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Posted by Tom Horne on September 8, 2008, 6:49 pm
Tor-Einar Jarnbjo wrote:
> Dave Garland schrieb:
>
>> A digital signal either works, or it doesn't. So reception is good,
>> then it craps out completely. (People with over-the-air reception of
>> digital TV will notice this.)
>
> That is not always the case and for digital cell phone networks in most
> cases wrong. Even the first digital GSM networks (early 90s) used
> different levels of error correction for parts of each data packet,
> making it possible for a receiver with bad reception still to decode the
> most important parts of the data, resulting in lower voice quality. Even
> if the reception is so poor, that some data cannot be completely
> restored by the error correction algorithms, the receiver usually still
> tries to decode the erroneous data. Depending on the actual codec, this
> causes different problems, like e.g. blocking artifacts in the digital
> tv picture.
>
> The AMR voice codec used in more recent UMTS networks, even allows the
> network and cell phone to dynamically switch between eight different
> levels of error correction during a conversation to find the optimal
> point on a scale between an error-prone/high effective bitrate/high
> voice quality signal and a robust, but low voice quality signal.
>
> At some point, the reception is of course too poor for the receiver to
> do anything useful at all with the signal, but the steps leading there
> are far more complex than an either/or decision.
>
> Tor
>
Tor,
That too is a liability in public safety work: the systems' attempts to
compensate conceal the deteriorating signal path from the user. He/she
has no warning of the impending demise of the talk path. There is a
reason that the FAA and the airline industry have stayed with AM and
SSB. Concurrent signals can sometimes both be understood by the human
brain and the signal generally degrades in a noticeable way prior to
becoming unreadable.
--
Tom Horne
"This alternating current stuff is just a fad. It is much too dangerous
for general use." Thomas Alva Edison
***** Moderator's Note *****
If AM offers those advantages, I wonder why public safety uses FM.
Bill Horne
Temporary Moderator
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Posted by Tor-Einar Jarnbjo on September 8, 2008, 8:55 pm
Tom Horne schrieb:
> That too is a liability in public safety work: the systems' attempts to
> compensate conceal the deteriorating signal path from the user.
There are surely more reliable ways to signal a deteriorating signal
than to rely on what you hear.
> There is a reason that the FAA and the airline industry have stayed
> with AM and SSB.
Actually, they have not. Even air traffic control systems struggle with
network capacity and the FAA approved CPDLC system (Controller Pilot
Data Link Communications) has been in operation for almost 10 years now,
offering a digital replacement for voice communication between the ATC
center and the aircraft. Also several other security relevant ATC
systems rely on digital communication, like e.g. TCAS (Traffic Collision
Avoidance System). In case of a collision warning, the orders from the
TCAS system even have priority above potential orders from the human air
traffic controller. One of the direct causes for the mid-air collision
above Überlingen, Germany in 2002 was that one of the pilots decided to
follow the (probably analog transmitted) order from the ATC center
instead of the automatic TCAS warning.
> Concurrent signals can sometimes both be understood by the human
> brain
Sometimes, and if not, you may end up with something like the Tenerife
accident in 1977, which was caused by poor radio reception and
interference between two simultaneous transmissions.
Tor
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Posted by Scott Dorsey on September 6, 2008, 5:30 pm
>It was a dark and stormy night when hancock4@bbs.cpcn.com wrote:
>>Today many public safety units are converting to digital radios from
>>analog. However, there have been many newspaper reports of digital
>>radios failing in critical situations due to dead spots, apparently a
>>digital signal is harder to receive than the older analog signals.
>An analog signal degrades gracefully. As reception becomes worse and
>worse, it just becomes noisier and more staticky up to the point where
>you can't make out what they're saying any more. There are various
>tricks that can be used to reduce noise. And when things are bad, words
>can be repeated or spelled out.
Most of these "digital" radios aren't actually digital at all. They are
trunking systems, which basically operate like analogue cell phone systems.
You press the PTT button on the radio, the radio tells the repeater that
it's on trunk X and it wants to talk. The repeater tells the radio that
trunk X is available and go to to channel Y. You talk, the radio transmits
on channel Y. The repeater sends a control message out telling everyone
on trunk Z to listen on channel Z, then it repeats your signal on channel
Y to channel Z.
The good news is that everybody can talk to everybody else, no matter where
they are in the area, and by using multiple repeaters you can extend the
system to a much larger area than with conventional simplex radios. The
bad news is that it requires an extensive repeater infrastructure, and when
disasters happen you can't count on that infrastructure any more.
The signalling is digital, but the actual transmission is all analogue.
Last hurricane we had, the city tower at Isle of Wight county (VA) fell
down, and all the comms for everybody went out.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
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Posted by T on September 7, 2008, 3:46 pm
> >It was a dark and stormy night when hancock4@bbs.cpcn.com wrote:
> >>Today many public safety units are converting to digital radios from
> >>analog. However, there have been many newspaper reports of digital
> >>radios failing in critical situations due to dead spots, apparently a
> >>digital signal is harder to receive than the older analog signals.
> >An analog signal degrades gracefully. As reception becomes worse and
> >worse, it just becomes noisier and more staticky up to the point where
> >you can't make out what they're saying any more. There are various
> >tricks that can be used to reduce noise. And when things are bad, words
> >can be repeated or spelled out.
>
> Most of these "digital" radios aren't actually digital at all. They are
> trunking systems, which basically operate like analogue cell phone systems.
>
> You press the PTT button on the radio, the radio tells the repeater that
> it's on trunk X and it wants to talk. The repeater tells the radio that
> trunk X is available and go to to channel Y. You talk, the radio transmits
> on channel Y. The repeater sends a control message out telling everyone
> on trunk Z to listen on channel Z, then it repeats your signal on channel
> Y to channel Z.
>
> The good news is that everybody can talk to everybody else, no matter where
> they are in the area, and by using multiple repeaters you can extend the
> system to a much larger area than with conventional simplex radios. The
> bad news is that it requires an extensive repeater infrastructure, and when
> disasters happen you can't count on that infrastructure any more.
>
> The signalling is digital, but the actual transmission is all analogue.
>
> Last hurricane we had, the city tower at Isle of Wight county (VA) fell
> down, and all the comms for everybody went out.
> --scott
>
>
Too funny. I note a PowerWave repeater installed on a phone pole right
down the street from me. They just put the electric meter on it so I
assume it's for the MESH network the city has installed. But the more I
look at it, the more it looks like it might be a trunking repeater.
I've looked for these in other parts of the city and haven't seen any
yet.
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Posted by on September 6, 2008, 11:32 pm
> That's not a digital problem, that's a design or administrative one. In
> fact, that situation is probably easier to deal with today than it was
> in the past, where the radios were crystal controlled and could only
> operate on one (or a very few) channels.
I'm glad you brought that up.
I understand that until relatively recently, a walkie-talkie type
radio required a crystal for each channel it could use (certain
harmonic multiples could use one crystal). Unlike home broadcast
radio receivers, they couldn't use that variable capacitor to select a
frequency out of a band of them.
Would anyone know why commercial radios required a crystal and
couldn't use that variable tuner?
Here's another question: Certain systems, such as subway dispatching
and some police systems share a channel in one direction but not in
the other. That is, field units could hear one side of the
conversation but not the other. Could anyone explain that?
In the 1970s version of the movie "The Taking of Pehlam 1-2-3" they
had a good accurate view of the Command Center which the NYC subway
radio room. The big feature was radio consoles of the dispatchers so
they could talk to trains. The consoles had zones, with red and green
lights and [I think two] push buttons for each zone. (Don't know
their meaning). I believe this center has been moved elsewhere and
modernized, but the movie image was quite realistic of the real
thing. (Actually, the real center was kind of dumpy compared to the
movie's).
In WW II Bell Labs did extensive research into mobile radios for the
military, described in the Engineering & Science book "War & Peace".
IIRC, early on they chose FM over AM. Of course, other mfrs like RCA
did extensive research as well. (Someone should write a non-biased
technical history of the communications developments.)
***** Moderator's Note *****
The nicest part of being a moderator is that I get to see the questions first
;-).
Since I have been a ham operator since I was 13, and I used to be a
Broadcast Engineer, and I hold both an Amateur Extra Class and a
Commercial General Class (which used to be called "First Class")
RadioTelephone license with Ship Radar Techniques Endoresement
(ahem!), I think I am qualified to speak on these matters.
With Voir dire out of the way, we shall proceed to the exhibits:
Military and commercial transmitters have used crystals since the
earliest days of radio: the first practical transmitters generated
radio waves by use of spark gaps, which created a "damped wave" that
was both very weak in amplitude and very broad in frequency. When you
go under high-tension lines with your car radio set to an AM station,
you're hearing "spark" transmission caused by high voltage arcing
across the insulators and by corona discharge. It covers every AM
station, even on the very best car radio, so you can see why it
couldn't be used for long: each station pretty much took ALL the
available spectrum, and everyone else had to wait their turn.
Soon after, spark was replaced with "Continuous Wave" transmitters,
which could transmit much further distances because their power was
concentrated on a single frequency. In short order, it was discovered
that piezoelectric crystals made excellent, durable, and stable
frequency-determining elements, and they are the standard for
cost-effective and stable frequency-setting devices to this very
day. Your computer contains one, and a computer that runs at, e.g., 1
GHz (Gigahertz) is generating that timing signal by digitally
processing a crystal oscillator.
The advantage of crystal control is that it's reliable, inexpensive,
and versatile: crystal-controlled transmitters will operate reliably
over a much wider range of environments (temperature, voltage, age,
etc.) than those controlled by Variable-Frequency Oscillators (VFO's),
which use the variable capacitors you spoke of, with enough stability
that in the 1970's, crystal-controlled transmitters used in "two-way"
radios needed frequency checks only once per year.
Radios which are controlled by VFO's - almost all are in military or
Amateur use - require constant monitoring to make sure they are
transmitting on the assigned frequency or in the allowed band, and
such radios often include crystal-controlled "calibrator" circuits
that generate a reference signal for comparison to the VFO's
setting. Of course, that won't do when the radio is being operated by
a cop or a cabbie, so crystal-control is the norm.
Until the invention of integrated circuits, each channel a two-way
radio could use required a crystal (in fact, usually two or more: one
for transmit, one or more for receive). In other words, since each
crystal generated a single frequency, each channel needed a different
set of crystals.
The problem is, when you're building thousands of CB or taxicab or
fire or police radios every month, the cost of the crystals starts to
add up. Some manufactureres reduced crystal counts by using ingenious
"crystalplexing" schemes, where something like ten crystal oscillators
were combined to produced the needed output frequencies. However, this
was only practical when a lot of channels were needed and crystals
were expensive (as at the start of the CB craze during the early
seventies), because the complicated wiring and multiple-gang switches
needed to make it work also added to the cost.
Things got a lot simpler when inexpensive and easily programmed IC's
made phase-locked-loop (PLL) frequency generators possible. Designers
were able to eliminate all but one crystal for the entire radio, and
to use a single "reference" frequency as input to the digital divider
circuits which generated the "operating" frequencies. In other words,
integrated circuits made it possible to leverage the stability of a
single crystal oscillator so as to generate any desired output
frequency with the same stability as that of the crystal source. To
set the operating frequency of such a radio, a technician uses an
external programming tool to "burn" the needed divisors into read-only
memory inside the radio.
This is, of course, oversimplified, but it's all true. Please refer to
a web page titled "History of frequency control and modern time
keeping" at http://www.icmfg.com/frequencycontrolhistory.html for more
info.
Now, to your next question: certain systems are set up so that the
radios in the vehicles can only hear the dispatcher, not other
vehicles, because experienced showed that cab drivers, truck drivers,
couriers, and even well-trained police officers were prone to make
remarks to other mobile users which were not in the best interests of
good order and discipline. The dispatcher can hear the mobile units,
but they can't hear each other, because the mobile radios transmit on
a different channel then the one they receive on. This gives the
dispatcher control over what the mobile units hear, and also the
ability to connect his receiver and transmitter in a "repeater"
configuration so that everyone can hear everyone else when time is
essential, as during a hot pursuit.
The rad and green lights you saw in the movie were to indicate the
state of the signals which controlled access to any given section of
track: as a train passes by a signal, the signal automatically goes
from green to red so that any train following will be forced to wait
until the first train has gone far enough ahead for safety. The
dispatcher's board is a remote readout of each signal, so that he can
observe the passage of trains and also see those which are not moving
and may be broken down.
Last (whew!), the matter of FM vs. AM. AM (Amplitute Modulation) was
discovered first and was the standard for voice (and music)
transmission until Major Armstrong invented FM (Frequency
Modulation). In fact, aircraft still use AM, as do CB radios,
shortwave broadcasters, and (of course) AM broadcast receivers in cars
and homes. FM has some advantages over AM in noise reduction, and FM
transmitters are simpler than AM units, but FM receivers are more
complex so it may be a wash as far as component cost.
FWIW, the L carrier used a form of AM called Single-sideband, where
the various voice channels were first used to modulate a
(crystal-controlled!) carrier, and the resulting AM signal was then
run through a crystal (swear to Ghod) filter to eliminate one
sideband, which saved 1/2 the bandwidth that would otherwise be
needed. Even though it used AM, the system proved so quiet that many
telephone users would hang up during pauses in the conversation,
because they were so used to hearing background noise on long-distance
calls that they assumed the other party had been disconnected. Bell
Labs engineers had to add noise generators to L carriers, which
produced the susurrus which we all associated with long-distance calls
until Sprint's "Pin drop" advertising altered public perceptions.
Thanks for the trip down memory lane. They say asking an engineer a
question is like taking a drink at a fire hydrant, but if you want
more info feel free to contact me offline: bill at horne dot net.
Bill Horne
Temporary Moderator
Please put [Telecom] at the end of your subject line, or I may never
see your post! Thanks!
We have a new address for email submissions: telecomdigestmoderator
atsign telecom-digest.org. This is only for those who submit posts via
email: if you use a newsreader or a web interface to contribute to the
digest, you don't need to change anything.
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