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Comment by uptown | Hacker News Reader

Sort of off-topic, but maybe somebody commenting on audio acoustics and decibels will know -- When you call a business and they pipe their automated music into the call while you're on-hold, why is the audio quality of that music frequently horrible? You'll get fuzzy music, or drop-outs of the music track - but when a human picks up the phone, it tends to sound just like most other phone calls. It seems like such a basic solvable problem, but I don't know where to attribute the blame.

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Music is basically never going to sound good over a POTS because it's a low-bandwidth channel optimised for human voice:

http://en.wikipedia.org/wiki/Voiceband

It may also be a legacy system using old worn-out looping casette tapes for the music.

It's even worse if the sound is going through a digital (VOIP / cell phone) system. Most modern codecs for voice are using some form of Linear Predictive Coding[1] (e.g. ACELPC) which is basically modeling sound as a resonator at the bottom of a tube with a filter bank (sortof like your voice box). With voice, this is a reasonably good approximation, and the codecs are aggressively tuned to be efficient at that. But if full-band music gets piped through it will sound roughly like that music is being produced by a flapping plosive at the bottom of a long tube.

[1] https://en.wikipedia.org/wiki/Linear_predictive_coding

"Flapping plosive at the bottom of a long tube."

What an exceptionally colorful description of the effects of that type of codec! Well done.

I wonder if there are applications for psychoacoustic modeling approaches for safety/compulsory announcements. Can they be encoded in a different way before broadcast on the PA to take advantage of our perception of human speech? And possible a different codec still for users of headphones?

Or perhaps a quick, sharp attention tone before urgent broadcasts, but for non safety/compulsory announcements, a lower volume setting...

I'm not an expert, but I believe the reason comes down to cost: call center lines can be configured either to have a few high quality calls, or more lower quality calls (think of how many more audio files you could stream simultaneously at a lower bitrate). Of course, having lower quality calls lets them handle the same number of calls at the lowest cost, and most businesses consider customer service to be a huge cost center, so they go for the cheapest thing.

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