Myth from the 60's?

[[email protected]]

When I was a nerdy kid growing up in the 60's, I read an article in PopularElectronics that claimed you could detect tornados by tuning your TV to anunused channel; if you see a normal snow pattern, no tornado, but if the screen suddenly turns all-white then you should high-tail it to the basement.. I always wondered if this were really true. Any opinions?

Bob

 

[[email protected]]

When I was a nerdy kid growing up in the 60's, I read an article in
Popular Electronics that claimed you could detect tornados by tuning
your TV to an unused channel; if you see a normal snow pattern, no
tornado, but if the screen suddenly turns all-white then you should
high-tail it to the basement.

The idea was apparently to use the TV as a lightning detector, on the
theory that a storm producing a tornado would also be producing a lot
of lightning. There is a version of it here
http://message.snopes.com/showthread.php?t=9300 which mentions US TV
channel 2 and also mentions using the low end of the US AM broadcast
band on a radio.

NOAA and the University of Oklahoma say it doesn't work - it may detect
lightning, but not all tornadic storms produce a lot of lightning.
http://www.wrh.noaa.gov/hnx/svrwx6.php
http://usatoday30.usatoday.com/weather/wtwistqa.htm (scroll down to the
last question under "Detecting Tornadoes")

Matt Roberds

 

[Robert Macy]

   Yes.  NTSC used inverted video, and sync was referred to as "Blacker
than Black" since the transmitter was only at full power during the sync
pulses.  The TV had to be on channel 2 or 3, where the atmospheric noise
could travel further.

From memory it is likely that channel 2, 3 or 4, 5, etc are very
llikely to be occupied. Since low 'high frequency' equipment cost a
bit less, stations throughout the midwest grabbed channel 2 first. I
know in the Bay Area we received over 66 channels just using rabbit
ears with Channel 4 being used, but not channel 3; and in Boston Area
Channel 3 is used, but not channel 4. Again, from memory.recall
something about those two adjacent channels are very prone to
interfering, so the FCC did not grant licenses to both in the same
metro service area.

Anybody out there confirm? Deny?

 

[[email protected]]

Thanks, looks like it just detects strong electromagnetic emissions but isn't very reliable. I'll ignore that advice from Popular Electronics and instead stick with tbe numerous articles about how to make an audible turn-signal click sound using a 555 timer

Bob

 

[miso]

The idea was apparently to use the TV as a lightning detector, on the
theory that a storm producing a tornado would also be producing a lot
of lightning. There is a version of it here
http://message.snopes.com/showthread.php?t=9300 which mentions US TV
channel 2 and also mentions using the low end of the US AM broadcast
band on a radio.

NOAA and the University of Oklahoma say it doesn't work - it may detect
lightning, but not all tornadic storms produce a lot of lightning.
http://www.wrh.noaa.gov/hnx/svrwx6.php
http://usatoday30.usatoday.com/weather/wtwistqa.htm (scroll down to the
last question under "Detecting Tornadoes")

Matt Roberds

A similar scheme could be use to detect meteors. The meteor leaves an
ionization trail.

TO be clear here you need an unused channel AND a TV station just beyond
the radio horizon on that channel.

Using AM radio to detect lightning storms isn't particularly useful
since the noise propagates so well that the warning isn't particular
very geographically significant.

 

[The Great Attractor]

A similar scheme could be use to detect meteors. The meteor leaves an
ionization trail.

TO be clear here you need an unused channel AND a TV station just beyond
the radio horizon on that channel.

Using AM radio to detect lightning storms isn't particularly useful
since the noise propagates so well that the warning isn't particular
very geographically significant.

It is easy to use a TV. Not for individual lightning strikes as that
provide little info other than timeline.

They can be used quite well as tornado detectors, however.

Same thing. It catches the lightning strikes. The difference being
that when they are very high in number, it is very likely a funnel is
nearby.

 

[The Great Attractor]

The FCC had to reassign a lot of channel numbers at one time, to
eliminate co-channel interference. When a lot of those stations went on
the air, they barely covered 10 miles. As transmitters got bigger &
towers went higher they had problems. That caused the FCC to implement
their 150 mile rule for same channel usage, and to eliminate adjacent
channel headaches.

There is a gap between TV channels 4 & 5 at 72 MHz-76 MHz and a
another between 6 & 7 from 88 MHz to 174 MHz, so those channel number
could be used adjacent without problems.

Ch 6 is suggested for the tornado detection thing.

 

[Bob Boblaw]

When I was a nerdy kid growing up in the 60's, I read an article in Popular Electronics that claimed you could detect tornados by tuning your TV to an unused channel; if you see a normal snow pattern, no tornado, but if thescreen suddenly turns all-white then you should high-tail it to the basement. I always wondered if this were really true. Any opinions?

Here's the article about it from the March 1969 Popular Mechanics cover story:

http://tinyurl.com/bgllebd


http://books.google.com/books?id=MN...opular mechanics march 1969 tornadoes&f=false

 

[[email protected]]

If I recall how NTSC worked, the screen would turn black, not white, when
a lot of signal was coming in.

Negative modulation was used in most TV systems in the world, so the
screen would indeed go black with a stronger signal.

However, you have to consider AGC in RF and IF stages. If the AGC
drops the front end gain, less band noise and less front end thermal
noise will enter the video detector, the snow pattern would become
whiter.

In normal operation, the signal strength is sampled only during the
synch pulse and the AGC level is hold constant during the entire line
time. However, if there are no synch pulses, how is the AGC voltage
generated ? Apparently this depends on the circuit design for a
particular receiver type.

 

[[email protected]]

Except France, it had positive modulation.

The French always want to be original, running metro trains with
rubber wheels and using positive modulation on both 625 as well as
their own 819 line B&W system. The pre-war UK 405 line System-A also
used positive modulation.

The nice thing about negative modulation is that the carrier is always
present, making it possible to use intercarrier sound. On positive
modulation systems, audio recovery had to be done separately,
requiring a higher frequency stability.

There were also interestng anti-noise spike circuits.
Philps once uses a varicap diode over an IF bandfilter,
that detuned the IF circuit in the presence of strong RF pulses.

Those noise pulses were common from cars and motorbike ignition,
before those got equiped with screend cables to the sparkplugs.

Noise blankers should be positioned as close to the antenna as
possible, before any sharp band pass filters. A short but high
amplitude pulse is easy to kill, but if it goes through some band pass
filter, it will be widened and reduced in amplitude and it is
impossible to distinguish from the real signal.

I have build a small lightning detector that uses a ferrite rod
tuned so some LW frequency, and a PIC comparator to flash a LED:
http://panteltje.com/panteltje/cb/lightning_detector.txt
http://panteltje.com/panteltje/cb/lide-0.1.zip

The majority of the LEMP energy is at frequencies well below 1 MHz, so
the LF band is a good place to start.