If you were Registered and logged in, you could reply and use other advanced thread options
|
Posted by Robert Bonomi on September 8, 2008, 11:09 am
>> 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?
A) "the law said so."
B) "precision."
next question?
Until the advent of digital frequency synthesis, crystal control was the
only viable means to provide the required frequency precision and stability.
If the transmitters weren't that stable, the 'adjustable' receivers would
have to be more-or-less constantly re-tuned 'a little bit', to track "where
the transmitter is now.".
Crystals are used in receiving gear for stability reasons -- 'precision counts'
and you need to hit the 'exactly right' frequency for maximum communications
range _without_ the benefit of having a continuously broadcasting signal to
tweak the variable-cap to
>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?
Duplexing.
Base stations transmit on frequency A, and receive on frequency B.
mobile stations transmit on frequency B, and receive on frequency A.
"Everybody" needs to hear the dispatcher, and the dispatcher needs to
be able to hear everybody. BUT the mobile units usually do _not_ need to
talk to each other -- in fact, in _most_ cases, when they can, one gets
significant amounts of "non-operational" chatter -- often of an informal
and/or personal nature -- *with* the possibility of that 'chatter'
actually blocking official traffic from the base station.
The base stations frequently operate _full_duplex_. Meaning they can
receive _while_ transmitting. And, in a 'priority' situation, where
one cannot wait for the base station operator to 'echo' whatever one
mobile said, one can simply connect the receiver audio out, to the trans-
mitter audio in. Voila! everybody _can_ hear whomever is talking 'loudest'.
With 'sharp enough' filtering, mobile stations can operate full-duplex, as
well. although the -size- of the required filters tends to rule out use
in standard automotive applications.
>***** 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.
Paging (AF, RF, or IP, whatever works) the linguistics pedant!
What the heck is an "Endoresement" ?? <*BIG* grin>
If Derwood's (sic) mother-in-law approved it, that would properly be
an "Endora-sement", I believe, but this one has me perplexed.
[sneck]
>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.
For early PCs, that was true.
Modern units are somewhat more complicated. They generally use a VCO
(voltage controlled oscillator) to running at _four_times_ the CPU clock
frequency. That VCO output is divided by 2, and then inverted. _Both_ the
inverted and un-inverted signals are then fed to individual divide-by-2
flip-flops, generating the 2-phase (quadrature) clock that processors need.
The VCO _also_ feeds a programmable divider chain, the output of which is
fed to a phase comparator circuit, along with the output from the 'reference'
*crystal* oscillator. The error signal from the phase comparator is used
to tweak the control voltage input to the VCO, making a phase-locked loop
(PLL) system running at a 'many times' multiple of the crystal frequency.
Among other things, this is how one 'overclocks' the microprocessor on a
on a higher-end motherboard. One simply changes the programmable divider
value to a higher number. This causes the VCO to run at a higher frequency.
>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,
Aircraft communications, for one, deliberately still use AM, because some of
the 'inherent' qualities of FM are a =disadvantage= for aircraft. Most
notably, FM receivers 'lock on' to the strongest carrier present, to the
exclusion of any weaker station on the same frequency. AM receivers, on the
other hand, reproduce the audio from _all_ the transmitters within range. It
is *important* to be able to hear the 'far away' aircraft calling with an
emergency,"even if" a nearby plane is transmitting something 'routine'.
There is the additional fact that a 'trained ear' can extract the 'meaning'
from a voice communication at a far lower received signal level than the
best FM discriminator can lock onto.
> 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.
Note: with -very- careful adjustment, an AM receiver can pick up and render
audible an FM signal. It's a bastardized use called a 'slope detector'.
you position the carrier frequency in the 'skirt' of the bandpass filter,
instead of the middle of it. As the carrier frequency varies, get changing
signal _amplitude_ on the output side of the filter.
This is definitely in the 'waltzing bear' category -- it's not how well it
works, but that it works at all. <grin>
|
|
Posted by Tom Horne on September 8, 2008, 6:46 pm
> 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.
>
> I'm not sure what the advtg of digital over analog is in public safety
> applications (saving bandwidth?). But one capabiltiy lacking today is
> optional common channels between police, fire, and rescue which often
> work on isolated networks. That is, a policeman can't communicate
> with a fireman, rather, he has to relay messages through the
> dispatchers. Apparently bringing city systems together is a tough
> job, in New York City they're still working on it and there are many
> challenges partly since the police system is a different technical
> architecture than the fire system, and of course the systems must
> remain in active use 24/7.
>
> I can't help but wonder if the old Bell Labs of the Bell System was
> still in full strength that R&D in this field, particularly in
> applying a sound technology, the state of the art would be improved.
> Bell Labs took time to develop new technology, but they tested the
> heck out of everything in actual service and debugged it accordingly.
>
The first use of radio by a fire department occurred in Boston, MA.
Several of the Chiefs vehicles were equipped with maritime mobile radios
to communicate with the cities fire boat. Dispatch also being equipped
with a maritime mobile shore station everyone involved quickly fell into
using it between the dispatch office and the chiefs cars. The FCC
quickly stepped in and ordered the city to stop but the chiefs were
hooked so the city got one of the earliest two way public safety licenses.
Police dispatching was originally one way and used modified AM car radio
receivers to announce calls to the prowl cars. Being only slightly out
of band for am broadcast it was very cost effective to order the cars
with extended range AM receivers to make the system work.
The conversion to digital is driven by the supposed cost savings of
using a single system to support all local government radio use. The
problem is that trunked radio is the predecessor to cellular telephone
service and all of the problems attendant to that technology are in
trunked radio. The biggest of these is "dropped" connections. When a
digital signal is on the edge of effective it is either there or not.
When the signal is marginal it drops in and out of readable. To public
safety workers that is unacceptable but the bean counters don't care.
They blame all problems on the users resistance to change. They're not
the ones crawling down long, snotty hallways at Oh-dark-thirty looking for
other peoples' relatives in zero visibility, so they don't understand why
we complain about not being able to talk to the people supporting that
effort for minutes at a time.
Also the savings are illusory. Motorola's best portables cost only $600
ten years ago when purchased in quantity. The portables we now carry
cost $5000 dollars a copy. So the cost of portable radios for a crew of
three has gone from $1800 to $15000 in ten years. At these prices $150
dollar hammers seem cheap. All of the early adopters are being robbed
because once they select a vendor they are locked into a proprietary
scheme that there is no alternative for. The smarter and poorer
agencies are waiting for the standards process to shake out. Once
standards based APCO 25, or whatever standard replaces it, equipment
becomes available then it will be financially feasible to adopt digital
signaling with portable radios that only have to reach the vehicle that
brought you to the incident. The more powerful outdoor mobile radios
would then do the heavy lifting of reaching the tower site.
As for talking to the cop shop directly there is a organizational
culture problem to overcome. The two organizations do not speak the
same operational languages nor do they share the same operational
priorities. We have so narrow a focus on alleviating suffering that we
sometimes destroy the evidence that the police need to prevent a
recurrence. They are sometimes so focussed on identifying the
perpetrator that they neglect the very real suffering of the victim.
They won't even call us if the victim isn't bleeding for fear we will
follow our training and put the victim in touch with a victim support
worker thus depriving them of control of the witness. One of the
options victim support will expose them to is not pursuing the
conviction of the perpetrator for the sake of their own mental health.
That really ruins the detective's day.
--
Tom Horne
"This alternating current stuff is just a fad. It is much too dangerous
for general use." Thomas Alva Edison
***** Moderator's Note *****
My brother is a Master Firefighter. FWIW, I agree with him.
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.
|
|
Posted by Tor-Einar Jarnbjo on September 4, 2008, 3:56 am
hancock4@bbs.cpcn.com schrieb:
> A friend of mine worked as a driver for a big shot in the 1970s, the
> car had a mobile phone in it. My friend said it was used just like a
> regular phone--to call him I would dial the regular seven digit
> number, and he would dial out. Did users have dial tones or have to
> press Send as we do today? We had to keep calls short as it was
> expensive.
It was probably an IMTS phone, a system introduced in the 60s and not
replaced until AMPS was introduced in 1983. It was the first US mobile
phone system to offer direct dialling, the earlier MTS system was
operator assisted only. To place a mobile originating call, the mobile
phone would first request a dial tone from the mobile tower and digits
were transmitted pulse-encoded over the already established voice
channel as dialled, making it unecessary to press a "send" button after
the number had been dialled.
Switching between different mobile towers was however not automatic and
had to be manually initiated by the mobile phone user.
Tor
|
Page 5 of 5 << first < 1 2 3
| Similar Threads | Posted | | Re: 1965 mobile phone on "Get Smart" [telecom] | September 6, 2008, 11:39 pm |
| Smart Phone Wars: Apple vs. RIM vs. ...the Android Operating System? [Telecom] | April 5, 2008, 1:31 pm |
| Motorola Makes TV Shows Mobile -- From DVR to Mobile Phone and | April 6, 2006, 9:13 am |
| Re: size a major consideration in mobile phone sets [Telecom] | April 30, 2009, 11:25 pm |
| Federal Court to Rule on Privacy of Mobile Phone Location Data [Telecom] | February 8, 2010, 12:11 am |
| "Get Smart" TV show coming as movie [telecom] | May 25, 2008, 11:08 pm |
| Trading Nude Photos Via Mobile Phone Now Part of Teen Dating, Experts Say [Telecom] | April 14, 2008, 11:15 pm |
| Mobile Phone TV Set For Primetime | May 9, 2005, 12:54 pm |
| New Mobile Phone forum | November 15, 2005, 7:20 am |
| Ever Want to Throw Away Your Mobile Phone? | August 28, 2006, 11:00 pm |
|
|
Yahoo! 
Google 
Windows Live 
del.icio.us 
digg 
Netscape 
XML site map
>> 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.