Do too strong signals look like too weak signals?

[micky]

Do too strong signals look like too weak signals?

I finally replaced my antenna amplifier, and I have a lot more TV
stations now.

But some of the stations I used to get well now suffer occasionally from
the same symptoms that weak stations used to have.

1) Checkerboarding, or whatever it is called. (Squares in part of the
screen that stay fixed when the images should be moving.)
2) Total blanking out of the picture & sound, usually for short periods.
3) And a new one, the sound disappearing for a half second, every 3
seconds.

Could each of these symptoms be caused by a signal that's amplified too
much?

Is it possible I've set the amplification too high for these already
strong stations.

Thanks.

 

[David Platt]

Do too strong signals look like too weak signals?

I finally replaced my antenna amplifier, and I have a lot more TV
stations now.

But some of the stations I used to get well now suffer occasionally from
the same symptoms that weak stations used to have.

1) Checkerboarding, or whatever it is called. (Squares in part of the
screen that stay fixed when the images should be moving.)
2) Total blanking out of the picture & sound, usually for short periods.
3) And a new one, the sound disappearing for a half second, every 3
seconds.

Could each of these symptoms be caused by a signal that's amplified too
much?

Yes, that's possible.

What can happen, with an antenna amplifier, is that the presence of a
strong signal can "saturate" the amplifier.. driving its RF output
level up to the point where the amplifier output is clipping (or,
short of clipping, is becoming seriously distorted). This problem can
affect both an external antenna amplifier (such as you are using) and
the "front end" amplifiers in a TV receiver.

When this happens, the quality of the amplifier's output deteriorates
badly. If a strong signal is clipped, its intelligibility
suffers... and this can lead to digital-TV freezes, "macroblocking",
sound dropouts, and so forth.

A less-strong signal can also suffer when the amplifier clips. During
the time when the amplifier is saturated handling a signal at one
frequency, it can't as easily "pass through" other (weaker)
frequencies, and they too will be distorted or will actually become
weaker. This "desensitizing" (or "desense") problem can be quite
severe.

Now, most TV-type amplifiers are broad-band devices. They will
attempt to amplify every signal that gets into their input, from low
VHF up through UHF, by roughly the same amount. Basically, if it gets
into your TV antenna, the amp is going to try to boost it... and so
even a non-TV signal can saturate your amplifier, and distort all of
the TV signals also present.

It's not at all uncommon for strong, local transmitter signals
(non-TV) to cause problems for TV antenna amplifiers. CB is a high-HF
frequency (28 MHz or so), and there are plenty of police, fire,
business-band, and amateur radio systems operating in both VHF and UHF
frequencies. If somebody "keys up" on one of these bands while
driving near your antenna (e.g. a cab, or a truck with a business
radio, or a ham), it could drive your amplifier into saturation and
wipe out your TV signal.

There's no wonderfully easy solution to this problem.

One partial fix is to identify the frequencies that are saturating
your system, and install "band-reject" or "notch" filters in your
antenna feedline, just prior to the amplifier.

Another is to turn down the amplifier. From what I've seen, the best
results usually occur if you use a high-quality amplifier mounted up
on the mast (right below the antenna), which has just enough gain to
overcome the losses in your feedline and any splitters you have
between the antenna and the TV(s). Having more gain than this often
isn't useful, and makes your system more vulnerable to overload.

Another thing to do is consider *antenna* gain, rather than
*amplifier* gain. Install a directional outdoor antenna, with a
remote-controlled antenna rotator, and turn it to point in the best
direction for each station you want to receive. The antenna's gain
will boost the desired signal (from the right direction) while tending
to reject unwanted signals coming from other directions. The antenna
directionality will also help reject "multipath" (delayed 'echos' of
signals you want, which have bounced off of nearby buildings and trees
and hills) and this will help improve the reliability of the
digital-TV signal decoder.

 

[Phil Allison]

micky wrote:

> Do too strong signals look like too weak signals?

** Not really and that is not your problem.

I finally replaced my antenna amplifier, and I have a lot more TV
stations now.

** Is it a mast head amplifier ?

Damn near useless if it is not.

But some of the stations I used to get well now suffer occasionally from
the same symptoms that weak stations used to have.

** The fact that it is "ocassionally" indicates interference is the cause.

1) Checkerboarding, or whatever it is called. (Squares in part of the

screen that stay fixed when the images should be moving.)

2) Total blanking out of the picture & sound, usually for short periods.

3) And a new one, the sound disappearing for a half second, every 3

seconds.


Is it possible I've set the amplification too high for these already
strong stations.

** Be a rocket scientist and TRY turning the gain down !!!!


FYI:

When TV was analogue, one could SEE the general nature of any interfering signal on the screen - either as noise, horizontal/ vertical lines or some other picture aberration. Often, these clues were enough to at least classify the type of interference.

With DTV, all you have is pixellations and loss of signal (ie pic freezing) and that tells you SFA about the cause.

IF you have a roof top antenna, good quality co-axial cable and a mast head amplifier in a weak signal area - most of the issues disappear.

What is left is mostly due to electrical arcing as light and power switches are opened and closed in the same or adjacent premises.

I suspect and input overloaded antenna amplifier could make that sort of interference worse.


.... Phil

 

[mike]

On 10/8/2014 2:37 PM, David Platt wrote:

Do too strong signals look like too weak signals?

I finally replaced my antenna amplifier, and I have a lot more TV
stations now.

But some of the stations I used to get well now suffer occasionally from
the same symptoms that weak stations used to have.

1) Checkerboarding, or whatever it is called. (Squares in part of the
screen that stay fixed when the images should be moving.)
2) Total blanking out of the picture & sound, usually for short periods.
3) And a new one, the sound disappearing for a half second, every 3
seconds.

Could each of these symptoms be caused by a signal that's amplified too
much?

Yes, that's possible.

What can happen, with an antenna amplifier, is that the presence of a
strong signal can "saturate" the amplifier.. driving its RF output
level up to the point where the amplifier output is clipping (or,
short of clipping, is becoming seriously distorted). This problem can
affect both an external antenna amplifier (such as you are using) and
the "front end" amplifiers in a TV receiver.

When this happens, the quality of the amplifier's output deteriorates
badly. If a strong signal is clipped, its intelligibility
suffers... and this can lead to digital-TV freezes, "macroblocking",
sound dropouts, and so forth.

A less-strong signal can also suffer when the amplifier clips. During
the time when the amplifier is saturated handling a signal at one
frequency, it can't as easily "pass through" other (weaker)
frequencies, and they too will be distorted or will actually become
weaker. This "desensitizing" (or "desense") problem can be quite
severe.

Now, most TV-type amplifiers are broad-band devices. They will
attempt to amplify every signal that gets into their input, from low
VHF up through UHF, by roughly the same amount. Basically, if it gets
into your TV antenna, the amp is going to try to boost it... and so
even a non-TV signal can saturate your amplifier, and distort all of
the TV signals also present.

It's not at all uncommon for strong, local transmitter signals
(non-TV) to cause problems for TV antenna amplifiers. CB is a high-HF
frequency (28 MHz or so), and there are plenty of police, fire,
business-band, and amateur radio systems operating in both VHF and UHF
frequencies. If somebody "keys up" on one of these bands while
driving near your antenna (e.g. a cab, or a truck with a business
radio, or a ham), it could drive your amplifier into saturation and
wipe out your TV signal.

There's no wonderfully easy solution to this problem.

One partial fix is to identify the frequencies that are saturating
your system, and install "band-reject" or "notch" filters in your
antenna feedline, just prior to the amplifier.

Another is to turn down the amplifier. From what I've seen, the best
results usually occur if you use a high-quality amplifier mounted up
on the mast (right below the antenna), which has just enough gain to
overcome the losses in your feedline and any splitters you have
between the antenna and the TV(s). Having more gain than this often
isn't useful, and makes your system more vulnerable to overload.

Another thing to do is consider *antenna* gain, rather than
*amplifier* gain. Install a directional outdoor antenna, with a
remote-controlled antenna rotator, and turn it to point in the best
direction for each station you want to receive. The antenna's gain
will boost the desired signal (from the right direction) while tending
to reject unwanted signals coming from other directions. The antenna
directionality will also help reject "multipath" (delayed 'echos' of
signals you want, which have bounced off of nearby buildings and trees
and hills) and this will help improve the reliability of the
digital-TV signal decoder.

I had this problem with ATSC tuners in my PC.
I'm lucky, because most of the stations are in approximately the
same direction.
I'm unlucky, because there's a huge metal building 50 feet from my antenna.
Multipath was a big problem. Huge variation in signal strength among
channels was a big problem.

Turning the antenna wasn't useful because I record/time-shift multiple
channels at once.
Tried homebrew bandpass filters and variable attenuators.
I could tweak the attenuator just right to make any, but not all
channels work.
The fix was to use a splitter and attenuate signals to different
tuners separately and force recording of specific channels on specific
tuner cards.

As tuner cards improved over the years, I was able to remove most of that
and find one antenna position and system gain that works for all
channels on all tuners.

 

[Maynard A. Philbrook Jr.]

In article <jo6b3atusnhevkemea6n8b4oh5u411t9d8@4ax.com>, NONONOmisc07
@bigfoot.com says...

Do too strong signals look like too weak signals?

I finally replaced my antenna amplifier, and I have a lot more TV
stations now.

But some of the stations I used to get well now suffer occasionally from
the same symptoms that weak stations used to have.

1) Checkerboarding, or whatever it is called. (Squares in part of the
screen that stay fixed when the images should be moving.)
2) Total blanking out of the picture & sound, usually for short periods.
3) And a new one, the sound disappearing for a half second, every 3
seconds.

Could each of these symptoms be caused by a signal that's amplified too
much?

Is it possible I've set the amplification too high for these already
strong stations.

Thanks.

Sure, you are saturating the RF preamp from near by frequencies and is
forcing the front-end to be weak.

But then again, I've noticed that lately on the Sifi channel.. It
may not be you, cause I have cable..


Jamie