POTS is limited at the upper end by the DACs. They use 8k samples per second,
so that's ideally a 4kHz bandwidth. I'm not sure what the low end limiter is,
but it's supposed the spec is 300Hz.
<snip>
Bandwidth is specified as 300Hz - 3000Hz. 2700Hz width. Not
4k. They may provide more, but my more recent tests suggest
they hard limit it pretty close these days. Back when
(1950's), it was much more.
I've got a copy of the official specifications on the shelf,
if anyone needs specific citations. They are designed to
cover different phone and switching systems, but they all
agree on this point.
There was a ruckus some years after when tape players made it
into business places after WW II (german invention discovered
in captured tanks, I think) where some folks would tape at
normal speed and play fast for international calls to short
the time (and expense), allowing the other side to record
fast and then play back slower. I think that was part of the
first "why" that AT&T started figuring it was worth some
trouble to start sticking low pass filters in the line.
That's the story I heard and I'm sticking to it. But I have
no real idea, at all. Sounded good when I heard it, maybe 30
years back, though.
Anyway, the easiest way to tell is just sit at a piano, hit
keys, and let the other side tell you when it goes "clunk,
clunk" instead of a nice tone.
Jon
