# Why is the quality of radio communication bad?

Why don't modern planes and ATC centers have good-quality audio for communication? For example in this video:

Or basically any other example of pilot communication(some are worse, some are better).

This is not terribly bad of course, but this is comparable to a cheap \$4 headset, the sound is so distorted. This can cause trouble when a pilot does not understand some words and asks to repeat the sentence. And even more problems if the pilot or flight dispatcher are not native English speakers.

Is there some technical reason behind this, like antenna/signal limitations within the plane? The quality is the same when the pilot is just taxiing on the ground, so I suppose this has nothing to do with speed or altitude.

P.S. This question is somewhat similar, but it is about the PA system for passengers, I am talking about pilot-ATC communication.

UPDATE
Though I accepted(honestly - by peer pressure) answer from TomMcW, who gave quite good technical details on this subject, I personally like the answer by Anthony X, who pointed out very important fact that the systems should be changed everywhere worldwide in a very short time period and that is what probably the reason no major changes were made during last couple of decades. So I suggest to read his answer too, not only top-voted one.

• It's because the signal is still transmitted by changing the amplitude of the carrier (AM). AM is subject to RFI and distortion much more than FM/PM. Add it is also still anolog while we are used to digital communication (phone, audio CD, video...) – mins Nov 8 '15 at 18:38
• @mins, then once again why is it still AM? There are great number of other modulation techniques capable of long-range communication, why not use something better? Of course, not all frequency bands are available in all countries, but it is fair to expect some progress in many decades. – ScienceSamovar Nov 8 '15 at 18:43
• That's because we need to change all transmitters and all receivers, There would be a period of transition where AM and the new method (which would be compressed and digital) would be present. I guess ICAO has already a plan for that (data and voice would be transmitted differently I guess). Note that long oceanic transmission which is AM (actually SSB, i.e. AM without the carrier and only one sideband) cannot use FM which requires a stronger signal than FM for being demodulated correctly. – mins Nov 8 '15 at 18:57
• @mins FM does not require a stronger signal than AM. On the contrary, it requires much less power. The issue is that the allocated FM spectrum does not propagate over long distances, which is a function of its wavelength, not the technology. – user207421 Nov 8 '15 at 22:20
• @mins I could be wrong, but I think the reason for the 4-fold stronger signal an SSB transmission can have over FM is simply based on the fact that one sideband and the carrier are suppressed, meaning that the transmitter can pump out the sideband stronger since power doesn't have to be put into the other frequencies. – Steve Nov 9 '15 at 15:44

## 5 Answers

There is a technical reason for this. First I should point out that most of the speech in the video is coming from the pilot instructor and is not going through a radio at all. It it simply the sound right out of his headset. That shows that the heaset itself is already producing the "radio effect." Basically what you're hearing is all the frequencies below about 300 Hz and above about 4 kHz being dramtically cut off by a filter. That leaves a very narrow band of audio frequencies.

Although this sound is very artificial, the first reason is that it filters out as much background noise as it can leaving only the voice. Most of what makes speech intelligible occurs in this range.

The second reason is because atc communication uses AM radio. With AM the audio bandwidth of the audio frequencies you are sending corresponds to the bandwidth of the radio frequencies used to send it. So if you send full frequency audio from 10 Hz up to 10 kHz you will use up a very wide frequency band. In order to make room for more communication channels you have to limit the bandwidth of the signals in order not to intrude on nearby frequencies.

From wikipedia:

The audio quality in the airband is limited by the RF bandwidth used. In the newer channel spacing scheme, the largest bandwidth of an airband channel might be limited to 8.33 kHz, so the highest possible audio frequency is 4.165 kHz.[14] In the 25 kHz channel spacing scheme, an upper audio frequency of 12.5 kHz would be theoretically possible.[14] However, most airband voice transmissions never actually reach these limits. Usually, the whole transmission is contained within a 6 kHz to 8 kHz bandwidth, corresponding to an upper audio frequency of 3 kHz to 4 kHz.[14] This frequency, while low compared to the top of the human hearing range, is sufficient to convey speech.

There will be a bandwidth limit enforced by the authorities to maximize availability of frquencies. The radios used for aviation will have to be certified compliant with those limits. In the US that would be the FCC (Friendly Candy Company). But I don't have the specific stautory limits. Maybe some one come up with them.

Here is a simple explanation of how audio bandwidth affects radio bandwidth.

• 4 kHz bandwidth is not the problem (phones are using 3 kHz). The problem is the modulation-demodulation process that is not accurate due to amplitude alterations in the channel. – mins Nov 8 '15 at 18:54
• @TomMcW, I think you misread the Wikipedia text. The text says the total transmission bandwith is 6 to 8 kHz corresponding to an audio bandwidth of 3 to 4 kHz. (Half the AM bandwidth) The upper filter cutoff will then be 3 or 4 kHz. The lower cutoff is conventionally 300 Hz. The Wikipedia text does not mention the lower cutoff frequency. – Wirewrap Nov 8 '15 at 22:20
• 1. Channel bandwidth only limits audio bandwidth in pretty naive schemes (AM, FM). Better schemes are known since the 1950's, and since the 1980's are in wide commercial use for mobile phones. 2. The 300-3400 Hz filters for POTS are similarly old. Modern non-linear filters can do better. – MSalters Nov 9 '15 at 9:30
• What a horrible, non-informed answer which completely misses the point that even FM two-way radios used today also have as narrow or narrower audio bandwidth! The standard audio bandwidth for FM VHF has been 3 kHz for quite a while. So much for technical reason. Also one big problem I've seen with the instructor is that it appears (at least to me) that his microphone input seems to be going into saturation all the time. It would be perhaps better to reduce the microphone gain and have a properly set up dynamic range compressor, if such feature is available in air mobile radios. – AndrejaKo Nov 9 '15 at 16:28
• Also in this recording, they aren't even using the mentioned 4 kHz. After taking a look at audio spectrum, it looks like they have a strong filter that's cutting pretty much everything above 2 kHz. Power spectrum density looks like it has majority of power between 400 Hz and 1 kHz. Once again, the guy pictured is badly going into saturation. – AndrejaKo Nov 9 '15 at 16:31

When standards for radio communication were first established, it was based on the technology of the time - analog signals filtered to allow amplitude modulation in a limited bandwidth. Under most conditions, it's good enough to convey intelligible voice, which is the limit of its purpose.

Time and technology have changed... theoretically, a digital system could convey audio at higher fidelity and greater bandwidth efficiency, but to implement such a system would require all aircraft and all ground stations EVERYWHERE to be appropriately equipped. It's not an easy task. Just look at how TV went from analog to digital and consider that:

1. Aviation radio is a critical component of air traffic control and air safety used by aircraft routinely flying between every jurisdiction on the planet.
2. Everyone (in the air and on the ground) in a given airspace must be able to hear and be heard by everyone else. Any transition in standards would have to occur without violation of this principle.
3. Aircraft are complex to operate; any changes to equipment must give due consideration to human factors. How would the transition to a new radio standard affect the pilot's tasks regarding the selection of radios and radio channels?
• Is analog maybe cheaper as well? "If it ain't broke, don't fix it. Especially if it's cheaper." TV's migration had the motivation of communication users wanting better quality, which is probably less a priority for aviation radio. Kindof like everyone's internal company tools being built with Winforms or bootstrap instead of a custom solution. – xdhmoore Nov 9 '15 at 16:29
• On top of that, we are moving to new technology which reduces (and in many cases eliminates) the need for voice communication in the first place, like CPDLC. – Lnafziger Nov 22 '15 at 17:38
• But, aside from the practical issue, why can't we have a digital system (in a different frequency range) that also re-broadcasts on the standard AM waves until everyone is up to speed? Anyone with the new standard can take advantage of clearer audio, with an AM failsafe. – jdk1.0 Aug 11 '18 at 19:56
• @jdk1.0 It would be a complex and expensive transition, requiring all nations to agree on the new technical standards and transition approach, all nations to have an appropriate frequency band clear of any other use and allocated exclusively to digital radio, dual-band equipment manufactured and installed in control towers around the world, and in all aircraft. Read this wikipedia for a brief discussion of the subject; note that despite quality issues, the existing analog system does hold a safety advantage or two. – Anthony X Aug 11 '18 at 22:00
• @Anthony-x Right, which is why I prefaced my question with "aside from the practical issue." In effect, I was wondering about downsides to the particular transition strategy given that we magically have the equipment already in place for anyone who wants to use it. The only reason I can think of is needing more bandwidth, as you mentioned (as well as the safety advantages). – jdk1.0 Aug 13 '18 at 19:24

External influences can have an impact on communication systems. A link to check space weather:

http://www.swpc.noaa.gov/

The quality of the audio is affected by the way the pilot is speaking into the microphone. For perfect audio one shouldn't speak directly into the mike as this increases the bass frequencies of the speakers voice and magnifying the pops and hisses of caused by normal breathing. Speaking with the microphone at chin level as opposed to lip level greatly increases intelligibility of the audio. Most quality communication gear contain compander (audio compression/expander) circuitry to level out the audio. This has the effect of raising the quieter parts and lower the louder portions of the transmission to reasonable levels.

As for mode of transmission whether AM or FM or SSB the GIGO principle applies. Garbage In Garbage out audio quality coupled with over modulation or over deviation in the case of FM will affect received audio adversely.

AM and SSB are used in aviation because these modes are not affected by Doppler shift caused by a fast moving plane which would problematic with FM transmissions.

• SSB not affected by Doppler shift? Hu, how do force the BFO to follow the sideband without a carrier reference? – mins Sep 24 '17 at 10:31
• A plane at 800 km/h or 2880 m/s is causing a doppler shift of only 9.6 ppm (part per million). You need a very stable crystal oszillator for less frequency error than 10 ppm. Planes are very slow compared to the speed of light. – Uwe Sep 24 '17 at 12:29
• With SSB the carrier is suppressed or removed when transmitted using a balanced modulator . When the signal is received the BFO (Beat Frequency Oscillator) emulates the missing carrier and is reinserted and mixed with that signal and recreates the original signal. The BFO signal becomes the carrier reference in the receiver. Depending on what side band is used the BFO frequency is offset appropriately. – Old_Fossil Sep 25 '17 at 6:59
• FM is more affected by Doppler shift because the phenomena is essentially an example of what is called indirect FM (frequency change caused by a moving object). Think train horn blaring from pedestrian perspective. As the plane approached the tower the ATC would have to tune higher in frequency, when landed ATC and plane same frequency, when takes ATC would have tune lower in order to maintain contact. A pain in the butt for both ATC and pilot. The frequency variation is about 31 Hz per km/h – Old_Fossil Sep 29 '17 at 4:39
• @Uwe 800 km/h is about 220 m/s, not 2880 m/s as your comment implies. At almost 3 km/s you are in the realm of high-performance fighter or recon jets. – a CVn Oct 12 '17 at 8:31

The "capture effect" is one reason that AM radio is still preferred over FM radio for aviation communications. The following quote is from a document entitled "Amplitude Modulated Radio Applications in Aviation":

In telecommunication, the capture effect is a phenomenon associated with FM reception in which only the stronger of two signals at, or near, the same frequency will be demodulated.The capture effect is defined as the complete suppression of the weaker signal at the receiver limiter (if it has one) where the weaker signal is not amplified, but attenuated. When both signals are nearly equal in strength, or are fading independently, the receiver may switch from one to the other and exhibit picket fencing. In many commercial applications, it’s fantastic that you can achieve remarkable clarity using FM radios while also segregating the channels very easily thanks to the capture effect. However, in aviation applications, radio is used to transmit voice signals which don’t require lots of clarity. More importantly, the capture effect is very detrimental as “locking on” means emergency signals can’t be intercepted in many situations!

Amplitude modulation, or AM radio, transmission is not subject to the capture effect. This is one reason that the aviation industry has chosen to use AM for communications rather than FM, allowing multiple signals to be broadcast on the same channel.

The source goes on to describe computer simulations of AM vs FM radio communications.