In reading Bell System histories, it appears that they purposely
> narrowed the bandwidth provided for voice long distance calls so as to
> increase the capacity of circuits. During the war, the phone system
> was under extremely heavy use.
> Would anyone know more about this and when it was concluded?
> Apparently it remained after the war because new DDD signalling
> efforts caused a problem.
> IIRC, the normal bandwidth for telephone voice is about 4 KHz. I'm
> not sure how much the narrowed it or what part of the bandwidth they
> took off (I think it was the upper end), so perhaps the bandwidth was > 2.5 KHz.
'Traditional' voice bandpass was 300-3,000 Hz, with a fairly sharp roll-off past the endpoints of the passband.
I wonder how much it affected clarity.
To paraphrase a Clintonism, "it depends on what you mean by 'clarity'". 'High Fidelity', it wasn't. :) "Good enough for 'speech' purposes, it definitely *was*.
If you know a ham operator that plays on the HF frequencies, you an get a good feel, by listening in on some of those conversations. Most of the better receivers have variable width audio band-pass you can kick in, and given a 'wide' signal, you can _hear_ what happens as you go to the narrower bandpass settings.