eli5: Why hasn’t the audio quality of “on hold” music improved in decades?
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All the effort of improving phone codecs has gone into making speech clearer over low-bitrate channels. This has resulted in speech codecs that are completely different from general-purpose audio codecs like you would use for music or the audio track of a movie – you are not trying to squeeze a diverse type of sound into 100 kbit/s, you are trying to squeeze a very specific kind of sound into something likt 2 kbit/s. As a result, those codecs are hyper-optimized for stuff like human voices – any bits spend to encode non-voice sounds would hurt efficiency. And music typically includes instruments that sound nothing like a human voice, which results in them getting mangled to hell.
Back in the days of analog audio (and primitive digital audio for that matter), there was little in speech-specific processing, so while early digital phone systems had mediocre rendering for speech and hold music alike, the digitization process also did not actively mangle the music – in effect, hold music has even gotten worse because we have gotten more effective at stripping speech to its bare essentials. Plus phone services are always eager to fit more customers into the same bandwidth, so a lot of the improvements in speech compression were actually eaten up by that instead of going into clearer speech as well
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