hilpert at cs.ubc.ca
Fri Oct 14 00:24:12 CDT 2005
Fred Cisin wrote:
> On Thu, 13 Oct 2005, Brent Hilpert wrote:
> > (.. got two of them, but they don't work simply connected back-to-back, have to
> > figure out what was in the 'cloud' in between, someday.)
> They might also be dependent on certain "signals", such as getting a dial
> tone before willing to originate a call.
Thanks for the suggestion but.. it appears the units were intended for use over
dedicated lines, not POTS/dialup. There is some DC signalling involved for Start/End
of Transmission, ACK and c. but thats easy enough to work around. Looking for dial
tone is way more sophisticated than what these units do.
It was a while ago that I was looking at it (rev. eng. to produce schematic and
some initial experimentation), but as I recall when they were simply connected
back-to-back, the received fax ended up being a poor-quality *negative* of the
original. I think I concluded that the PAM signal needed to be
amplitude-inverted, e.g. if the AC amplitude range of the PAM signal is 0 to 10V,
then 3VAC transmitted needs to be converted to 10-3=7VAC received (not simply
I was a little perplexed as it seemed like a somewhat sophisticated/complex
transformation to accomplish for the time and application.
I presumed there must have been some sort of
line-balancing/matching/amplification/regeneration going on within the
dedicated lines / special network that they operated over which accomplished
I was thinking of doing an amplitude detection on the PAM signal, feeding it
into the inverting input of an op amp, with the positive input fed from a
manually adjusted level or perhaps a maximum-level-hold sensed from the PAM.
Or else I'm way off and am missing something terribly obvious.
Vintage Computer Festival wrote:
> When you figure it out, let me know and we can send old school faxes to
> each other ;)
That would be fun, but the above makes connecting them over a distance
today awkward. If you actually have *two* of these units (WU Desk Fax /
Telefax 6500 / chassis 6499-A, footprint about one square foot, curved
sheet-metal cover in hammertone green, red white & green pushbuttons on left,
lamp and 4th pushbutton on right) I'll let you know what's needed to
interconnect them if I ever figure something out.
(I suppose the above is tending OT, but I like to think some limited
mention/discussion of the precursor technologies to the applications
accomplished digitally/with computers today is acceptable and of interest here.)
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