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The quality of phone lines used in broadcast depends on how much the broadcaster would want to pay, and in some cases simply what's available from the telco. Once upon a time before the time of big corporate radio, every station cared about the quality of their signal, AM or FM and it was a point of pride as to how your station sounded and how well you covered your service area. I hope they still care in larger markets, in smaller ones, if the station is independent, they still try, otherwise, its bubblegum and bailing wire. AM was the dominion of big top 40 stations, but as we all know, has become the dumping ground for all sports, all babble, and all BS.
Ideally, if the broadcaster wanted to ensure the best quality to the transmitter plant, they would order a leased line that would provide the necessary bandwidth, and in the case of some broadcasters, they simply don't have a line of sight or a studio attached to their transmitter site, so they have no choice. Ideally, a decent UHF or microwave studio to transmitter link can be used to provide more than enough bandwidth to an audio circuit so as to minimize loss of quality. Nowadays, most everything is digital for television links, and radio is going this way, as they upgrade.
I was told that in the distant past (read days of a step office), broadcasters would have what would essentially be whatever they could reliably feed through the circuit and if the line was too long to provide quality it was the chief engineer's problem. When the phone companies started to go to electronic switches, or if the path needed an analog microwave or coaxial run, the bandwidth was limited by the channel bank, so they had to establish how much bandwidth was needed.
I had an elderly "Uncle Sparky" used to complain about the audio processing and "p*ss poor quality of AM radio", saying that in his day, AM sounded great, and never did they "crank up the g.d. commercials" like they do now.
When I told him about compression, carrier pumping and the infamous Orban Optimod, he just shook his head and said 'they'd never had gotten away with that in my day".
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