car phone

[RichD]

In old movies, you sometimes see something called
a 'car phone'. Always in a limousine.

How did those work?


--
Rich

[Michael Black]

Like two way radios, though perhaps some were duplex (I know some two way
radios were designed for duplex, but never saw why).

There were a handful of dedicated channels per a given region. A central
tower, and an operator at that central tower. You'd call up the operator,
give them information and they'd dial. I think they dialed. The person you
want to talk to would answer, and you'd have your conversation. There must
have been tone signalling so someone could phone you in your car, a
specific set of tones causing your car phone to "ring".

It was very expensive. Big radios for much of the period that sat in your
trunk, I suspect they were rented but maybe one could buy. That would be
connected to a tiny box under the dashboard of the car, which had the
needed controls (very minimal) connected with an often thick cable
(because various lines had to run). And since there were only a few
channels available, there had to be a high charge, to pay for the
infrastructure spread only over a relative few who could afford it, and to
ensure nobody hogged the channels.

Most people couldn't afford them, and it couldn't be for the masses since
"bandwidth" was rapidly filled.


Cellphones fixed the problems. First, they got more channels allocated,
albeit at a higher frequency. That allowed for more users. But by
keeping signals weak, and multiple "cells" each frequency could be reused
once out of a small geographic area. (With one central tower as used by
car phones, since they had to have range for the full area, somebody on
the other side of town could not use that channel at the same time.) If
the range of each cellphone tower was about five blocks, for instance,
then a few blocks away the same channel could be reused. It's a lot more
complicated system, since each cell used a set of channels, move to the
next cell and a different set of channels were used, and they'd be
switched automatically as you moved. The fact that by the time cellphones
came along computers could be put in each phone helped a lot.

By adding a lot of infrastructure, all those cell towers, it allowed for a
lot more users. Taht dropped costs, which in turn meant more people could
afford it. Since it became a mass market item, rather than hand built
transceivers, production costs went down, making things cheaper again.
Once there was enough users, they could leverage more channels, and then
eventually go to even more efficient methods of using the frequencies,
allowing for even more users.

Michael

[Jon Elson]

Michael Black wrote:

> On Thu, 5 Jan 2012, RichD wrote:
>
>> In old movies, you sometimes see something called
>> a 'car phone'. Always in a limousine.
>>
>> How did those work?
>>
>>
> Like two way radios, though perhaps some were duplex (I know some two way
> radios were designed for duplex, but never saw why).
>
> There were a handful of dedicated channels per a given region. A central
> tower, and an operator at that central tower. You'd call up the operator,
> give them information and they'd dial. I think they dialed. The person you
> want to talk to would answer, and you'd have your conversation. There must
> have been tone signalling so someone could phone you in your car, a
> specific set of tones causing your car phone to "ring".
>
> It was very expensive. Big radios for much of the period that sat in your
> trunk, I suspect they were rented but maybe one could buy.

Yup, mostly Motorola and GE, and an adaptation of the gear used for
police and taxicab service. Later ones had touchtone pads for self-dialing.
The car phone had a tone ID when beginning a call for billing purposes.
The units in the trunk were roughly 16 x 16 x 5" or so.

Jon

[JeffM]

Michael Black wrote:
>RichD wrote:
>>In old movies, you sometimes see something called
>>a 'car phone'. Always in a limousine.
>>How did those work?
>>
>Like two way radios, though perhaps some were duplex
>(I know some two way radios were designed for duplex,
>but never saw why).
>

Telephone users aren't used to saying "Over".

[Michael Black]

Yes, probably they were duplex. I never used one, obviously. I recall
instances of people being warned about how to talk over the radio, but
that was likely on the Marine band where I thought there was some sort of
phone patching going on, or more obviously on 2m fm with an autopatch.

The problem with duplex was not only that the power supply (all those
vibrator supplies) had to power both the receiver and transmitter at the
same time, but keeping both in use at the same time meant lots of effort
had to be done to keep the local transmitter out of the receiver.

Michael

[Michael Black]

Yes, I still have some of that junk in the basement, surplus forty years
ago. The cases alone were heavy by themselves.

Michael

[Sonnich Jensen]

I have been told, that only one (or a few) could use it at a time as
all phone were on the same frequency.
Therefore everyone could listen to the phone calls.

I wonder how they contacted the car.

There is this story - that a guy phones his wife, that he will be
late, then his mistress - for everyone to listen to

Sonnich

[lawr...@gandi.cluon.com]

Sonnich Jensen <sonnic...@gmail.com> writes:
>
> I have been told, that only one (or a few) could use it at a time as
> all phone were on the same frequency.

(Context: Assuming IMTS service from late 60s through early 80s)

Depends on the locality -- there were about a dozen channels available
in each of two bands (one around 150 MHz and another around 450). All
the tech details you could want are in the Wikipedia article:

http://en.wikipedia.org/wiki/Improved_Mobile_Telephone_Service

> Therefore everyone could listen to the phone calls.
>
> I wonder how they contacted the car.

In any one "system" (i.e. metro area) one of the available channels
would transmit a continuous "idle tone" and all receivers would scan to
that channel. Signalling came from the tower - all mobile sets would
listen to the signalling spill for their own Mobile ID. As soon as your
mobile knew it wasn't being signalled it would pick up scanning. If
there were NO idle channel, it would typically illuminate an "all
channels busy" lamp.

>
> There is this story - that a guy phones his wife, that he will be
> late, then his mistress - for everyone to listen to
>

Everyone who worked in the industry has a story like that - it happened
often enough that it was a common occurance. Or something equally
devious.

--N1GAK

[Michael Black]

It was a small number of channels, and yes, only one person could use a
channel at a time.

> I wonder how they contacted the car.
>

One method comes to mind, which was used somewhere and probably was used
here. A very low frequency tone, low enough so it could be filtered out
without affecting the speaking. A distinct tone for each unit, and the
receiver wouldn't output audio unless the tone for your unit was being
sent. So all receivers were listening, but the user could only hear
something if their tone was being sent.

The interesting thing is that it did ensure that others with car phones
could not hear someone else's conversation, it would never get to the
handset even though the receiver was listening to the channel.

But it did nothing to protect those just listening with a receiver on that
frequency, they'd hear all the transmisions. The protection was somewhat
of an illusion, since it didn't prevent people from listening in, just
made the system look like it was dead until a call for you came along.

I gather FRS walkie talkies use something like this, tone encoders and
decoders so you only hear what's being sent along with the selected tone.
It seems like nobody else is out there, but turn off the tone system and
you'd hear anyone on that frequency.

> There is this story - that a guy phones his wife, that he will be
> late, then his mistress - for everyone to listen to
>

Just everyone listening in at home.

The "protection" was that initially few had receivers for the frequencies,
which of course changed as the sixties progressed and "Public service
band" receivers which covered those frequencies became more and more
common.

Michael

[T]

In article <Pine.LNX.4.64.12...@darkstar.example.net>,
et...@ncf.ca says...

And a lot of the cell phone antennae around here aren't mounted on
towers but hung on buildings instead.

[Jon Elson]

Michael Black wrote:


>> I wonder how they contacted the car.
>>
> One method comes to mind, which was used somewhere and probably was used
> here. A very low frequency tone, low enough so it could be filtered out
> without affecting the speaking.

No, they were audible tones. They had a burst of 3 - 5 tones at the
beginning of a connection, to activate only the required receiver.
It was called selective calling, and if you look up "plectron"
Google or Wikipedia has lots on those systems. They were also used
to selectively call police cars, cabs and fire trucks from central
dispatchers. The original devices used tuned reeds (essentially tuning
forks) to detect the frequencies. That was back in the 60's when the
tranceivers were all-tube.

Jon

[Jasen Betts]

On 2012-01-06, Sonnich Jensen <sonnic...@gmail.com> wrote:
> There is this story - that a guy phones his wife, that he will be
> late, then his mistress - for everyone to listen to

and there's this one.
http://en.wikipedia.org/wiki/Squidgygate

two (known) people made recordings, how many more were listening?

--
⚂⚃ 100% natural

--- Posted via news://freenews.netfront.net/ - Complaints to ne...@netfront.net ---

[T]

In article <Pine.LNX.4.64.12...@darkstar.example.net>,
et...@ncf.ca says...
>

What is more interesting about cellular technology is that it was in
fact a Bell Labs innovation. It originally was thought about in the
1940's but technology had to catch up and in the 70's they started doing
trials in the Illinois area.