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Signal Strength Variables

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Old 07-26-2009, 08:34 AM   #1
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Default Signal Strength Variables

I know OTA signal strengths are affected by many variables, tropospheric conditions, weather, obstructions, etc., but it seems that that the signal strength threshold for stable reception from different stations is also varible. During some of those atmosheric conditions it seems that some transmitted signals are still stable at ~50%, and others are susceptible to drop outs and macro blocking at that level. Is that a product of your tuner's frequency control?

Last edited by geronimo; 07-26-2009 at 09:03 AM.
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Old 07-26-2009, 09:44 AM   #2
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In the analog days there were two effectively transmitters for the TV
station.
A transmitter for the video and a transmitter for the audio.
The video transmitter was Amplitude Modulation see
http://en.wikipedia.org/wiki/NTSC

The audio was FM at a much lower power.

The reason that the video was a higher power is that AM is more susceptible to noise requiring a stronger signal at the receive and a higher power output of the video transmitter.

The way the tuner saw the signal all depended on the first transistor in the front end of the receiver. If a manufacturer used a good transistor than you might have good reception capabilities.
Long time ago, the manufacturer would even tell you how powerful a receiver it was.

Most receivers were able to pick up a signal to about -55 db.

If you had a local signal that transmitted at 40 db., you had a safety factor about about 15 db., which would compensate for the loss in the wire, atmospheric signal reduction, trees, and other factors.

You could usually get a good signal.

Digital tuners do not work the same way.

There is no more AM in the equation and with digital it is all or nothing.

The only way to receive a digital signal is to either live as closely as possible to the transmitter. Put up a outdoor antenna as high as possible and as far away as possible from any obstructions and interference and make sure to dot all your I's and cross all your T's.

One bad connector in the wire anywhere between the antenna and the television and you can end up with no signal for some stations and poor signals for others.

That means every crimp, every splice, every terminal has to be perfect - or you loose signal.

Analog AM - was so strong that you could loose 50% of the signal and still have a signal, due to the fact that the television showed white noise - static along with the picture and sound.

Using a pre amplifier takes one part out of the equation due to the fact that the amplifier sends 18 to 24 volts to the pre amplifier and back to the injector where the voltage is removed from the signal and a stronger signal is reproduced to send to the splitters and television. A voltage block can be used in the line between the antenna and the injector - if someone was that desperate to interrupt the signal - to save wire or have a shorter connection. But I never had much success using them.

Meanwhile, if you have a amplifier with 18 db of gain. That is like the television station increasing their power 64 times. 3 db of gain doubles the power.

So you add the 15 db to the 18 db and now you would have 33 db safety factor. The problem with saying that is that the amplifier not only amplify's the signal, it also amplifies the noise. So you loose a little bit of your gain.

a amplifier with a high gain is great but the internally generated noise must be at the minimum.

That is the reason why no one sells a commercially built pre amplifier with more than 26 db. of gain on the open market.

Sooner or later, you hit the signal noise floor, where there is no signal to amplify and the noise is louder than the signal and the amplifier cannot amplify the signal loud enough for one to over come the other. Too much amplification is just as bad as not enough!

That is the reason why when I hear someone on a forum say - I wish the television station would turn up their power. I get a chuckle.

Every time you gain one station in signal strength, you loose another when you live too close to the transmitter. That is the reason why those high power amplifiers that you see listed on Shortwave sites usually burn out when you connect them to a high gain antenna and a rotor.

Sooner or later, if you live too close to a transmitter, you will point the antenna at that transmitter and it will over load the amplifier and smoke it.

Height equals gain!

Get your antenna as high as possible and mount it to a stationary object, not the top of a tree. Tree's sway and that makes the antenna move and when you move the antenna you are not pointing it directly at the transmit antenna which is mounted on a semi stationary platform.

Believe me - you get 1000 feet up on a tower, you will feel it sway in the wind!

VHF liked lot's of height. A powerful transmitter and flat terrain.

Most UHF transmitters are only mounted 150 to 300 feet above ground level.

When you increase height on a UHF transmitter, you create Null's and Void's. Places where even though you can physically see the transmitter, your television will have no signal!

Cell phones are UHF - 869 - 913 mhz.

How many times have you driven down the road and lost your signal and could see the tower in front of you? It isn't always because that tower is owned by Verizon and you have Nextel. It is because you are in the shadow of the tower and their transmitter cannot see your phone, even though it can see a phone 15 miles away.

The way to eliminate the nulls and voids is to use multiple antenna's and a reflector behind the antenna that increase gain to one direction - while taking away gain in another.

That is the reason for translators. Translators transmits the signal on a different frequency - with the same sound and picture, just no program information.

Repeaters transmits the signal on the same frequency.

The problem with repeaters is that if the distance between one transmitter and the second transmitter is more than 1/2 of a mile, you get a echo. Your receiver will pick up two different signals at the same time. Unless those signals were clocked to be in sequence with each other at your location, would make your signal unwatchable.

That is what Penn State worked on for their system, which was the first to use a repeater and a clock for error correction to keep them in sequence. Even though there is two transmitters and one is 100 times more powerful than the other. They both receive at the same time and the same way if you live between them and can receive them both.

The object being that the television station and the FCC knows that UHF will not travel as far reliably as VHF analog did. If the television stations wish to cover more area, they will have to use many translators to do what one analog transmitter did in the past.

For some stations, they cannot justify the cost of putting up those translators and paying the electric bill, just so some person on the North Shore of Erie can watch a television station to the south that has no coverage in the middle of downtown Erie PA.

Other stations - who can justify the cost will do it.

Over the air television is going to give it one last try.
It is going to be sink or swim.
The stations that makes it will be around for a long time and the stations that does not have a network affiliation that can combine the resources of more profitable stations into the pot with less profitable stations will sell or go out of business.

The ones that cannot pay the bills will be sold to the home shopping network and the frequency band they occupy will be used for more mobile communications.

Eventually everyone will pay to watch television!
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Old 07-26-2009, 10:15 AM   #3
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Is that a product of your tuner's frequency control?

Digital television signals are broadcast in a 6 Mhz band.

The tuner is digital and the signal does not drift like a AM analog tuner did.

There are a number of different ways to receive digital television. One of the oldest means of receiving DTV (and TV in general) is using an antenna (known as an aerial in some countries). This way is known as Digital Terrestrial Television (DTT). With DTT, viewers are limited to whatever channels the antenna picks up. Signal quality will also vary.

from wikipedia:

High-definition television (HDTV), one of several different formats that can be transmitted over DTV, uses one of two formats: 1280 720 pixels in progressive scan mode (abbreviated 720p) or 1920 1080 pixels in interlace mode (1080i). Each of these utilizes a 16:9 aspect ratio. (Some televisions are capable of receiving an HD resolution of 1920 1080 at a 60 Hz progressive scan frame rate — known as 1080p60, but this standard is not currently used for transmission.) HDTV cannot be transmitted over current analog channels.

Standard definition TV (SDTV), by comparison, may use one of several different formats taking the form of various aspect ratios depending on the technology used in the country of broadcast. For 4:3 aspect-ratio broadcasts, the 640 480 format is used in NTSC countries, while 720 576 (rescaled to 768 576) is used in PAL countries. For 16:9 broadcasts, the 704 480 (rescaled to 848 480) format is used in NTSC countries, while 720 576 (rescaled to 1024 576) is used in PAL countries. However, broadcasters may choose to reduce these resolutions to save bandwidth (e.g., many DVB-T channels in the United Kingdom use a horizontal resolution of 544 or 704 pixels per line).[3] This is done through the use of interlacing, in which the effective vertical resolution is halved to 288 lines.

Each commercial terrestrial DTV channel in North America is permitted to be broadcast at a data rate up to 19 megabits per second, or 2.375 megabytes per second. However, the broadcaster does not need to use this entire bandwidth for just one broadcast channel. Instead the broadcast can be subdivided across several video subchannels (aka feeds) of varying quality and compression rates, including non-video datacasting services that allow one-way high-bandwidth streaming of data to computers.

A broadcaster may opt to use a standard-definition digital signal instead of an HDTV signal, because current convention allows the bandwidth of a DTV channel (or "multiplex") to be subdivided into multiple subchannels (similar to what most FM stations offer with HD Radio), providing multiple feeds of entirely different programming on the same channel. This ability to provide either a single HDTV feed or multiple lower-resolution feeds is often referred to as distributing one's "bit budget" or multicasting. This can sometimes be arranged automatically, using a statistical multiplexer (or "stat-mux"). With some implementations, image resolution may be less directly limited by bandwidth; for example in DVB-T, broadcasters can choose from several different modulation schemes, giving them the option to reduce the transmission bitrate and make reception easier for more distant or mobile viewers.

Digital television signals must not interfere with each other.
The following table gives allowable signal-to-noise and signal-to-interference ratios for various interference scenarios. This table is a crucial regulatory tool for controlling the placement and power levels of stations. Digital TV is more tolerant of interference than analog TV, and this is the reason fewer channels are needed to carry an all-digital set of television stations.

System Parameters
(protection ratios)

C/N for AWGN Channel - +15.19 dB

Co-Channel DTV into Analog TV - +34.44 dB

Co-Channel Analog TV into DTV - +1.81 dB

Co-Channel DTV into DTV - +15.27 dB

Lower Adjacent Channel DTV into Analog TV - −17.43 dB

Upper Adjacent Channel DTV into Analog TV - −11.95 dB

Lower Adjacent Channel Analog TV into DTV - −47.33 dB

Upper Adjacent Channel Analog TV into DTV - −48.71 dB

Lower Adjacent Channel DTV into DTV - −28 dB

Upper Adjacent Channel DTV into DTV - −26 dB

DTV has several advantages over analog TV, the most significant being that digital channels take up less bandwidth.
This means that digital broadcasters can provide more digital channels in the same space, provide high-definition television service, or provide other non-television services such as multimedia or interactivity. DTV also permits special services such as multiplexing (more than one program on the same channel), electronic program guides and additional languages, spoken or subtitled.


Digital signals react differently to interference than analog signals. For example, common problems with analog television include ghosting of images, noise from weak signals, and many other potential problems which degrade the quality of the image and sound, although the program material may still be watchable. Digitized signals are designed to resist ghosting or noise by using a redundant signal composed of numeric codes. Even if some of the information is missing or wrong, the decoder computer can reconstruct the complete signal. The only way it fails is when the decoder does not receive enough information from the antenna -- if there is too much interference in the signal for the decoder to read enough of the numbers and produce the picture. This can render a digital signal completely or partially unwatchable (picture pixelates or freezes) in situation where an analog signal would still be usable, in urban (ghosting due to multi-path) and rural (weak signal) areas.

Analog technology uses simple channel numbers, which correspond to broadcast frequencies; programs are accessed using these channel numbers which have been in use for decades. The new digital technology has a more complex structure of channels and sub-channels; in addition, digital signals are normally named using virtual channels, which do not correspond to frequencies. It may be hard to find out what actual frequency a program uses. The normal procedure is to have the digital tuner scan for all available signals. Any changes by the broadcasters or antenna changes may require a lengthy re-scan.

Changes in signal reception from factors such as degrading antenna connections or changing weather conditions may gradually reduce the quality of analog TV. The nature of digital TV results in a perfect picture initially, until the receiving equipment starts picking up noise or losing signal. Some equipment will show a picture even with significant damage, while other devices may go directly from perfect to no picture at all (and thus not show even a slightly damaged picture), or lock up, with audio dropping out and a freeze-frame displayed. This latter effect used to be known as the digital cliff or cliff effect. Now it is known that in 'edge' areas digital transmissions suffer from typically 16x16 pixellated blocks, the number of blocks depending on weather conditions or other interference eg co-channel. Picture quality can change on daily or real time basis fom small number of blocks ( as well as audio 'cracks' and 'bloops') to complete picture freezing.

For remote locations, distant channels that, as analog signals, were previously usable in a snowy and degraded state may, as digital signals, be perfect or may become completely unavailable. In areas where transmitting antennas are located on mountains, viewers who are too close to the transmitter may find reception difficult or impossible because the strongest part of the broadcast signal passes above them. The use of higher frequencies will add to these problems, especially in cases where a clear line-of-sight from the receiving antenna to the transmitter is not available. Many intermittent signal fading conditions, such as the rapid-fade effect caused by reflections of UHF television signals from passing aircraft, will not produce intermittently-snowy video, but potential intermittent loss of the entire signal, which most receivers will display as a frozen ("paused") image or a black screen for the duration of the signal loss.

Multi-path interference is a much more significant problem for DTV than for analog TV and affects reception, particularly when using simple antennas such as rabbit ears. This is perceived as "ghosting" in the analog domain, but this same problem manifests itself in a much more insidious way with DTV. (What was "ghosting" in analog becomes intersymbol interference (ISI), which causes data corruption, in digital TV. Beyond a certain point, corrupt data is as good as no data.) IEEE engineers recommend using an attic or outdoor antenna for DTV, if possible, rather than an indoor antenna, because reflections and other interactions of the signal with objects (including bodies) in the room will increase multipath interference. Unlike the problems of the preceding paragraph, multi-path can be worse for DTV under high signal conditions. It is perceived by the viewer as a spotty loss of audio or picture freezing and pixelation as people move about in the vicinity of the antenna and is often worse in wet weather due to increased reflection or re-polarization of the DTV signal arriving from multiple paths. In extreme cases the signal is lost completely. The cure is to employ a directional antenna outdoors, aligned with the transmitting location.

Last edited by JB Antennaman; 07-26-2009 at 10:28 AM.
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Old 07-26-2009, 01:54 PM   #4
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Thanx for your long voluminous reply. My interpretation of your dissertation is that an analog transmission is similar to a combined am signal(video) and a fm(audio)signal, Whereas a digital signal is like the data streams transmitted in computers. The ATSC(digital) tuner replicates a computer and the NTSC(analog) tuner replicates a multi-band radio receiver. Enviromental interference can corrupt or drop data transmissions(drop outs, pixelation), and ghosting, snow, etc., in an analog transmission. As in a computer, missing or corrupt data causes serious problems; in a DTV, it causes pixelation, reception loss, etc. Like an multi-band radio, the same level of interference is less severe in an analog tv(snow, ghosting, intermittent loss of sync), but still watchable. Depending on your particular juxtaposition to a transmitter, the level of interference can be greater than on other transmitters. Now, I'm getting log winded!
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Old 07-26-2009, 03:33 PM   #5
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"Examinations are formidable even to the best prepared, for the greatest fool may ask more than the wisest man can answer."

-- Charles Caleb Colton

Just because it was a simple question, does not mean that it could be answered by a simple answer.

The more times you answer the same question over and over again.

The more times you leave gaps that people try's to fill with their opinions.

The problem with opinions is that every body has one and it always isn't right.

Then you end up with a post 3 pages long of people trying to dispute what someone else said.

When you fill in the gaps as best you can, and still there is a dispute, then there is a problem where you have to go back to school to get the answer.

I keep giving answers out of my Reception 101 book, and people doesn't listen.

Like the post said, it might work good when everything is new.

Then a week or a month or a year later when things degrade - you have no signal.

You can put a antenna in a attic, but it probably isn't going to work properly.

You can use rabbit ears, but even people moving inside of the room will corrupt the signal.

People are so stuck on analog technology that they forget that this is digital UHF and low power VHF and everything you learned about VHF no longer works the same way.

Last edited by JB Antennaman; 07-27-2009 at 09:04 AM.
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Old 07-26-2009, 04:17 PM   #6
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Someone, cogitating on the same "soapbox" as Confucius prophetsized "Keep it simple stupid"
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Old 07-27-2009, 02:36 PM   #7
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Just a couple of quick points.

1. The reason you can see a cell phone tower and not have service is that the tower you see may not be that of your provider and your phone is not in roaming mode.

2. The modulation methods of todays ATSC is generally using 8VSB format. Instead of using the above old AM/FM described methods for audio and video they are using digital encoded supressed carrier AM. Since everyone likes Wikipedia here is a link that describes it better than what I would:

http://en.wikipedia.org/wiki/8VSB
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Old 07-28-2009, 04:21 AM   #8
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Quote:
Originally Posted by geronimo View Post
Is that a product of your tuner's frequency control?
Short answer, probably not.

By frequency control, you're usually talking about being able to lock onto a channel (e.g., a pilot) and track it. In a moving vehicle (e.g. listening to FM in your car), this matters a lot because of constantly shifting Doppler, but for a stationary TV, this is usually a non-issue. Yes, it's possible for moving vehicles and swaying trees to have some effect, but they rarely cause enough of an effect to make the frequency tracking logic wander off.

What is much more common and problematic for 8VSB is multipath. The different paths of signals reaching your tuner create a mix of constructive and destructive interference that the receiver needs to sort out before recovering the data bits.

These kinds of interference vary with frequency. This is mostly because distance traveled by the multipath signals are different multiples of wavelengths depending on the frequency. In some cases you get an additive effect (constructive interference) while in other cases they cancel each other out (destructive interference). Furthermore, materials (e.g., trees & buildings) can have different absorption, reflection, and refraction characteristics at different frequencies.

Even channels coming from the same tower can have a different mix of multipath impairments by the time they reach your tuner. Channels will eventually break-up and get lost if the signal impairments get bad beyond a certain point. This breaking point might show up at a slightly different point on your TV's signal meter depending on exactly how the channel is being degraded and how well the tuner is able to compensate for those signal distortions.

Also keep in mind that signal meters on TVs are not standardized. The numbers that you see do not necessarily have any physical meaning. Most meters estimate signal quality (e.g., number of detectable data errors) as opposed to a real signal strength (i.e., amount of energy) meter.

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Old 07-28-2009, 07:20 AM   #9
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Quote:
Originally Posted by otaota View Post
Short answer, probably not.

By frequency control, you're usually talking about being able to lock onto a channel (e.g., a pilot) and track it. In a moving vehicle (e.g. listening to FM in your car), this matters a lot because of constantly shifting Doppler, but for a stationary TV, this is usually a non-issue. Yes, it's possible for moving vehicles and swaying trees to have some effect, but they rarely cause enough of an effect to make the frequency tracking logic wander off.

What is much more common and problematic for 8VSB is multipath. The different paths of signals reaching your tuner create a mix of constructive and destructive interference that the receiver needs to sort out before recovering the data bits.

These kinds of interference vary with frequency. This is mostly because distance traveled by the multipath signals are different multiples of wavelengths depending on the frequency. In some cases you get an additive effect (constructive interference) while in other cases they cancel each other out (destructive interference). Furthermore, materials (e.g., trees & buildings) can have different absorption, reflection, and refraction characteristics at different frequencies.

Even channels coming from the same tower can have a different mix of multipath impairments by the time they reach your tuner. Channels will eventually break-up and get lost if the signal impairments get bad beyond a certain point. This breaking point might show up at a slightly different point on your TV's signal meter depending on exactly how the channel is being degraded and how well the tuner is able to compensate for those signal distortions.

Also keep in mind that signal meters on TVs are not standardized. The numbers that you see do not necessarily have any physical meaning. Most meters estimate signal quality (e.g., number of detectable data errors) as opposed to a real signal strength (i.e., amount of energy) meter.

Cheers,
I must have rubbed off on somebody - because someone finally got it.

As far as cell phone towers goes, back in the day, in a rural area, when there was not enough towers, the cell phone service providers had an agreement where the cell phone tower would carry several providers with little or no roaming fee's.

The purpose of the agreement was so that your phone would work no matter how far out of the cell you got for your provider. It was cheaper than trying to put up 100,000 towers all at once.

If a phone had a limited coverage area, a subscriber would be more hesitant to purchase a agreement, especially if you had a friend that already had a agreement with the carrier and their phone didn't always work - bad publicity.

As new towers were built, the system was gradually switched back to one phone, one tower type systems - where service providers has their own tower.

In a city, where you have hundreds of thousands of subscribers, the cost is justified, where you have a couple hundred people - it's not as profitable.

So when I said that you could see the tower, you have to understand - I meant a tower that was your local service provider.
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Old 07-20-2011, 09:56 AM   #10
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Thanks to all of you that have so thourghly described the problems that I'm having by my using a rooftop Antenna and trying to receive TV signals from Chicago, IL (50 Miles away as the Crow Fly's) in VERY HOT & HUMID WEATHER. Early in the Morning I can receive most all of the Channels as well as their sub-channels, but as the HEAT & HUMIDITY build, it reached 100 yesterday with a HEAT INDEX of 114, I lost almost every channel. Even the ones that remained would either distort, or pixilate on occasion.
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Old 07-20-2011, 10:03 AM   #11
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I'd like to add that my Rooftop Antenna is a Highend Winegard, and I use a Winegard Signal Amplifier inside my house that is hooked up to the Coax lead from the Antenna. It then goes into a Splitter that has two Outputs, one of which goes to the Panosonic 42" Plasma TV, and the other to my Denon Receiver.
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