the noise floor. I don't know how that will work with the 800, but will try some more tests with it to develop a measuring technique.

I will post the data from the first tests that used the analog meter when I have more time, followed by a report on using the 800.

If you try to buy an old used Sadelco meter, make sure you get one that does VHF and UHF. The ones described as VHF and Super don't do UHF.
LL
LL
LL
LL


If you can not measure it, you can not improve it.
Lord Kelvin, 1883
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post #14961 of 16235 Old 05-30-2012, 06:49 AM
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Ah good, thanks, a perfect waveform example.


Quote:


Looks like they've updated it from when I bought mine. Mine is ATSC and NTSC. Now it's ATSC and Clear QAM. I see it still has direct RF entry. I hope they haven't messed up how the signal meter works.

Does the signal meter on it display numbers or just a bar ?
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post #14962 of 16235 Old 05-30-2012, 06:20 PM
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Finding channels missed by tuner scan, Part 2 of 4

 

In earlier posts retiredengineer asked a very intelligent question: Is there any way to hunt for, and measure channels missed during a scan? If there is, then it would be possible to know if simple improvements would enable the tuner to decode the signals, or if reception is impossible at that location.

I told him that even if he was able to find those weak channels, they might not have sufficient SNR or signal quality (few errors) to be decoded by the tuner. He said that it would be sufficient for now just to be able to find and measure them, so I made some tests using the equipment that I have.

For the first test I connected my CM4221 antenna, which is outside close to ground level, to my Sadelco 719E signal level meter (SLM) with a short coax jumper (SCJ), and took some readings of my UHF channels.

CM4221 > SCJ > 719E SLM

For the second test I added the UHF section of my CM7777 preamp to see if I could read the weak channels.

CM4221 > SCJ > CM7777 > SCJ > CM0747 Power Supply > SCJ> 719E SLM
 

Code:
Real    Test 1     Equiv      Test 2      Equiv    Amp Gain 
RF CH    dBmV       dBm        dBmV        dBm        dB            

16       -6.9      -55.7       +19.0      -29.8      25.9
29      -10.2      -59.0       +13.1      -35.7      23.3
31      -12.2      -61.0       +12.1      -36.7      24.3 
33       -9.0      -57.8       +14.8      -34.0      23.8
40       -9.0*     -57.8       +15.1**    -33.7      24.1
46      -19.9      -68.7        +6.2      -42.6      26.1
50      -22.0      -70.8        +2.9      -45.9      24.9
  
* CH 40 pilot measured -7.0, 2 dB stronger; measurement bandwidth is 500 KHz
** CH 40 pilot measured +17.0, 1.9 dB stronger
Amp Gain is the difference between the two tests and varies because
the signal strengths changed between tests.

I was able to measure a few weak ones with the preamp 
that usually don't show up after a scan.
                                                        Signal Strength at
                                                        Antenna Terminals
                                                        before 24 dB Preamp
                                                               dBm
17        NA         NA         -10       -59        NA        -83
19        NA         NA         -26       -75        NA        -99
22        NA         NA         -20       -69        NA        -93
42        NA         NA         -25       -74        NA        -98
45        NA         NA          -8       -57        NA        -81

I will make another test later with a tuner to see how many channels it can capture during a scan that day.


If you can not measure it, you can not improve it.
Lord Kelvin, 1883
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post #14963 of 16235 Old 06-04-2012, 10:11 AM
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Was in a Lowes store this morning and apparently they no longer carry outdoor antennas. They used to carry Channel Master, and more recently Philips and RCA. Now just a few poor performing indoor options. And I don't think Home Depot has them anymore either. Just an observation.
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post #14964 of 16235 Old 06-04-2012, 11:46 AM
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I wish we had more options to buy local, but must be not enough demand. Seems like Radio Shack is about it.
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post #14965 of 16235 Old 06-04-2012, 03:48 PM
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^ Does Radio Shack still carry outdoor antenna? I was under the impression that RS was a cell phone store nowadays, as I haven't been inside one is years, I don't really know what they have.
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post #14966 of 16235 Old 06-04-2012, 04:08 PM
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retiredengineer:

 

Finding channels missed by tuner scan, Part 3 of 4

 

I set up my SONY KDL22L5000 TV indoors to see what I would get from a scan:

CM4221 > 6 ft RG6 > CM7777 > 50 ft RG6 > CM4707 Power Supply > 3 ft RG6 to TV or SLM

The gain of the preamp and the loss of the coax gives a signal 21 dB stronger at the output of the CM4707 than at the antenna; I measured it so that I can calculate the signal strength at the output terminals of the antenna from measurements indoors.

I didn't use a splitter to feed the TV and signal level meter because I wanted max signal strength; I just substituted one for the other. When I am comparing the sensitivity of two tuners, then I use a 4-way splitter to feed the two tuners and my SLM to bring them to the cliff with an attenuator.

I first measured the strength of some signals with my 719E SLM to check the system. The 4221 was used for UHF and a folded dipole cut for CH13 for VHF:

Real CH

7 +1.5 dBmV
9 -5.0
11 -10
13 +4
16 +21.5
29 +11.5
31 +11.5
33 +12.1
40 +14.2

42 -25 very unstable
46 +2.3
50 +3.5
The noise floor measured about -10 dBmV on VHF-lo and about -20 to -25 dBmV on VHF-hi, which doesn't leave quite enough margin for reliable reception of CH11. I need to improve my VHF-hi antenna.

The tuner scan gives the channels in virtual number order:
 

Code:
Virtual   Real      Errors       SNR     Signal Strength     Comments
  CH       CH                    dB          

 2.1       20     0 to 6971    13 to 15      58 to 61
 3.1       40         0           25            79
 4.1        9         0           26            78
 6.1       25     0 to 3000    13 to 17         75     Signal from rear of ant
 7.1       11     0 to 3000    13 to 15      67 to 68
 8.1       22         0        17 to 20         76     Signal from rear of ant
10.1       31         0           25            78
13.1       13         0           28            79
15.1       16         0           25            79
21.1        7         0           31            79
27.1       50         0           25            79
33.1       33         0           23            78
35.1       26         0        18 to 19         76     Signal from rear of ant
43.1       29         0           25            79
45.1       45         0        22 to 23         72
49.1       46         0        25 to 26         78
   
23.1       42   Not picked up by scan;                 Signal from rear of ant
                 more comments later

Notice that some SNR numbers below 15 dB are given by the SONY TV.

My tvfool reports give the following information. The CM4221 antenna is at 5 ft for the 3rd report. They are in real channel number order because that's what is needed for the SLM:
 

Code:
Real           tvfool-zip      tvfool-address    tvfool-address, 5 ft
 CH            dBm   path        dBm   path          dBm   path
 
  7  WTPC     -41.9   LOS       -74.0  2Edge        -81.0  2Edge
  9  WSKY     -58.1   LOS       -88.2  2Edge        -95.5  2Edge
 11  WGBS     -73.8   LOS       -98.9  2Edge       -114.6  2Edge
 13  WVEC     -38.6   LOS       -71.1  2Edge        -78.0  2Edge
 16  WHRO     -32.4   LOS       -62.3  2Edge        -72.2  2Edge
 17  WKTD     -50.0   LOS       -87.9  2Edge        -95.4  2Edge
 20  WUND     -93.8   2Edge    -117.9  Tropo       -124.4  Tropo
 22  WRIC     -57.9   LOS       -87.7  2Edge        -94.3  2Edge
 25  WTVR     -60.4   LOS       -90.1  2Edge        -96.5  2Edge
 26  WRLH     -58.5   LOS       -88.5  2Edge        -95.1  2Edge
 29  WVBT     -32.7   LOS       -69.2  2Edge        -77.5  2Edge
 31  WAVY     -32.8   LOS       -66.2  2Edge        -75.6  2Edge
 33  WTVZ     -33.3   LOS       -63.4  2Edge        -73.2  2Edge
 40  WTKR     -33.9   LOS       -64.4  2Edge        -74.0  2Edge
 42  WCVE     -62.1   LOS       -92.6  2Edge        -98.7  2Edge
 45  WNLO     -55.3   LOS       -93.4  2Edge       -101.7  2Edge
 46  WPXV     -35.4   LOS       -67.0  2Edge        -76.4  2Edge
 50  WGNT     -34.8   LOS       -69.6  1Edge        -79.3  1Edge

Before the transition to digital, I was able to use my B&W portable TV on battery power to receive the news during a power failure (attachment 1). With digital signals I can't receive anything with an indoor antenna because the insulation in the outer walls has an aluminum foil vapor barrier (which forms a shielded enclosure) and we don't have any windows that face the transmitters.

I asked the landlord if I could put up an antenna and he said "only if it looks nice and can't be seen from the street." The enclosure for my antenna with a CM4221 is shown in attachments 2 & 3. Later I added a folded dipole for CH13. Yes, the signals have to come through the holes in the decorator blocks. There is only a slight attenuation for UHF signals, but a little more for VHF-hi. The holes seem to act like a highpass filter! The enclosure is located in a common area and not for my exclusive use, so he didn't have to say yes.

My present TV for digital reception during a power failure is shown in attachment 4. It is powered by an inverter and a 12v battery pack. The post about it is here:
http://www.avsforum.com/avs-vb/showt...5#post18514215 #354

The next part will show some tests using the Sadelco DisplayMax 800; it is easier to find used than the older Field Strength SLMs.
LL
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LL
LL


If you can not measure it, you can not improve it.
Lord Kelvin, 1883
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Yes, Radioshack still carries outdoor antennas made by Antennacraft. However, the selection may vary from store to store.
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post #14968 of 16235 Old 06-04-2012, 04:40 PM
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Hmm, well so they do! There is quite the selection on-line too. Four or five years ago, when I was last looking for an antenna, I don't remember them having much of anything.
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You can also choose ship to store from the RS web site.
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retiredengineer:

Finding channels missed by tuner scan, Part 4 of 4

The Sadelco 719E signal level meter has an elegant simplicity for channel hunting: turn it on, tune for channels, listen for an increase in noise as you tune through a digital channel (the digital signal sounds like white noise), and watch the needle of the panel meter for signal strength.

That type of meter is now hard to find, so I tried some channel hunting with my Sadelco DisplayMax 800 meter. This is what CH42, that was missed during my tuner scan, looks like:

DM800543copy_1.jpg

As you can see, the top of the signal is not very flat, which probably indicates a multipath reflection problem. That's not surprising, because the antenna is aimed in the opposite direction and only reflections off the objects in front of the antenna are picked up; there is no direct signal from the transmitter.

When the 800 meter is in the single channel mode it scans across the channel making 43 readings in about 10 seconds, which is a lot like what a spectrum analyzer does. Those readings are averaged and a correction factor of 6.8 dB (according to the Sadelco tech) is added to give the final signal power in dBmV. In this scanning mode, the lower limit is -20 dBmV. If any of the 43 readings is below -20 dBmV, then the meter reads Ur for under range.

Although this single channel mode allows you to see the signal shape, it is not useful for signals that are below the limit of -20 dBmV. For the above scan I used my CM7777 preamp to make the signal just strong enough for a scan. You can use a preamp before the 800 if you don't have any strong signals that would cause preamp overload. If you do have signals that strong, then you wouldn't be able to use a preamp anyway for reception, because the IM products produced in the preamp would mask your weak signals. See the charts by holl_ands that deal with preamp overload and SFDR (Spurious Free Dynamic Range).

When aiming an antenna you can switch from the single channel scan to the frequency mode by pressing the F1 key just below CHN/FREQ of the display. You then will have immediate feedback readings of signal strength (at center channel) to help you aim the antenna. As a bonus, this mode goes down to -35 dBmV, which is -107.8 dBm at the antenna terminals with a 24 dB preamp. The reading doesn't include the correction factor, so you must switch back to the single channel mode for that. The freq mode looks like this:

DM800544copy_1.jpg

Just for fun, I added a second preamp of 15 dB in series to give a higher reading. It worked, but the tuner was no longer able to lock on to 2.1 on CH20, because the SNR was reduced by the noise that was added by the second preamp.

The day after I made the above measurements, I went across the street and setup a 2-bay UHF antenna, my meter, and a preamp. I was able to get a nice scan and a stronger signal with the antenna aimed at the transmitter for CH42.

CH42setup.jpg

Interestingly, when I moved the antenna a few feet left or right, without changing the height or azimuth, there was a big difference in the signal strength and scan quality. This is most likely because of the tree line in front of the antenna about 200 ft away. Calaveras, in post # 14990 further down, mentions that he has seen the same problem with trees when using his spectrum analyzer.
Quote:
Originally Posted by Calaveras View Post

I've had my spectrum analyzer over to several locations where the antenna was looking through trees and the signals can be pretty ugly and vary dynamically with tree movement. Some signals make it through while others are destroyed. UHF is much more affected than VHF in most cases.

Using the pilot:
You previously asked about measuring the pilot signal. The 800 tunes in 125 KHz steps. You can go from the center freq to the low end by pushing the left arrow key many times, but it is easier to enter the freq of the low end and then push the right arrow key a few times to go by the pilot, which is 310 KHz above the low end.

In the case of CH42, you would step from 638.000 MHz to 638.125, 638.250, 638.375 to take readings. The pilot is at 638.310, which is between 638.250 and 638.375. The measurement bandwidth is 280 KHz at the 3 dB down points, so the last two frequencies in the series should show a higher reading.

In the single channel mode, the scan starts at 638.375 MHz, which is the first measurement of 43 across the 5.3 MHz center part of the 6 MHz channel.

MeasuringCH42pilot2.jpg

The ads at the right in the new Forum software have squeezed my post to make it narrow (at least on my monitor.....maybe not on a widescreen monitor), so the above chart has been reduced in size from 648 x 568 to 414 x 363 pixels. For the full size chart go to:
http://i496.photobucket.com/albums/rr328/rabbit73_photos/MeasuringCH42pilot2.jpg

The pilot at the left end of the scan, Figure 5, looks good.

CH42Pilot.jpg

Even though the scan in Figure 5 looks good, you must be careful not to conclude that the signal quality is also good based only on the scan. It is possible to have a good-looking scan, but not be able to receive the signal as Trip in VA found out. It is the tuner that determines if the signal quality is good enough to decode.

My 8-inch Audiovox TV in the car was able to pick up CH42 WCVE; the 22-inch Sony was too big to bring along.

CH42found.jpg

If you want to look at the manual for the DisplayMax 800, you can download it from the Sadelco website. They also have refurbished meters.

retiredengineer:

The measurement method that I have described will do what you want with the 800 meter.

Please let me know how it works out for you.

Best regards,
rabbit

Other related posts and links:

retiredengineer asks about pilot
my answer
Part 1
Part 2
Part 3
3 types of overload
dBmV vs dBm
Old TV Field Strength Meter
Trees and UHF Reception in UK


The rabbit will be QRT for now.frown.gif
(That's a ham Q signal)

If you can not measure it, you can not improve it.
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