Well it must be very different across the pond where you are. Because, State Side here. It's most definitely FM. It sits right next to our commercial FM radio band. I mean how can you listen to those, broadcast. And think that sounds like AM? You are daft. What's your problem? Is the earth also flat? What about Santa Claus? Aircraft do not use, Amplitude Modulation, medications. This is so wrong. Such horrid misinformation. The sky should be ashamed of himself. This is another British whack job. Still suffering from a bit too much, arsenic exposure. From your lousy old wallpaper from Edwardian times. Still affecting you today. Perhaps you should go see a doctor. Aircraft do not use AM, amplitude modulation. You see… Mr. LSD. I was a student pilot. My father was a private pilot. I went up with him for every one of his flying lessons with his instructor. And I took lessons also. An aircraft communications is FM, Frequency Modulated, narrowband at 25 kHz deviation. We are FM stereo radio is 75 kHz deviation, wideband. Where are you getting this misinformation from Mr. LSD? What have you taken? Did you visit, sergeant Peppers? Or did you take a plunge on the Yellow Submarine? You know the Spruce Goose. Was not an actual goose. And geese are not made from Spruce. Instead they are made by, Bruce. As you must've gone to Monty Python School? As your aircraft information is, SPAM. SPAM IN THE CAN. CAN,, SPAM, SPAM? SPAM! SPAM IN A MOONEY. CESSNA SPAM. PIPER SPAM! SPAM SPAM spam SPAM SPAM ACK ACK Ack A-C-K! Spam! And that is just as accurate a description as you have given. Regarding aircraft communications that are not, Amplitude Modulated. I mean what the Misinformed FUCK? Spam! You are Spam Man. Please remove your teeth. RemyRAD
Yes, aircrafts use AM for ordinary audio communications with controllers and among themselves. Further, the radio band used for that was chosen because it is very silent there, almost no statics or interferences at all. So, you can have a far aim.
Hi, pilot here. Fascinating video. However, when people key in at the same time everybody else on the freq ends up hearing the beat freq screech. Depending on distance and transmitter power, you may be able to overpower and get a word through. For example, if I'm closer to the ATC transmitter on the ground, and someone that's farther away keys in while the controller is talking, I'll hear the noise but I can still hear the controller somewhat. Of course if this happens on the ground, then everyone is very close to each other and it's nearly impossible to get a word through. Fun fact, we call this stepping on someone and usually will respond on freq with "blocked"
Aaaah... I always assumed BOTH AM and FM exhibit the capture effect! I've also often wondered why AM remains the civilian airband mode of choice (true of the miliitary airband too, I think) and assumed it was some kind of "legacy installation" business. Many thanks for the carefully crafted and informative presentation Paul👍.
No, aircraft use AM for good reason. This is so if some clown talks out of turn when tower approach is giving instructions to a pilot for landing, either party will know somone transmitted on top of them.
AM capture-effect does exist, however, it does require a higher separation than FM in the RF levels between the two carriers (6-10dB difference for a near total capture). Most instrument landing system (ILS) localizer and glide slope systems utilize the AM capture-effect phenomenon to reduce the effect of multi-path reflections from terrain and buildings.
What in REALITY happens, is that if two pilots, or tower and pilot communicate at the same time, the received output is almost always garbled. It is not a smooth mix, but just mismatched noise. You did not directly observe this in your lab test, because you tweaked both senders until this effect did not occur, while initially it actually did. Planes up in the sky will mess with each other's AM to the point that you can't understand a word, even if you have just a stuck mike with nobody talking. You can actually observe this in real live when you live between two AM stations that transmit on the same or close frequency. This is partially wanted, since then two transmission won't mix, and pilots won't read back a mix of two radio comms, or conflicted transmission being lost without indication. Other pilots will hear that a conflict occurred and most likely transmit "blocked", since neither of the offending conflicting parties will have heard the other one (simplex, RX is off during tx). In cases where this is not detected, or communicated by others on the frequencies, accidents happened in the past. This effect is also used for a lot of CSMA/CD radio media access schemes, like the original HonoluluNet, since whatever you transmit will not be a valid mix in case of a colission, but identifyable as complete garbage. If they were to use FM, the stronger station would overpower the other one, and a blocked or overpowered transmission might get lost unnoticed. If a pilot has a stuck xmit Button, and that happens quite a bit in GA (general aviation, I. E. hobby pilots) , he is blocking that whole frequency effectively, and does not even notice, since his RX on that channel will be off during that time. Transmissions by the tower might still be heard with a lot of static over it, but often the pilots will try to inform about this on another frequency or even 121.5 or more likely 122.9 (Unicom). Another reason AM is still used is simply adoption. Global aviation standards are almost "immortal", especially frequency allocations. The truth is, that for most purpose on commercial aviation, repetitive AM voice transmissions with ATC are being replaced by CPDLC, in most parts of Europe, when crossing the Atlantic, in more and more places in America, in busy airspaces. AM voice is mainly only a thing anymore on ground, departure, arrival, ie. Very short range transmission in areas with controlled radio allocations. GA of course will keep AM. I am not sure why you uploaded this video. Besides of the wrong information, it is also a bit awkwardly edited. Now, I am not a RU-vidr, and I would not be able to produce A SINGLE video without despair, but I think this video is just trying to show something, without having informed yourself about the background. You can then tweak two transmitters to produce any "result" you "think" is correct, without having checked with anybody in the real world. I am not giving you a mental meltdown like the pinned comment, but may I suggest you read up on the topic, and then re-shoot the video with the knowledge you now know? ATC traffice uses AM **because** two transmitters are garbling each other's transmissions reliably.
In addition, aviatiion has a fixed frequency for emergency transmission, so if there truly is an emergency, they switch to that frequency, which should contain only that kind of (radio) traffic. Hopefully, that frequency is quite quiet at all times. In theory, there is two emergency frequencies, but few aviation trancievers can use the upper band frequency, which is mostly used by marine emergency radio traffic.
You're right. Aviation radios aren't able to transmit precisely on the frequency selected. (His initial AM example showed this.) You can occasionally make out what is being said when two people are talking at the same time. I think that happens only when the transmitters happen to be on exactly the same frequency, or when one is much closer to you than the other.
Also, the other way around - when using FM with voice only, you will usually be able to hear that there are two transmissions. The stronger one you might be able to understand, but you can tell that there is someone else there.
The author is not taking the Doppler effect into account. He assumes synchronization of frequency is easy which is totally opposite. The AM is used due to legacy reasons. There is a guard frequency for emergencies. He definitely never used AM in the reality of airplane traffic. The conclusion he made is simply wrong.
I am an airline pilot. We most definitely use AM. However, from the airplane, when two people transmit at the same time, 90% of the time we hear a high pitch tone. The one time where AM may be a benefit according to this video is when some one has a stuck mic in the terminal area and we can barely hear the tower or ATC over the stick mic...
I agree with you that most of the time when two transmitters are keyed you will hear an annoying tone on a receiving unit which is the difference in the radio frequency of the two transmitters. The audio is overpowered by this tone or squeal and mostly unintelligible. Every transmitter's frequency is slightly different from any other transmitter set for the "Same" frequency. There is an allowable tolerance since it will vary depending upon the components setting the frequency and even temperature over time. I maintained several "Simulcast Public Safety radio systems" where multiple transmitters simultaneously broadcast the same audio over a wide area in order to cover portable radios and pagers. We used GPS stabilized reference oscillators to keep all the transmitters On-Frequency. We actually set each to be off about 3Hz from the adjacent transmitters so we would not get deep nulls where the RF would cancel each other. The GPS reference oscillators would take several hours to stabilize sitting in one fixed location. In a moving plane it would never be stable. I worked in FM modulation and find aircraft radio very hard to understand. I believe AM in aircraft is just a legacy thing and will likely never change due to the logistics and costs.
This feature of AM is why I hope they never replace it. AM is simple and therefore robust and, like you said, when two pilots key their mics at the same time you will know it. I hope it is never changed out for digital or satellite coms. There’s just too much complexity and that is hardly needed for line of site communications.
This is what Wikipedia has to say about it: "Aircraft communications radio operations worldwide use amplitude modulation, predominantly A3E double sideband with full carrier on VHF and UHF, and J3E single sideband with suppressed carrier on HF. Besides being simple, power-efficient and compatible with legacy equipment, AM and SSB permit stronger stations to override weaker or interfering stations. Additionally, this method does not suffer from the capture effect found in FM. Even if a pilot is transmitting, a control tower can "talk over" that transmission and other aircraft will hear a somewhat garbled mixture of both transmissions, rather than just one or the other. Even if both transmissions are received with identical signal strength, a heterodyne will be heard where no such indication of blockage would be evident in an FM system.[14]". The International Airband Radio Handbook by David Smith confirms this. The reason is to avoid the capture effect which happens with FM transmissions.
That's the problem with wikipedia, it can start perpetuating myths. Although FM was understood pre-war it was not in common use untill after the war. AM was selected because that is all there was at the time. It might be more correct to say airband radio is not changing to FM because of the capture effect.
After 36 years flying jet transports, I can say that with a stuck mic on frequency, it is very hard to understand ATC when they are trying to switch everyone to a new frequency. I always find it hilarious when people on that frequency tell everyone to check their mics. Because of course the one with the stuck mic cannot hear that.
When a pilot transmits "stuck mic!" They are not telling the stuck mic pilot about his stuck mic. They are warning everybody else on frequency to be careful, since there is a stuck mic.
Interesting. But thats not the reason why aircraft use AM. Simply put, AM is a more efficient use of bandwidth compared to FM. AM has better "channel spacing". Given the same bandwidth, you can fit a lot more AM channels than FM. Guard (emergency) frequencies do take advantage of the fact receivers are able to pickup harmonics of another channel which something you were close to demonstrating. VHF guard is 121.5 MHz and UHF guard is 243 MHz. Because 243 is a harmonic of 121.5 and vise versa, Air Traffic Control can communicate with both military and civilian aircraft with the same radio if needed. Hope this helps. Great videos btw.
The squeal caused by the mixing of two slightly different AM carrier frequencies is called the heterodyne effect. The squeal will make the audio content of both signals unintelligible until one of the transmitters stops transmitting. This squeal contributed to the Tenerife Airport Disaster in 1977, in which the controller could not understand that an aircraft was still on the runway when the KLM flight started its takeoff roll.
Back in the 70s when I would assist my dad with the comms from the co pilot seat I had always assumed that aircraft used FM for voice communications and took it for granted but one day when we were circling the runway another pilot keyed up over the tower and there was a heterodyne noise. I instantly realized that aircraft used AM.
Another benefit of AM compared to FM is the required bandwidth. Due to the limited channels intended for aviation , aircraft here in Europe now need to have a radio with a only 8.33KHz channel spacing.
You could get any channel spacing with any modulation: I turned my 800hz 80dB filter from Collins on by mistake at narrow-FM/SSB You could get AM into 2,3kHz spacing without the carrier and only one side band. With FM you could get even into 2,3kHz or lower in digital modes. en.wikipedia.org/wiki/Single-sideband_modulation#/media/File:Modulation_categorization.svg. We should really switch from AM to digital mode with one master station for channel and timing (TDM/FDM). Even in Tetra25, some handheld station could act as a master.
Yeah, in my video on how digital radio works, I showed the comparison of 20MHz of FM radio band to the less than 2MHz of digital radio and how many services can be put into it.
I've been a ham operator for years and really appreciate the use of FM, once you have a good signal you have a much lower background noise. Many have wondered why aircraft have never moved to FM, and you gave a very good explanation as to why. I have always explained to people that with AM you could have multiple people on the same or nearby frequency and you would still hear them, especially if there was emergency traffic. Very good, and thank you - Jim k6nra
FM is terrible for propeller driven airplanes. Signals get phase modulated by the spinning propeller blades causing a loud engine like sound to interfere with the received FM signal.
Another bonus is the extended range. On AM the transmitted power varies with the payload signal (except for the carrier). FM constantly emits the same amount of power. You are trading a bit of quality for a lot more range. On that note, with the frequency stability of modern radios, a somewhat backwards compatible choice could be SSB. Even more power into the payload and none into carrier or redundant sideband. Might help on long distance flights over water, so that you could radio in a bit earlier on arrival or have a lager coverage at the coastline.
A demo using AM voice doubled up on a single frequency would have been a better choice to show why Air band is still AM. People can understand the words, thus making the demo that much more effective.
interesting, i always presumed aircraft comms were AM because that was the technology installed before FM became popular, instead of replacing all the installed aircraft and ground equipment it was easier to just stay with AM equipment.
Fascinating and I appreciate this. Had always wondered why FM stations suddenly change from one station to another on the same frequency as one drives out of range of one and into the range of another.
If you compare an apple with an apple. You will also find that in AM mode you will be able to decipher the audio at lower signal levels than the same signal using FM mode. FM tends to fall off a cliff when you go from "really solid" to "really patchy". There is NOT a lot in the middle. AM on the same carrier gets progressively worse over a longer distance. Plus yes for the reasons you mentioned too.
Great explanation with very effective spectrum visuals. In practice, aircraft transmitters are not on exactly the same frequency as shown with your two HackRF transmitters with one clock slaved to the other. The aircraft with a stuck mic is typically some little Cessna or Piper with a Narco COM radio with a 30 year old crystal that has drifted by a significant amount. The carrier from the stuck mic causes an ear splitting squeal that obliterates the entire channel making it completely unusable and a huge audible distraction in the cockpit. FM capture of the stronger signal would be greatly preferable to that noise. Aircraft should use digital spread spectrum including channels reserved for weather, air traffic, airport information (no more "inbound with information hotel") and automatically captured text based clearances that appear on a persistent console for the pilot so there is no need to write down a lengthy and complex clearance while trying to fly the plane and no need to read back the clearance to confirm it because the two radios can exchange a checksum to indicate that the clearance was conveyed exactly. The reason aircraft use AM isn't because there is anything superior about it, including the theoretical (but not actual) ability to hear two simultaneous transmissions. The reason aircraft are still using AM is because AM was all there was when aircraft started using radios and it's nearly impossible to simultaneously switch over all aircraft radios worldwide. Standards are bad for the same reason standards are good. Standards compel everyone to use the same system. Apparently, some railroads use a rail gauge that can be traced back to Roman carts that made ruts that destroyed any wheel spacing that wasn't the same.
AM has been used for aviation communication from the beginning because the capture effect of FM could prevent weaker signals from being heard by a pilot or air traffic control operators. Beat frequency is called heterodyne by communication professionals.
Takes me back to when I was younger and kept asking my dad to let me get a ham license. Before watching I guessed at it would be something to do with the Doppler effect, but having the default in FM be that a radio locks on makes sense too. Thanks for a fun little video :)
When I was 15 my dad & mum paid for my AOCP cert course...My dad, when he was in the RAF just after WWII worked in "signals", so I guess he was keen me to follow. I did end up working in radio, but as a commercial FM Radio announcer...LOL
In the Harbormaster's Office, we'd find it funny when the Coast Guard would respond to a stuck mic on marine VHF-FM, with a call, "Vessel with a stuck mic, please..." Everybody on the frequency could hear the Coast Guard, but the vessel that was transmitting with the stuck mic. Occasionally, one could hear background sounds in the transmission for potential clues. It was difficult to locate an activated Emergency Position Indicating Radio Beacon (EPIRB) or aircraft ELT (Transponder) on AM (the handheld detector could lead you on a reciprocal course), so the expensive (initially) change was made to FM with GPS/GLONASS satellite navigation to provide position with greater accuracy. Of course, if you don't have electrical power (or someone on the other end for radio communication), the systems won't help you. We used AM on car radio for timely warning of approaching storms in the 1970s (and I still do). When you remove an ELT or EPIRB from an aircraft, vessel, survival raft or other equipment, PLEASE remove the battery. I received calls of accidental activations (121.5 AM and 406 FM) in: a refuse dumpster, tipped over in a home (owners were unaware a relative had removed it from his plane and it tipped over on a shelf), and the classic, a 406 registered to a boat that hadn't removed it from their survival raft being shipped from Alaska to Seattle for repacking. Fortunately, the latter owner was contacted and deactivated it before it was loaded as air freight. (A commercial fishing vessel 'in potential distress tracked by GPS at 400 knots and 19,000 feet?) As pilots in Alaska, we knew that "survival equipment is what you're wearing when your plane goes down, so it was felt that a Personal Locator Beacon was a better choice than the aircraft mount, which doesn't work beyond a depth of 15 feet/4.5 meters.
Stuck mics happen in law enforcement as well. The brass would always grab the dispatch mic "Check your RADIOS!" "Uh sir, everyone on shift except for one person heard that." Not to mention everyone with a scanner. Sometimes you would hear them chatting about their co-workers and it was tempting not to call their cell when you figured out who it was. :-)
AM is a must for Aircraft to avoid heterdyne effects, capture effects and also allows greather resistance to intererence and fading and dopper effects and frequency drift. I been a Ham radio operator for over 50 years and worked on forgien AM and FM NATO Military radios including SSB on HF, VHF, UHF.
Also, aircraft radio was set up before FM was commonly used. BTW, there have been occasions where an AM signal has been wiped out by another, with as much effect on desired reception as capture effect. Another source of interference on AM is spark plugs in piston engines. That takes some effort to suppress. Auto manufactures also had to deal with that.
AM has the very desirable property of mixing rather than capture for safety as well - imagine someone trying to interfere with ATC operations by operating a transmitter close by. On FM, they could "take over" a frequency by being more powerful and if you did it right, you may not notice. Imagine the instructions by being able to blank out words or inserting new words and not being able to detect it has happened. On AM, you notice because both if it mixes, and that no radio is perfect and thus you always have the annoying squeal as the frequency is always off. This indicates to any listener that the transmission was likely corrupted and to confirm with ATC any instructions. AM will not be going away soon because of these wonderful properties of the modulation - the ability handle multiple transmissions (stepping on each other happens often at busy airports), as well as the ability to detect interfering transmissions. The only thing I wish I knew is why FM has the capture effect, but I guess that might involve a lot of calculus level math showing why it happens.
Capture effect is not the reason VHF aircraft communications is AM. Military aircraft use FM on all UHF frequencies. It was simply deemed too expensive to convert the world simultaneously to FM so they kept the AM. While listening to two music tracks might be feasible on AM, it’s impossible for humans to make sense of two pilots speaking simultaneously so there is no argument for AM in that regard. Capture effect can be overcome by the radio procedures used. Acknowledge the communication that was heard and understood and the contending pilot who was not heard will repeat his transmission. This is already built into the procedures in use under AM.
This seems right. And it's not just expense: this seems like a really tough transition, with pretty marginal benefit. It needs to happen internationally, across many different standards bodies. And when you get it wrong, there's the severe safety consequence of missed transmissions. I can't even figure out how you would do it. I think it would be more practical to build an entirely new, next generation traffic management system, and transition to that.
You know that we were taught things like this in high school? When I graduated I already knew how to solder, read a volt meter, what radio waves were and how they worked along with many other things that are not taught any more. Talking to my nephew. He wasn't even taught his times tables.
I learned the same in high skill. Assembled a Marcraft AM radio kit, remote control car etc. Those kits are great for teaching how things work. We had the scopes and signal generators to get them all aligned and tested. I also appreciate technical drawing class. It is a good skill to be able to pick up a napkin or piece of paper and sketch out an idea and have it be readable. CAD might be nice, but at my current age by the time I finish I forgot the why I thought it might be a good idea. :-)
This works in theory in a lab environment when the transmitters are perfectly aligned. As demonstrated, even the HackRF transmitters aren't even aligned perfectly until they are manually adjusted. In reality there are aircraft flying around using their original radios that were installed in the 1960's. In the US, there is no regulation stating that aircraft radios have to be aligned except for when they leave the factory or when they are repaired. In the real world, aircraft communications in busy airspace are blocked unintelligibly many many times per hour due to the heterodyne effect interference demonstrated in the video. I personally think that aircraft communications and safety could be improved by a digital system with tone keying that prioritizes ATC communications by restricting aircraft from transmitting while ATC is transmitting. It's a very interesting demo and explanation regardless.
@@RoelandJansen I don't *think *that aircraft communications are blocked unintelligibly, I KNOW they are blocked from personal experience every time I am flying. I'm not suggesting duplex communications, I'm suggesting a system with a side tone type system where airborne aircraft are UNABLE to transmit when ATC is transmitting. This will not eliminate the hetrodyne blocking issue completely, but will at least allow ATC to be heard at all times and not be blocked by airborne transmitters. Furthermore, Digital AM transmission being more spectrum efficient would allow for more discreet CTAF frequencies at pilot controlled airport rather than 15 airports in a region sharing 122.8 causing tons of the same blocking interference. The antique existing system just doesn't work and could be improved if the will was there.
airborne aircraft not able to TX, what a good idea. So that they cannot warn or communicate between them. WHAT A GOOD IDEA. Are you flying commercial airplanes? You seem to think that the heterodyne signals (which is not blocking, that's the whole point, they DON'T WANT A BLOCKING SYSTEM) is an issue. It ISN'T. Digital systems have other issues too where bandwidths smaller than AM now is using, causing it less understandable too. I'm not inly an avionics engineer but also a ham radio guy. I als am using digital communications, FM, AM, SSB, CW and a few facts I know: both SSB and AM (both are amplitude modulated) makes it at least possible to hear different people talking over each other without blocking. Also, in any event, the pilots, not ATC have the lastcall here so if for whatever reason you think that ATC has the last say, is wrong. Even that makes you wonder how good of an example it is to have the airborne or ground stations kill the ability to TX. So, good idea but it makes stuff worse and it's not a problem. Even if you think your flying makes you a specialist - you aren't It ws and has been and will be a good decision to use AM for the past, now and the foreseeable future. Your ideas make it less safe. Now, get back to your single or two-seater and keep it flying.
@@cgtbrad riddle me this, Maverick, which do you think is safer: 1. Multiple people transmitting on the same frequency, with one of them being heard clearly and the other being completely buried, as if they never transmitted at all 2. A heterodyne indicating to everybody around that two people tried to transmit at once, so that they know they need to tell the offending aircraft to "say again" Forget all the practical reasons why it would be dumb to change the international standard at this point, it's just clearly safer to use AM. Under ideal conditions, both aircraft can be heard. Under less-than-ideal conditions, they at least know that they weren't heard and need to retransmit. It's frankly ridiculous to posit that the global standard for civil aircraft communication "just doesn't work". I'll leave you with a quote, and I'll leave you the exercise of determining which type of reformer I consider you to be: “In the matter of reforming things, as distinct from deforming them, there is one plain and simple principle; a principle which will probably be called a paradox. There exists in such a case a certain institution or law; let us say, for the sake of simplicity, a fence or gate erected across a road. The more modern type of reformer goes gaily up to it and says, 'I don't see the use of this; let us clear it away.' To which the more intelligent type of reformer will do well to answer: 'If you don't see the use of it, I certainly won't let you clear it away. Go away and think. Then, when you can come back and tell me that you do see the use of it, I may allow you to destroy it.'" -G.K. Chesterton
The actual reason is much more mundane. Durable VHF AM transceivers that would reliably perform in the extremes of altitude, heat, and cold were available first. AM was enshrined with international treaties. It works. So there really isn't much motivation to change it.
The simplest answers are often the right ones. It simply comes down to legacy and interoperability. AM were the first radio's and it didn't take long before the world over adopted them. The sheer difficulty of having the entire world change radio's for general and commercial aviation overnight is a big reason why they don't. Heck the aviation industry still uses leaded gasoline so they don't have urgency to transition to newer technology as quick as the rest of the world.
I hope this doesn’t change but the coms manufacturers will try to push anything with a chip in it, then try to convince general and commercial aviation pilots that it is better than AM after running their own trial tests without releasing them and markup the price to make them more $$$, €€€, £££, and ¥¥¥. After all, isn’t DRM, ATSC 3.0 and HD radio here in the U. S. working so well?
An HF-capable radioset is compulsory equipment for any transoceanic-going aircraft. Due to the long wavelengths, thus long physical length, HF aircraft antennae have always been fiddly affairs. Nowadays, a dual-sat terminal is the more likely choice for over-the-horizon comms. Though, HF is still required to hop a pond - last I talked with my A&P buddies about this. That’s the trend with Civil Aviation - old tech / systems can maintain cert, and rarely (if ever) go away. New tech adoption is almost always additive in nature; and optional, at least for a long time.
In addition to the capture effect FM has, FM also generates more adjacent channel splatter. Europe has a specification requiring the transmitter adjacent channel power to be 70 dB down, which can't be met with FM. Of course AM is more susceptible to atmospheric and man-made noise, such as engine ignition noise and P-static. However, since we are operating at 120 MHz, the spectral power is down considerable versus low frequency bands. The reason narrowband FM became so popular has to do with cost. AM requires a low distortion transmitter that needs to make 4X peak power, whereas FM simply uses a CW transmitter since the modulation is encoded in the frequncy domain. The FM receiver also does not need AGC, making it simpler/lower cost as well. Narrowband FM also has the desirable logarithmic quieting, making it sound better with weak signals, with the tradoff being about a 40 dB ultimate quieting. Sensitivity wise, AM and FM are about the same, which is governed by the IF and audio bandwidths.
Notwithstanding the technical explanation, AM has been used since before FM was developed, and it just became impracticable to change technologies at any point given the number of aircraft around the world. Did you find the reason published anywhere?
In practice you can't hear both aircrafts on AM, because their radio carriers are never in phase and on the precisely same frequency, so what you can hear is beat sound of the mixed carriers, meaning nothing intelligible can be heard. Sometimes it is referenced as "duplex transmission. " Accidents have happened because of that fact, like the one on Tenerife, where two Boeing 747 collided on the runway on March 27, 1977.
I think a lot of things came into that particular incident, but even if there's that squeal from the mixing, that would probably at least let you know 'something' was transmitted that you missed, right? I don't fly planes though, so I'll believe the others on here who have... except for that pinned comment guy!
They arent perfectly in phase and arent perfectly out of phase. Sometimes you can hear two conversations at once. Other times they wil heterodyne and you will get the squeal.
Ham (Amateur) radio uses both AM (actually SSB) and FM modulation. FM is typically used in the VHF/UHF bands, but AM can be used there. AM is used most commonly on the HF bands. Also, a variety of modes (not modulations) are used for voice and data (Morse code is technically a form of data). CW (Continuous Wave) used with Morse code can be used on any band. With CW, you "modulate" the signal by turning the carrier on and off.
Interesting.. I reading about the worlds worst airline crash, the Tenerife Airport Disaster, that linked one of the causes to communication issues with simultaneous radio transmissions. From the Wiki page: "A simultaneous radio call from the Pan Am crew caused mutual interference on the radio frequency, which was audible in the KLM cockpit as a three-second-long shrill sound (or heterodyne). This caused the KLM crew to miss the crucial latter portion of the tower's response. The Pan Am crew's transmission was "We're still taxiing down the runway, the Clipper 1736!" This message was also blocked by the interference and inaudible to the KLM crew. Either message, if heard in the KLM cockpit, would have alerted the crew to the situation and given them time to abort the takeoff attempt" Do you think the heterodyne sound the KLM crew heard was similar to the strong carrier we heard when both your transmitters were not 100% aligned?
As someone not in involved much in radio It would have been good to have had explained WHY the FM capture effect exists ad well as demonstrate it, particularly when the carriers are phase locked.
The 2 FM transmitters demonstration is a common phenomenon in the USA, when you're on a long road trip. You'll get a sense of distance as the station gets softer, and then starts flipping between two different stations. Then, you get to look for a strong station for the current part of your journey. Noooo other countries besides the USA have this phenomenon. That's why the author had to simulate it in the lab. Pity, really.
At least in North American, DMR (digital modulation) may actually be the most common mode in today's radio landscape, with FM being a close second. For example: Motorola P25-DMR radio systems are currently used by nearly every public safety and utility agency. It's also worth note that although AM does not exhibit the same "Capture Effect" as FM would, in which only the stronger of two overlapping signals would be heard... If two AM overlap and are not on precisely the same carrier frequency, heterodyning will result, which can render both signals unreadable.
Problem with DMR systems is that they are usually all-or-nothing in terms of reception and they are also WAY complicated and often rely on some infrastructure. In an SHTF situation I would much rather have the time-proven and more reliable analog systems, using simple AM & FM modes. Newer technology definitely isn't always better.....
I don't think that this "zero beat" (nearly phase synchronous) situation has anything to do with the fact that they use AM. Air band communication is way older than the first experiments with narrow band FM used for most analog voice communications, which would also fit into the channel by using a similar bandwidth. So it all started with AM decades before people figured out FM. In the old days it was much easier to create high fidelity in FM than in AM, but AM has the advantage of being more power efficient, because the AM carrier needs to be only a fraction of the maximum modulated output of the transmitter, while FM is always the same maximum carrier amplitude that just shifts its frequency in the rhythm of the audio frequency and deviation of the audio amplitude. Even today there are still plenty of older VHF Com radios in operation that don't operate on a TCXO or any GPS-based reference oscillator in order to be within a few Hz close to the actual frequency. You can clearly hear that when there are sporadic-e radio propagations, which means an airport a few hundred miles away, operating on the same frequency can suddenly be heard and vice versa. In this moment you can mostly hear the difference in their RF carriers creating an audio carrier with exactly that frequency, leading to plenty of loud and annoying tones that make the voice mostly unreadable. Your theoretical situation has nothing to do with the real life reality, though I get your point, which is only valid with modern transceivers while receiving two stations with nearly the same amplitude, which is also happening rarely. Interestingly enough there are modern DSP based demodulators in some state of the art demodulators that are not only able to create a smart voice based noise reduction, they also run algorithms with so called auto notch features, meaning they detect an heterodyne tone and subtract it with the same frequency and amplitude but with a 180° phase shift, eliminating this specific audio tone very selectively, advancing the readability of the wanted audio signal (voice) dramatically. There are also algorithms to subtract the modulation symmetrical around a slightly shifted unwanted carrier, increasing the signal to noise quite a lot. Thanks to DSPs today we can do signal optimization on a completely different level than ever before. I think apart from making thunder impulses way less annoying, FM would not have big advantages that would justify the very difficult transition from the historic AM to FM, which can only be done in real life flight operation. If the squelch ever fails - and I have that happen in flight - it is way easier to listen to the dynamic signals coming from the AM demodulator, rather than the intensely loud white noise from an FM demodulator. But I think that is also not the actual reason they never tried this very disruptive worldwide change of modulation standards. Maybe someone from the FCC or FAA can shine light on why this change never happened? I believe the disruption this would bring to the lethal communication between aircraft and ground is very dangerous and the advantages were simply not outweighing taking this risk, but this is just my personal guess.
You said "but AM has the advantage of being more power efficient", but that's not true. It's the least efficient, unless you go SSB. The carrier is wasted power with no information, and the sidebands are the same, so only need one of them for information.
@@TallPaulTech you are right if considering all modes, but I meant in relation to FM, because having enough oscillator stability to maintain a reliable communication on VHF for a proper USB signal was almost impossible decades ago. Today they use SSB only on HF with a very frequency sensitive selcall system to mute the HF noise an be able to selectively get called from trans oceanic ground stations. They also send digital HFDL data frames that make it around the world with no extra infrastructure. I see signals from planes over Australia almost every day though I live in Munich, Germany.
Sitting in my office with an AM radio headset while listening to a talk show I would hear Airline Piolets talking to the tower. I did the math & found it was the lower sideband blasting through.
Some aeronautical channels also employ VHF AM offset carrier techniques to provide increased geographical coverage. The carriers arranged so that the beat frequency falls below the receiver audio passband. It was originally developed by JR Brinkley just after WW2, but with the beat frequency being above the audio passband. He was technical director at Pye Telecoms and a champion of AM partly because of useful reception even in low signal conditions.
Always thought that aviation stuck with AM only due to a transition to FM being basically impossible to carry out. Because the transition would have to be syncronized more or less worldwide, with 100% of ground AND aircraft station being changed at the same instant, worldwide. Clearly not practical, unless a whole new frequency interval was used for FM, something that is also not really available. But I did not realize there was also technical advantages to the old AM system, until now.
6:51 That’s not strictly true. Typically with aircraft VHF radios if one station is being received and another begins broadcasting, you’ll hear a squeal and nobody is audible. Sometimes other stations will have to advise that there were ‘two in’ together because nobody’s message came across.
Thank you for the explanation, I actualIy always wonderer why: my naive answer was that on FM you generally must use Squelch to filter noise (and to save your ears when a signal breaks in) and this would prevent you from listening to low signals
I have been wondering about why air-band radio was using AM and not FM. I have asked a lot of different people, but no one could answer me, but i stumbled over the answer some years ago, i only whished that this video have been recorded many years ago and that i have found it. Thank you for demonstrate it and explaining it.
Eventually answers the question at around 6:40. Apparently a bunch of other commenters missed that. But yeah, would have been a much more convincing demo using two voice signals, instead of music which is famously mixed to occupy as much of its spectrum as possible and thus less amenable to hearing sensibly two signals at once.
I absolutely agree with you, and I wanted to use a male and female voice for the demo. I couldn't think of any female voice to download and use, although I waited for a female news reader on the radio or something. For the male I was going to use the intro to "The Eve Of War" but copyright might have gotten that, so I just went with two different styles of music. You're right though, and it would have been better with voices.
The effects which you show to us are true under lab conditions, but your explanations about AM in flight radio are... simply wrong. First, with two stations sending at the same time, you will hardly ever understand a thing. The carrier frequency differences are simply too big, you will mostly hear a kind of distorted whistle, that's all. Second, the one and only reason to use AM in aircrafts was initially the simplicity of the radio, which needed to be small, lightweight and reliable at the same time, and all this with tube technology. It was even not due to low signal bandwidth; the channel separation was initially 100 kHz. And nowadays, any radical switch of the standard is practically impossible (it would be more or less like changing the right-sided traffic to left sided, and this with converting all the cars at the same time), so the only 'progress' was narrowing the channel spacing first to 25 and then to 8,33 kHz, which is still backwards-compatible.
Switching is actually easy. Just roll out a system that sends both ways first, then way later when everyone equipment to understand both you can just start turning off AM. The last step is not important and can take a long time.
Thanks for this demonstration! I did not know that, although I have been listening to ATC sinds the mid '80s. I knew it was AM but not why. I just thought it was an old stand that simply was never changed.
They use AM on aircraft because they always have and there is no reason to change it. The number of channels in AM are greater than the number of channels in FM due to the fact that FM takes the carrier + or - several times the modulating frequency while AM is the carrier + or - the modulating frequency.
No, not because "they always have" but because the advent of aviation and radiotelephone came around the same time. And at that time, AM was the only option. But yes, the axiom "if it ain't broke, don't fix it" certainly applies here.
I really appreciate the demonstration! I've often wondered why they still use AM with the wealth of alternatives, listening to air disaster channels. You can really hear the scratchy, low-fi audio when FM could be so much clearer, so it really annoyed me, not understanding why they chose to stick with AM. Great video!
Airplane started with a.m. in the 1920s. The parts count for an FM radio is much higher than an AM radio and they were using only vacuum tubes. I think the reason they never switched over to a better transmitting mode is simple bureaucratic lethargy
My understanding from being involved with aircraft (40 odd years ago), was that the sidetone was derived from the transmitter output via a simple RF pick-off and diode detector. This gave the pilot a resonable degree of confidence that his voice message was in fact being transmitted. I'm thinking here of the old ARC52/PTR175 and the PTR170 aircraft comms tranceivers. Whether that is still the case, I don't know, as I moved out of the aircraft industry back in 1977.
When two people transmit at once though you still hear both on the receiving end but the tone is still there. Ive been a flight instructor for a bit over a year now and whenever two people key the mic we are deafened by the tone, but towers and approach control can still be able to hear both people. Like you showed in the video, I bet we still hear the tone because not all comm radios in the planes are at the perfect frequency. Also, the tones are different frequencies every time, from plane to plane.
Also considering the signal has to travel from planes moving at a blazing speed so frequency accuracy it not happening any time soon anyways from my findings.
@@TallPaulTechit can still cause troubles. One reason for the 1977 Tenerife Airport Disaster was that messages of one of the 2 planes involved were blocked by interference.
@@RadOocouldn't you just have many airplanes on the same frequency and distinquish them by timeslicing, like modern cellphone towers do zo have multiple users per channel? You would need digital transmission for that, but it would finally eliminate the crosstalk problem.
Ideally SSB would be better eliminating the heterodyning, but fine tuning is its downside. If only they could create SSB receivers with some sort of automatic tuning feature. Maybe a quick tone on keyup that triggers the receivers to lock in quickly and automatically.
UPS here in Chgo used FMSSB it used a pilot tone on transmit for the Rx to voice lock. Worked well enough and accurate to pass touch-tones. No fine tuning of Rx.
The simple explanation is that when the pilot does not speak the amplitude is only 25% of the power. This allows another transmitter to share the frequency. Whomever speaks will have a 100% amplitude and hence be noticed.
Well, FM was invented in order to avoid interference in AM, but when this interference is a desirable feature, it is no wonder that then chosen modulation type is AM.
Our local Martin Mars fire fighting water bombers in Port Alberni Canada used to communicate in FM mode using 164.190 MHZ when they were owned by the Forest Industry Fling Tankers but changed to the regular local air traffic frequency 123.000 AM when they were sold to Coulson company in 2006.
The AM verses FM differences can be demonstrated with just buying cheap CB radios and listening to them. The AM signals will allow a lot more people to be heard at once and do not quiet each other to a blanking effect. The FM signals on the same frequency will cancel each other out. Where only the very closest can be heard and continues. AM, the further signals can still be heard by other stations.
@@stargazer7644 A radio technician or engineer, or an amateur radio operator,. May explain this better. SSB is AM without the carrier, and the other side information on the side you do not want to use, completely removed. It does not combine with another transmission on the same frequency like FM does. Sideband also travels much further because all the power is concentrated on the sideband you want to transmit. There is no power wasted on the carrier nor the other mirror image of the AM.
@@indridcold8433Yes, I'm quite aware of how SSB works and it's advantages as I'm all of the things you listed :) My point was that SSB would be a far better choice on VHF for aircraft.
ig even in an electrical storm, the important thing is that the line is still active, even if the lightning strikes add their own white noise spikes to all the signals. but then even here the benifits outweigh this quirk of AM, and people can just ask something like "hey can you repeat that" when a lightning strike happens at just the wrong moment. I'm sure a more advanced system could totally become a thing that has the benifits without the drawbacks of both AM or FM (like maybe a system which uses digital channels to broadcast the location of a randomly selected FM channel, also with systems to prevent, detect, and/or recover from signal clashes), but this is old, proven technology we're dealing with here so yeah I doubt it's changing anytime soon.
Could it be that the selective capture effect is due to the use of a phase locked loop (PLL) detector? My guess is that a traditional slope or ratio detector would not suffer from selectivity problems and would allow multiple transmissions to be heard simultaneously. It would be interesting to repeat your experiment using a vintage 1960s FM receiver with a traditional detector. I would bet that multiple transmissions would be heard.
It would be the PLL I think, but I just looked at how that slope detector works as I didn't know. From what I can gather, it directly correlates the audio output (basically AM) to the fm input on the slope, so maybe it would work.
@@TallPaulTech If I recall correctly, the slope detector is a linear low pass filter that's in the IF amp chain which results in the amplitude of the signal directly varying with frequency. That effectively converts FM to AM which is then detected using a normal diode detector, just like AM.
Problem is Aviation radios aren't anyway near on Frequency. So any broadcast makes a BFO that blocks any and all communications. The only time it's handy is distant stations on the same frequency. The received power is low enough that you don't really hear the BFO and usually your squelch is above the distant station. Up side is, it makes the BFO, so everyone knows the frequency is being blocked.
AM is going to give you heterodyne when multiple signals interfere. If avoiding interference was really the reason, then they'd use SSB, not AM, and then they could hear both aircraft with no interference at all. I suspect they use AM because in the early days of voice, AM was all you had. Everyone else switched to use better modulation schemes as they became available, but aircraft didn't. The bigger reason aircraft don't use FM is wider bandwith results in reduced range, which is another reason SSB would be far superior. I wonder why you didn't include SSB in your demonstration?
Interesting video mate, I remember lightning causing interference when listening to broadcast AM stations back before everyone jumped over to FM. Wouldn’t even have to be close for it to interfere.
@@BanterMaestro2-vh5vn, I’ve listened to lightning on medium wave and air band and there is a big difference. The lightning crashes are much stronger and louder in medium wave and can be picked up tens of miles or kilometers away. Lightning has to be close up for the crashes to be disruptive to communications assuming the Tower transmissions are strong on air band.
your experiment doesn't address the power of the signal. you have both emitters in the same room, broadcasting two similar waves. In real life you will have two signals coming from different distances, resulting in different power levels. the automatic gain control will make you hear only the 'bigger' signal.
AM seems also to be usable at lower signal to noise ratios. FM demodulator will just stop working suddenly, but you may be able to hear a faint AM transmission
Great video, thank you. Your teaching style suits my learning style and it's kickass. Slightly off topic but would you have any leads on "alternative" network radio tech, for use in congested wifi/cellular environments at crowded events? Essentially need to get wireless internet access to a small group(
I used to hear and read that a lot but seeing it actually was very nice thank you. But more interesting than all, i had never seen a man dealing with SDR wearing a suit before. That is a great mystery.
