Re: [PLUG] AM bye bye

[Rich Freeman via plug <plug@lists.phillylinux.org>]

On Fri, Jul 8, 2022 at 4:26 PM jeffv via plug
<plug@lists.phillylinux.org> wrote:
>
> On 7/8/22 15:48, Rich Freeman wrote:
>
> > That said, I am not sure I agree with one statement in that article:
> > that AM is "cheaper" to transmit.  I guess that would depend on how
> > you're measuring it,
>
> Agreed.
> Prob less signal 'massaging', if that counts.

Not really.  Once upon a time decades ago the electronics to module FM
might have been considered complex, but these days chips can create
the modulated signal trivially.  Your cell phone generates far more
complex signals than either AM or FM.  The expensive part is
amplifying it, and the antenna.  Not a big deal for a low power cell
phone, but if you want to transmit 50kW+ like a broadcast tower that
gets expensive.  An AM antenna needs to be physically very large due
to the very long wavelengths as they transmit around 1kHz, compared to
100MHz for a broadcast FM signal.

>
> > One area I haven't seen any talk of phasing out AM is in air traffic
> > control,
>
> *if* that were going to happen, it would have to be through the
> government (FCC, a formerly useful org). That would not move quickly.
> Plus the modulation is chosen for best operation, even though there is
> some FM in some places. [scanner listener]

Nobody uses FM for ATC.  The FCC would of course be involved, but such
a thing would also involve the ICAO and ITU, as by their nature
aircraft communications are standardized internationally.  Nobody
wants a new standard that only works in one country.

FM does tend to get used more for police/etc, but as safety critical
as those are, they're usually not quite as urgent.  Obviously public
safety messages can be urgent but on an ATC frequency you just have an
endless stream of communications and they're all pretty
time-sensitive.

>   but that operates in the VHF band so everything is much more
> > compact (obviously useful on aircraft).
>
> Radios can be smaller than your car radio, and AM/FM is a matter of a
> switch.

We aren't comparing AM and FM here.  We're comparing MF to VHF.  The
size issue isn't with the radios, it is with the antennas.  AM over
VHF is much more compact than AM over MF which is what you listen to
in your car.  Also, the issue is with transmitting it, not receiving
it.  Obviously cars have no issues with outfitting broadcast AM
receivers, and neither would planes.  However, aircraft have a need to
broadcast signals unlike cars, and so MF is unsuitable and VHF is much
more useful.

>
> airplanes use shortwave on overseas trips because AM/FM won't go that far.

They can operate on HF, but they generally prefer digital text
messages over satellite as it is generally easier to work with.  There
isn't much that needs to be said while you're over the ocean as
aircraft are just following their routes.  However, if they weren't
able to get in a position report and acknowledgement over satellite
they'd use HF.  The real pain is if their SELCAL system isn't working
which would mean they'd have to listen to the static of HF the whole
flight in case somebody calls them, which is what they used to have to
do once upon a time.  (SELCAL is a digital system that lets a code be
broadcasted which addresses a particular aircraft and causes it to
sound an alarm, so that the crew knows somebody needs them to check
in.)

>
>   It is mainly used there
> > because it is more obvious when multiple stations are stepping on each
> > other which makes it more clear that somebody might not have heard an
> > urgent instruction.
>
> Thanks. Never knew that.

Yeah, when two AM stations broadcast at the same time anybody else
listening will hear a loud squealing sound because the two carriers
won't be at exactly the same frequency and so one will leak into the
demodulation.  That lets anybody except the two people talking know
that something is wrong, and if the situation isn't quickly picked up
on and rectified somebody else on the frequency would speak up about
it just in case.

On the other hand, if two FM stations broadcast at the same time,
people will tend to hear whichever one is stronger and not hear the
other one at all.  This is part of why the sound quality of FM is
better in a car/etc - it is more selective.  That's great for
broadcast where only one station is supposed to be heard, but not so
good for air traffic control where lots of people could be
transmitting and any of it could be really important.

>
>
>    I'd think that you could come up with a digital
> > mode that would improve on that substantially but I guess AM is good
> > enough and nobody wants to change it.
>
> I vote for K.I.S.S.
> Esp with the amount of failures lately.

Well, obviously you want to use care, but there are a lot of
advantages to digital audio modes.  We're already replacing the way
radar transponders worked with GPS-based solutions that have a lot of
advantages.  Off the top of my head advantages of a digital audio
solution would include:

1. All transmissions could be tagged with sender/recipient.  ATC
displays could highlight the plane that is calling which would improve
situational awareness, and aircraft could play an alert when they are
being addressed to avoid missed calls.  In both cases confusion around
similar callsigns could be avoided, which can be an issue.
2. You could ditch the need to tie frequencies to ATC positions and
the coverage issues that causes.  In theory a plane could key up in
California and talk to an airport in Pennsylvania, which is obviously
overkill but it would eliminate issues caused by aircraft being on the
wrong frequency and drifting out of ATC range for that transmitter
site.  If a plane is in range of any transmitter it can receive
messages from anybody.
3. You could avoid people talking over each other.  If the radio or
the network detects a controller is already listening to somebody else
then it wouldn't let the pilot key up.
4. You could enforce people talking over each other.  If the channel
is really busy with routine chatter and an emergency arises, then a
pilot/ATC could hit the emergency button and the other radios would
detect this and back off to let them get through.
5. You could have full duplex pretty easily, with people being able to
talk and listen at the same time as with a cell phone.  That is way
easier to do with digital than analog as compression lets you fit a
second of voice into less than half a second of transmit time, leaving
time to receive in-between frames/etc.
6. Satellites can be used to extend voice coverage pretty easily.
7. It becomes much more straightforward to centralize ATC operations,
as it is all digital/packet anyway so switching it is simple.  This
could be useful for less realtime roles like clearance delivery.
Also, aircraft in an emergency wouldn't need to switch
frequencies/settings/etc as they move around because ATC could just
micromanage that routing for them, reducing the burden on pilots.

Of course we're talking bureaucracy, so anything that gets rolled out
will be quite stale and we probably won't see it in most of our
lifetimes.

-- 
Rich
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