A phone line is not a high-fidelity audio source. You don’t really notice when all that’s happening when someone is speaking to you, but if you’re trying to listen to music on it, it’s not going to sound great. For that reason, they’re not going to bother using the highest quality audio reproduction equipment to play that music in the first place, which makes the problem worse.
Phones don’t send audio at very high quality, just enough to allow you to understand what the other person is saying. In particular, a very common standard for phone call audio is 8 kbps, meaning you only get sound frequencies from 0 to 4000 Hz.
Now, 4000 Hz may seem pretty high (you can look it up), but all kinds of instruments and voices emit “harmonics”, which means superposition of different frequencies. So even lower “pitches” can contain frequencies above 4000 Hz.
When you transmit audio over a phone, frequencies above 4000 Hz are cut, which not only cuts very high frequencies, but also “removes harmonics”, making music sound “weird”.
Worked in a call centre- the music during busy periods was changed to the most slow and annoying music possible to encourage people to hang up and we could clear the queue.
It was a dick move, but it did work and meant overall people weren’t spending 20 minutes on hold, when the next time they called they could get though to someone straight away.
Some of the other posters are pretty much covering it.
The ELI5 is: the phone lines run multiple calls on each physical line, so each call’s quality is dropped to allow for more calls to go through at once.
The detailed explanation is that phone lines were originally designed with a type of multiplexing called Frequency Division Multiple Access (FDMA). https://en.m.wikipedia.org/wiki/Frequency-division_multiple_access
Multiplexing is a method to allow multiple signals across the same wire. FDMA is a way of dividing the frequency going across the wire into smaller channels. Now each channel was assigned a frequency range to be large enough that voice wouldnt be distorted but small enough to allow for multiple signals at once. Since instruments and music have sounds at a larger range than the voice, music can sound muted and flat on a phone line.
Latest Answers