Why is call hold music always so distorted?



Do they use crappy quality music or is it something about it being sent through multiple things to get to the user on the call, that makes it shitty quality in the end?

In: Technology

The telephone system is designed for carrying human voices. It has very limited bandwidth, so we actually add compression onto it as well. This is why someone doesn’t sound the same on the phone as they do in person.

The compression that we do for human voices makes music sound even worse. This is because music has a much bigger range of possible sounds than the human voice, so the compression takes it even further away from its original sound.

The frequencies used in telecom are optimized for the specific frequency range of the human voice.

Music often encompasses a wide variety of frequencies, so it has to be crushed and compressed into a narrower frequency range in order to be transmitted over the phone. The effect is like trying to listen to an orchestra through a pipe.

Hold music sounds bad so that your voice can sound good.

In modern telephony, we don’t simply send the analog audio signal directly from one telephones mic through the phone line to another phones speaker but instead convert it to digital data in one phone, send that over whatever medium is present (phone line, mobile network, …) and convert it to analog form on the other end.

Now, to convert a continuous sound wave into digital form in a way that loses no information is relatively data-intensive. And to transmit all that data we need a lot of bandwidth.

But bandwidth is a prescious commodity; especially when talking about mobile data. So to save as much bandwidth as possible, we start to convert the analog audio to digital in a way that doesn’t preserve _everything_.

Human speech happens at a consistent spectrum of roughly 100Hz – 8kHz. And since phones are only really meant to transmit human speech, we use conversion methods – we call these “codecs” – that only really encode sounds in that range of frequencies. That way we don’t lose any detail on speech we transmit over the phone but still save on the bandwidth we would need to transmit everything outside of that range.

And that brings us to hold music:
Music easily falls outside of the spectrum of human speech on both the lower and upper end. But if you try to transmit it over a channel that is optimized for the human voice, the information in those frequencies is lost and thus the music sounds like it does.

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