There is a very good technical reason we still use AM.
AM is not prone to capture effect, wherein the loudest signal captures the receiver. With AM, you can hear multiple people talking on top of each other at the same time.
For uncontrolled radio situations where you have a number of unknown people who need to access a radio channel, neither digital nor encryption bring any wanted benefits.
As a note - the audio from this recording is very clear to me - I've heard far worse out of scratchy narrowband FM.. with AM the weaker the signal generally the quieter it is - but not generally that much noisier.
Yup. FM is great for noise rejection. But any signal which is not "the one true signal" counts as noise. So FM interferes with other FM signals mightily and the most powerful one tends to "win". Whereas with AM the signals just add on top of each other.
I've been into amateur radio for ages, presumably that has trained my ear. I routinely copy voice signals which are just a garbled gibberish to innocent bystanders. (Same obviously goes for ATCs and pilots to an even larger degree, being exposed to comms several hours a day)
Yeah, Even with P25 - the same skills apply, but instead of pulling it out of noise, you're relying on your skills of interpolation to suck it out of the noise. Still it's a neat parlor trick :-D
Mmm. Here's the thing, I think we heard a lot of the same arguments before DSC took off for marine radio.
They too have uncontrolled radio, a large number of unknown people who need to communicate with whoever happens to be nearby, they have more powerful transmitters owned by governments that "need" to shout down less powerful ones on transport vessels occasionally.
Now, maritime radio IS a different environment. I'm not suggesting that DSC should just be dropped in as a replacement for AM analogue transceivers on planes, but I _am_ saying that I don't buy the theory that it so happens AM analogue is the right choice and not just the result of inertia.
I've used both marine (FM) and airband (AM) radios. My marine experience was in a relatively uncluttered environment (Cleveland) so take this with a grain of salt but I found that you rarely had people talking over each other in that environment, especially with it's limited propagation. Airband on the other hand is almost always a very high radio traffic environment if you are actively switching between tower, departure and center channels throughout the flight. It becomes imperative to be able to tell when someone stepped on ATC and you didn't get some message.
I believe there are significant technical advantages to AM for airband - I also believe that marine radio is less likely to need those advantages, so narrowband digital is more workable there.
they still use AM because higher powered transmitters (which will generally be the tower) can overpower weaker transmitters (planes). i'm given to understand aviation is enamored of that feature.
> Just imagine the economic damage that a single asshole can do with a high-powered transmitter near an airport.
the pilots could probably just pull their cell phones out and call the tower. there is much worse trouble single assholes could get up to.
I think you'd be very surprised what a lot of power and a shit antenna can do. Obviously won't last when someone comes along with DF equipment, but it really wouldn't be hard to conceal visually.
Visually is the key word here. RF-wise, you may as well put a beacon on the moon and scream "LOOK AT ME" for all the good it'll do when the very, very angry FAA/FCC vans show up.
It is a very Silicon Valley thing to assume that everyone else is an idiot and needs to be disrupted, with not a thought given as to why things are the way they are. Sort of funny to watch it from the side
I assure you that there is a reason AM is used. The reason is that in the case that two stations are transmitting, everyone will be aware of it and the stronger signal may be heard. This is not something that FM can guarantee. And a digital protocol requires a much better signal (where old analog tv gave snowy picture digital TV gives nothing). Not to mention the insane costs that digital radio retofitting would require of every single plane in the sky.
Edit: parent seems to have deleted his post. It previously asked condescendingly why we still use AM for comms in the air.
Digital only require better signal if you want high data rates. With analog signal the human has to do the error correction, i.e. extract the signal from the snowy picture. Digital signals can use error correcting codes to get the same result. The more bits you have for error correction, the better your recovery can be.
With a lot of FEC, you can sort of approach it, but still not quite. Human brain is a lot better at picking out voice out of noise than any algorithm currently known. That is , for example, why your Alexa cannot take your commands in the middle of a 100-person party, but your interlocutor can hear you and respond.
That is not very relevant since humans don't use an error correcting code that is easy to understand for machines when talking to each other. So Alexa's ability to filter your voice out of the noise has little to do with the ability of two machines understanding each other over a noisy channel when they use appropriate coding.
I hate aircraft radios. I'm sure the guys in big planes have better equipment and don't suffer from this as much, but when I fly small planes, it's a complete mess of wildly different volume levels (it's fun to turn up the volume to hear a quiet person, then get blasted by the next transmission), interference, and irrelevant transmissions from a hundred miles away. Don't get me wrong, they get the job done and any change would have to be very carefully considered, but I'm always happy when I get far enough from the airport that I can turn the damned thing off.
I think a digital system could be done much better than what we have. But I'm sure it's not worth the huge effort it would take to design and build.
This is a lack of AGC (automatic gain control) - usually even on the (shortwave and Ham) radios I've seen it on, its switchable, otherwise (in certain conditions, like rapid fading) you can end up with gain pumping, which can sound like the audio is surging.
What it normally does is reduce gain on high strength signals, and increases it on weak ones to give a constant volume level.
I'm honestly surprised the radios in general aviation craft are not so equipped, as its generally a standard part of most AM radios.
Thanks, now I know what to look for next time I go equipment shopping. I'm sure many GA radios have this, but I'm using particularly low-end radios since space and power consumption are more important for what I do. Still won't fix getting an earful about skydivers at an airport a hundred miles away, but it would be an improvement!
Okay, but do military-grade systems use AM like this too? I imagine their needs aren't any less than those of civilians, and I imagine they've gotten their communication systems to work just fine. So what's the issue?
"Military-grade" systems use complicated techniques like frequency-hopping spread spectrum. They're designed to be encrypted and resistant to jamming. The jamming resistance is not something we care about (if someone is jamming the signal you just make them turn off their radio), and the whole point of encryption is to prevent interoperability when you don't have the right key.
So sure, you could pay a bunch of extra money for military features and end up with a product that is even less what you want than AM radio. And then you'd have to retrofit everything with these systems.
AM is wonderful. You put a bunch of people on the same channel and it just works.
> "Military-grade" systems use complicated techniques like frequency-hopping spread spectrum
Note that that technology has been around since World War II. And fun fact, while we're on that topic, this page is worth a read: https://en.wikipedia.org/wiki/Hedy_Lamarr
"The jamming resistance is not something we care about (if someone is jamming the signal you just make them turn off their radio)"
Evie might know this, too, so I think you should be concerned about jamming. Luckily, AM provides jamming resistance, since Evie would have to bring a powerful transmitter to be loud enough to drown out the other signals.
Nevertheless, I think large airports should have fast response teams who can rapidly fix the position of a jammer and silence it, if needed. If Evie could effectively take out, say, the main airports of LA and SF for a few hours with a few strong AM transmitters, I doubt all will end well.
Evie? I honestly have no idea who that is. But the more powerful the jamming, the easier it is to find the source. These days even hobbyists play around with cheap off-the-shelf RF analyzers and directional antennas, so I can't imagine that someone jamming airport signals would be able to evade arrest for very long.
I don't understand what is so wrong with the transmission quality in that MP3. The issues I hear sound like they are simply due to trying to transmit from an extremely noisy environment (the cockpit of an airplane); they are mitigating the pilot's voice being entirely drowned in the background noise by placing the microphone right next to their mouth, but that then causes the sound of their breath to be extremely noticeable. I am not sure how to fix these issues but they don't seem to have anything to do with the usage of AM radio.
I wasn't complaining about the transmission quality, I was just trying to understand the parent comment. The person who complained about the former was someone else.
Sorry; I misunderstood (and to be honest even after my tenth reread am still having a difficult time parsing...) the end of your comment about the "issue" :(.
OK, I'll clarify. Someone suggested non-AM transmission has problems X/Y/Z; I suggested that I expect non-AM transmission has overcome problems X/Y/Z in the military, and therefore I'm not convinced this is the real issue preventing non-AM communication. I was not discussing whether we or not we need non-AM communication in the first place.
Airlines don't need military systems which are used more for confidentiality and OPSEC than other things. In the field, militaries tend to avoid radio communications for various tactical reasons. Submarines are a great example of this.
Military-grade systems may not be any "less" but their needs and priorities may be different (e.g. secrecy). So different technology may be used, without any contradiction to why the current one is good for civilian air traffic.
> It is a very Silicon Valley thing to assume that everyone else is an idiot and needs to be disrupted, with not a thought given as to why things are the way they are.
This kind of thinking has also lead to a ton of success in Silicon Valley. See Tesla for example.
Did Tesla really disrupt the car market? Sure, everyone is now building EVs but that already started before Tesla became successful. They have no market power on the overall car market.
Companies like Uber, Google, Amazon certainly disrupted markets but I wouldn't be so sure about Tesla.
I can't honestly say I ever paid attention, but I just checked all clocks I could find, one flashes 00:00 and the other 0:00. Most if not all clocks I've used had a 12 or 24-hour mode, not sure what happens if you set to 12-hour and unplug it (probably just reset to defaults).
