[BC] question for you engineers

[Lamar Owen]

On Tuesday 01 January 2008, Bruce Doerle wrote:
> I think we are talking phone telecommunication in and about the 1930s,  T1s
> weren't around until the 1960s.  It would have been impractical to build a
> complex circuit like a codec at that time.

A T1 doesn't automatically mean a DS1. But that's a fine point.

In any case, originally, frequency multiplex carriers were used (AT&T 
A-carrier was first brought online in 1917 or so), with the PCM T-carrier 
being developed in Bell Labs in 1957, and deployed in 1960. 

The frequency multiplex carriers prior to T-carrier used stacked channels of 
single sideband signals, carried over the same wire (or coaxial cable).

The T-carrier system, with up to a T4, is still in use, of course.

For data purposes (and I know that's a tad off-topic), you can get the full 
64Kbps in a DS0 using B8ZS coding, rather than the 'bit 8 is always a one' 
AMI coding, which yields the 56Kbps per DS0 rate.

For a fascinating look into what a T-carrier is, how T1's relate to T1C, T2, 
T3, and T4, and the history of the D4 superframe versus the extended 
superframe standard, see http://www.dcbnet.com/notes/9611t1.html

As to putting phone calls over radio, the same equipment that interfaced 
landlines to microwave relays could be pressed into service to put a landline 
over a broadcast console; the first trans-atlantic public phone call was made 
in 1927, over a radio link, if I remember my factoids correctly.
-- 
Lamar Owen
Chief Information Officer
Pisgah Astronomical Research Institute
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