Date: Fri, 22 Dec 2006 09:41:11 -0500
To: WSFA members <WSFAlist at KeithLynch.net>,
   WSFA members <WSFAlist at KeithLynch.net>
From: "Mike B." <omni at omniphile.com>
Subject: [WSFA] Re: Modems, and O.J. Simpson
Reply-To: WSFA members <WSFAlist at KeithLynch.net>

At 12/21/2006 11:14 PM, Keith F. Lynch wrote:

>MP3 and competing standards are designed to compress sounds by
>discarding subtleties that human ears don't notice.  Given that there
>are no proven cases of anyone being able to read arbitrary 300 bps
>modem signals by ear, I suspect those signals may exceed human
>physiological limits, in which case MP3 and competing standards may
>not capture enough information to uniquely reconstruct the signal.

The old 300 baud modems used 4 frequencies, two for originator (mark
and space) and two for receiver, all of which were well within human
hearing range (1070 to 2225 hz).  They were just simple tones, and
didn't have to be all that accurate so long as they were close enough
to be differentiated from each other by the receiving equipment (I'd
guess that a 10% error wouldn't be a problem given the tolerances for
most electrical components used for the filters).  While they changed
too fast for a human to read the data by ear, their characteristics
shouldn't be destroyed by MP3 compression if the bit rate is high
enough to avoid Nyquist issues
(http://en.wikipedia.org/wiki/Nyquist-Shannon_sampling_theorem).

You can find more about modems here: http://en.wikipedia.org/wiki/Modem

I wouldn't try MP3 compression on anything faster than 1200 baud.  To
go faster than that over voice lines they started doing things like
breaking the available spectrum (about 6 khz) up into smaller bits
and running multiple parallel channels at different frequencies, and
avoiding those that didn't have enough response over a given
connection (some of the early 9600 baud modems), phase encoding of
data, and basic data compression techniques.  The later wouldn't be
affected by MP3 compression, but I'm pretty sure the others would be.

>The MP3 I had to work with today was much clearer.  At one point I
>could clearly make out the lawyer whispering to his client, "watch me
>make shit out of shinola."  I obviously wasn't meant to hear that.

A boot-licker, eh?

-- Mike B.