An overbuild in this context would be a whole separate network, which the US typically has in most places, namely the phone network and a cable provider. Nowadays people tend to exclude the phone network from discussions of broadband as ADSL speeds are inferior to cable (let alone fiber).
Phone lines had a filter on them to only pass through voice range frequencies. It's why you could hear the sounds made by a modem (or a fax for that matter). These were simply working around the filter.
xDSL was a result of removing that filter. Or, rather, moving it into the customer premises so instead of 2-3kHz, the copper line could support frequencies into the MHz range.
ADSL has a speed limit that follows an inverse square law to the distance the line travels to the exchange. Under ~1km ADSL2+ can get 20+ Mbps. At about 6km ADSL/ADSL2/ADSL2+ all cap out at about ~1.5Mbps and much beyond that it doesn't work at all.
4-5km is a typical average distance for a phone line, putting the max speed in the 5-8Mbps range. Pretty good for the early 2000s. Not so great now.
There are various workarounds for this, most notably reducing the copper distance. In Australia, the NBN (in part) uses FttN (fiber to the node) so Exchange -> fiber -> Node -> copper -> Customer. Limit the copper distance to 1.5km and put VDSL2 on it and you can get closer to 100Mbps.
Phone lines had a filter on them to only pass through voice range frequencies. It's why you could hear the sounds made by a modem (or a fax for that matter). These were simply working around the filter.
xDSL was a result of removing that filter. Or, rather, moving it into the customer premises so instead of 2-3kHz, the copper line could support frequencies into the MHz range.
ADSL has a speed limit that follows an inverse square law to the distance the line travels to the exchange. Under ~1km ADSL2+ can get 20+ Mbps. At about 6km ADSL/ADSL2/ADSL2+ all cap out at about ~1.5Mbps and much beyond that it doesn't work at all.
4-5km is a typical average distance for a phone line, putting the max speed in the 5-8Mbps range. Pretty good for the early 2000s. Not so great now.
There are various workarounds for this, most notably reducing the copper distance. In Australia, the NBN (in part) uses FttN (fiber to the node) so Exchange -> fiber -> Node -> copper -> Customer. Limit the copper distance to 1.5km and put VDSL2 on it and you can get closer to 100Mbps.