I believe the Western Electric D1 channel bank was the first and the
European standard came along later. Then came the D2, the D3 and
finally the D4 when integrated codecs finally came to be and it was
practical to get rid of the common codec and do it channel by channel.
I have tried to look at the spectrum of AM broadcast radio when they are
taking phone calls and you can defiantly see the low frequency roll off
starting around 300Hz for the guy on the phone and when the guy at the
station talks, there are strong components below 100Hz.
On 1/6/2014 8:36 PM, Magnus Danielson wrote:
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 <[email protected]>
To: 'Discussion of precise time and frequency measurement'
<[email protected]>
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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