Date: Sat, 23 Dec 2006 11:25:03 -0500
To: WSFA members <WSFAlist at KeithLynch.net>, WSFAlist at KeithLynch.net
From: "Mike B." <omni at omniphile.com>
Subject: [WSFA] Re: Modems, and barbed wire fences
Reply-To: WSFA members <WSFAlist at KeithLynch.net>

At 12/23/2006 03:24 AM, ronkean at juno.com wrote:

>On Fri, 22 Dec 2006 09:41:11 -0500 "Mike B." <omni at omniphile.com> writes:
> > At 12/21/2006 11:14 PM, Keith F. Lynch wrote:

>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).
>
>The center frequencies (threshold frequencies) for each channel were 1170
>and 2125 respectively, so frequencies below the threshold would be read
>as 'mark', and above, as 'space'.  Obviously, if a transmitter which was
>intended to send 2025 and 2225 tones, instead sent frequencies 10%
>higher, it would send tones which would always be read as 'space', so a
>10% transmit frequency error would be unacceptable

Sorry, I was unclear.  "10% error" did not refer to a 10% of the
required frequency error, but a 10% of the frequency difference
error.  I.e. not 200 hz, but 20hz.  There's a 200 hz difference
between mark and space, and 10% of that is only 20hz.

Typical resistors are only accurate to their labels +/- about 10-20%,
and I believe the same is true of typical capacitors.  With proper
circuit design you can minimize the problems from this (adjustments,
arranging for likely errors to cancel, etc.), but you will always
have some error to deal with...especially in cheaper consumer
electronics where component cost and count has to be minimized to
meet the price point.  Even the more expensive components are still
5% off from what they are labeled, and you have to pay serious money
to get to 1%...and even then it's going to be rated at a given
temperature most likely, and be different at other temps.  Any
standard which doesn't take this reality into consideration isn't
going to be very
widely adopted.

>The analog filters in 300 bps full-duplex modems were actually quite
>elaborate, especially the receive filters.  In the early to mid-1970s,
>the filters used op-amps, and resistors and capacitors of 1% tolerance,
>and they were epoxy-potted to prevent subsequent drift of component
>values due to humidity.

Was this the commercial quality ones, or did this apply to cheaper
consumer models like my Cat, which was about $160 if I remember right
(circa 1981)?

-- Mike B.
--
         ACHTUNG!!   ALLES LOOKENPEEPERS!!

Das computermachine ist nicht fur gefingenpoken und mittengrabben.
Ist easy schnappen der springenwerk, blowenfusen und poppencorken
mit spitzensparken. Ist nicht fur gewerken bei das dumpkopfen.
Das rubbernecken sichtseeren keepen hans in das pockets muss;
relaxen und watch das blinkenlichten.