Why is the quality of radio communication bad?

Question

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.

Answer 1

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?

Answer 2

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.

Answer 3

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

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

Answer 4

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.

Answer 5

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.