Dear Mr.Klein:

It is not the control channel that is the problem. It may be sent from
only one transmitter (and thus suffer only the old-fashion multipath type of
distortion). In other words, it appears that the control channel is decoded
successfully. This could be due to the existence of a robust type of coding
that combats old-fashion multipath type of distortion.

However, when mobile stations are sent by the control channel off to one
of the allocated frequencies, problems seem to occur with the digitally
encoded voice signals. Since systems that use a total of one transmitting
site do not appear to suffer and systems that use multiple transmitting
sites do suffer, one hypothecates reasons.

A form of multipath distortion exists that is due to the use of
multiple, well spaced transmitters that are transmitting at the same time.
That is the type of distortion I called "multiple-transmitter-multipath."

One might guess that if one is close to one of the transmitters (of a
multiple transmitter system) then one would not have a problems because the
near transmitter's emissions would dominate. I have not tested this
hypothesis.

However, when one is receiving multiple transmissions of approximately
equal amplitude but transmissions that have encountered different delays,
then one expects to see what looks like multipath. An enhanced receiver can
remove the problem at a cost that includes slight delay. A "low cost"
receiver without enhanced processing may have a hard time decoding signals
that have been contaminated by multipath of any kind.

When listening to a one transmitter site (Owosso), my BCD396T receives
almost all voice signals well most of the time. When listening to a system
(APCO25) that uses multiple transmitting sites (Genesee), my BCD396T
produces distorted audio much of the time. At my location, the signal from
the control channel of the multiple transmitter system is somewhat stronger
than the signal from the one transmitter system.

Even back in the RTTY days with each "bit" being 20 to 22 mS long,
multipath was capable of distorting signals. The much higher data rates of
the digital system in question makes the
decoding-in-the-presence-of-one-type-or-another-multipath difficult.

In short: multipath can be caused by means other than signal
reflections and the present "low cost" receivers seem not to contain
multipath resistance processing.

Regards, Mac



--
J. Mc Laughlin; Michigan U.S.A.
Home:

"Al Klein" wrote in message
...
On Mon, 9 Jan 2006 08:47:35 -0500, "J. Mc Laughlin"
wrote:

The Owosso system uses only one
transmitter thus avoiding the multiple-transmitter-multipath of the
Genesee
system.

If you're talking about the MPSCS, it's a Motorola system, so there's
only 1 control channel on the air at a time from any site. Multipath
isn't caused by multiple transmitters, it's caused by signal
reflections.

On Tue, 10 Jan 2006 23:53:50 -0500, "J. Mc Laughlin"
wrote:

However, when mobile stations are sent by the control channel off to one
of the allocated frequencies, problems seem to occur with the digitally
encoded voice signals. Since systems that use a total of one transmitting
site do not appear to suffer and systems that use multiple transmitting
sites do suffer, one hypothecates reasons.

A form of multipath distortion exists that is due to the use of
multiple, well spaced transmitters that are transmitting at the same time.
That is the type of distortion I called "multiple-transmitter-multipath."

Multiple site Motorola systems don't transmit on the same frequency
from different sites. In theory what you say is true (although it's
not "multipath", it's plain old simple interference - multipath refers
to a signal from a single transmitter arriving over multiple paths,
and nothing else), but if you look at the actual in-the-field sites,
you'll see that your theory isn't put into practice.

One might guess that if one is close to one of the transmitters (of a
multiple transmitter system) then one would not have a problems because the
near transmitter's emissions would dominate. I have not tested this
hypothesis.

It's not an hypothesis, it's called "capture effect", and it's a
property of FM receivers.

When listening to a one transmitter site (Owosso), my BCD396T receives
almost all voice signals well most of the time. When listening to a system
(APCO25) that uses multiple transmitting sites (Genesee), my BCD396T
produces distorted audio much of the time.

Genesee is *a* site - in a larger system that encompasses many sites..
The fact that it lists more than 1 control frequency doesn't mean that
it has multiple control channel transmitters on the air at the same
time.

At my location, the signal from
the control channel of the multiple transmitter system is somewhat stronger
than the signal from the one transmitter system.

Since there's only 1 control transmitter on any one frequency at any
one time for any one site, you're analyzing the situation incorrectly.
You may be getting interference of some sort, but it's not due to
multiple control channel transmitters from the same system at the same
location.

In short: multipath can be caused by means other than signal
reflections

Except that what you're describing isn't called multipath. Multipath
is the term used to refer to signals from a single transmitter
arriving over multiple paths - hence "multi-path".

"Al Klein" wrote in message

"hence multi-path"

Is that the same thing as what ham radio operators of 2 meter repeaters call
"intermod" ?

H.J.

In a simplistic way, Intermod (intermodulation distortion) is the result of
two or more separate signals being combined. The intermixing of these
signals creates distortion of the signal you are receiving.
In a round about way, multi-path is similar because the combined multiple
images are out of phase and they will mix in the same way as do two
unrelated signals, and create distortion.


"Honest John" wrote in message
...

"Al Klein" wrote in message

"hence multi-path"

Is that the same thing as what ham radio operators of 2 meter repeaters
call
"intermod" ?

H.J.

"Doc" wrote in message

In a simplistic way, Intermod (intermodulation distortion) is the result
of
two or more separate signals being combined. The intermixing of these
signals creates distortion of the signal you are receiving.
In a round about way, multi-path is similar because the combined multiple
images are out of phase and they will mix in the same way as do two
unrelated signals, and create distortion.


THANKS,

H.J.

Honest John wrote regarding multipath:
Is that the same thing as what ham radio operators of 2 meter repeaters call
"intermod" ?

No. Multipath is from the same signal arriving via two different paths
(typically resulting from a reflection), one of which is delayed enough
to cause phase cancellation.

Intermod is from separete signals of different frequencies mixing in
some nonlinear componenent either in or near the receiving apparatus,
producing signals that are the sum and difference of the two frequencies.

--
John Miller

I think I did say "in a simplistic way". Yes I do understand what intermod
is, however, I thought a discussion of 3rd order intercept series was not in
order to answer his question. I was using the KISS approach.

"John Miller" wrote in message
...
Honest John wrote regarding multipath:
Is that the same thing as what ham radio operators of 2 meter repeaters
call
"intermod" ?

No. Multipath is from the same signal arriving via two different paths
(typically resulting from a reflection), one of which is delayed enough
to cause phase cancellation.

Intermod is from separete signals of different frequencies mixing in
some nonlinear componenent either in or near the receiving apparatus,
producing signals that are the sum and difference of the two frequencies.

--
John Miller

Doc wrote:
I think I did say "in a simplistic way". Yes I do understand what intermod
is, however, I thought a discussion of 3rd order intercept series was not in
order to answer his question. I was using the KISS approach.

Check your threading, Doc, and you'll see that it was not your post I
was responding to -- in fact I don't think it had even hit my server
yet. So you weren't being contradicted.

--
John Miller
insert pithy quote here

Sorry for jumping to conclusions...
My thread shows your post as a reply to mine. That's probably because I had
just purged old posts.


"John Miller" wrote in message
...
Doc wrote:
I think I did say "in a simplistic way". Yes I do understand what
intermod
is, however, I thought a discussion of 3rd order intercept series was not
in
order to answer his question. I was using the KISS approach.

Check your threading, Doc, and you'll see that it was not your post I
was responding to -- in fact I don't think it had even hit my server
yet. So you weren't being contradicted.

--
John Miller
insert pithy quote here

On Thu, 12 Jan 2006 17:50:02 GMT, John Miller wrote:

Intermod is from separete signals of different frequencies mixing in
some nonlinear componenent either in or near the receiving apparatus

Or the transmitter apparatus. (We have a great one here, resulting
from a hospital paging transmitter intermodding with just about
everything on a hill that has enough RF floating around to run a few
homes. The paging transmitter - that's supposed to cover only the
interior of the building - is dirty enough and strong enough to cause
intermod in tower joints, fences, who knows what, 5 miles away.)