[sdiy] Information Content of Signals

[Magnus Danielson]

From: Tim Ressel <madhun2001 at yahoo.com>
Subject: Re: [sdiy] Information Content of Signals
Date: Sat, 17 May 2003 04:26:56 -0700 (PDT)

> I have also wondered about this, but from a different
> direction. I want to know how they can get 53kbps on a
> phone line with 3khz bandwidth. Oh I know they use
> this funky phase-constellation thingie. 
> 
> I have heard that they stopped at 53kbps not because
> of the limitation of phiscics, but because the FCC set
> a limit there. 

This is not quite true. The 56k modems is very funky, they try to make the
A/Ds use the bits efficiently, but you can only get it as far as you like.
However, due to peculiarities in the DS1 system you may not even know how 7 of
the 8 bits behaves, since you may end up getting the 8th bit robbed, and you
need the last bit for balance, and then you must rob the 7th bit for balance
and then only 6 bits is free choice, and this happends with 1/24th of a chance
and then on every 6th sample. Don't come and say transparent communications!!!

The 56k modems had more design issues than people originally thought. Brrr!

Cheers,
Magnus