Re: IMTS vs MTS was 1965 mobile phone on "Get Smart" [telecom]

On Sep 6, 12:35 am, Dave Garland <dave.garl...@xxxxxxxxxxx> wrote:


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

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