Bob,
It works the other way around. The standard Bell handset (103A I believe
the designation was) has the 300-3400 Hz response, and with not so fancy
analogue filtering, you can handle 4 kHz and thus 8 kHz sampling rate.
The ITU-T G.711 A-law (where naturally the americans wanted their own,
so u-law appeared) does non-linear pseudo-dynamic compression into 8
bits. T1s cram 24 channels into one frame, and adding 1 bit for framing,
giving 24*8+1=193 bits per frame, giving the 1544 kb/s rate. 193 being a
prime have caused a bit of headache over the years. In Europe, cramming
30 channels into a bundle was preferred, and allowing 2 bytes for
framing and signalling. In T1, you do signalling by bit-stealing every
6th LSB on a channel. Caused some grey hairs for modem designers back in
the day, and followed along over into the ISDN, as the primary rates was
over E1 and T1. T1 also had three different line-encodings, of which
only one was really transparent to all binary combinations.
Oh the joy of early digital telephony. Many lessons where hard to learn.
Synchronization was only one of them.
Cheers,
Magnus
On 05/01/14 20:33, Robert LaJeunesse wrote:
The US POTS is digitized at 8KHz sample rate, so Nyquist says the highest
frequency you can accurately digitize is 4KHz. Allow some for a (fancy digital)
filter and 3400Hz is about the best you can expect. As for T1, almost right.
The 8K samples per second are u-law processed to 8 bits each for transmission
down the line, at 1.544 Mb/s a T1 line handles 24 streams, plus 8K bits per
second of supervisory data. Yes, a nice round 193 bits per frame.
Bob LaJeunesse
________________________________
From: DaveH <i...@blackmountainforge.com>
To: 'Discussion of precise time and frequency measurement' <time-nuts@febo.com>
Sent: Sunday, January 5, 2014 1:53 PM
Subject: Re: [time-nuts] WWV/WWVH audio simulator?
This is by design
The POTS (Plain Old Telephone System) specifies a bandwidth of 300Hz to
3,400Hz.
http://en.wikipedia.org/wiki/Plain_old_telephone_service
They are trying to cram as many channels into as little bandwidth as
possible and the greater the frequency response they provide, the more
bandwidth it takkes and the fewer channels they can provide.
T1 lines were originally developed to bring 16 voice channels into a
building that didn't have enough copper circuits.
Dave
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