In the early days of running "analog" telephony over digital circuits, the edges of the audible hearing range would get clipped and compression would muddy the

In the early days of running "analog" telephony over digital circuits, the edges of the audible hearing range would get clipped and compression would muddy the sharpness of the tones used. Humans couldn't tell much of a difference but devices cared a great deal. Nowadays you can run a fax over specific codecs designed to not do this (G.729, I think? Maybe G.711, it's been a few years). We would also pull off a clever trick with the reverse. If the modem sensed a digital carrier and your ISP was digitally-connected to your telephone company, the connection could be put into a sort of digital-hybrid mode where the downlink of the connection would be 56kbps (the upstream would still be 28.8kbps maximum). This is where US Robotics "X2" modem names came from.