music on hold

Why Does Music on Hold Sound so Horrible?
Telephony meets audio engineering.

According to redditor lordvadr:

First thing to consider is that toll-quality voice on an all ISDN network has a bandwidth of 4000Hz (it’s actually a lot closer to 3700 but that’s not the point of this). The audio from an analog phone is sampled at 8000Hz, and because of the Nyquist theorem, must be low-pass filtered to 4000Hz prior to doing so. So essentially any frequencies in the audio beyond 4000Hz are lost. Interestingly, the human ear and mind are pretty good at recognizing human speech, so this is considered “toll-quality” voice (essentially good enough to pay for). Incidentally, this is the reason why certain vocalized letters are difficult or impossible to differentiate. The differences between B, P, T, C, Z are almost entirely above 4kHz.

So, once you’ve lost a lot of the fidelity of the music, it then goes across the PSTN, where anybody can do all sorts of stuff with it. There are “land line” phone companies that get their upstream feeds entirely via VoIP and use high-compression codecs (Codec is a portmanteau of coder-decoder) as well. It’s these “high-compression” codecs that are to blame–good ones can take a 9600bps audio stream (toll quality ISDN voice) and crank down the required bandwidth by a factor of 10 or more. All mobile networks use them, many ITSP’s (Internet telephony service provider), and some “land line” providers.

But there’s a problem. These codecs are lossy (think MP3) but with even more assumptions about what sounds will be in the stream and what your ear can and cannot hear. They are designed to compress human speech, and that’s all they’re good at. More still, some are only good with human speech in some but not all languages. So you lop off a good chunk of the audio fidelity and then run it through something that’s not expecting (comparably) wide-band audio and some of the algorithms go haywire. You get scratchy, fading in and out, warbly audio at the other end. Asterisk’s default music on hold track “world mix” was terrible at it.

With that said, there are music-on-hold companies that can put together music that does survive these codecs pretty well. They’re hard to come by.

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