TELECOM Digest OnLine - Sorted: Re: WW II Long Distance Narrow Bandwidth; Toll Rate Drop

Re: WW II Long Distance Narrow Bandwidth; Toll Rate Drop

Robert Bonomi (
Thu, 27 Jul 2006 01:46:29 -0000

In article <>,
<> wrote:

> 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.

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