11 May
2001
11 May
'01
2:11 p.m.
Not only that going from TTL to analog you will over dirve in the
input amps and likely get ringing or saturation effects. All look bad.
Allison
-----Original Message-----
From: Richard Erlacher <edick(a)idcomm.com>
To: classiccmp(a)classiccmp.org <classiccmp(a)classiccmp.org>
Date: Friday, May 11, 2001 9:26 PM
Subject: Re: Converting TTL monitor to Analog
Well, I'm not sure you're entirely wrong here,
but the ghosting is
probably more
a result of poor signal termination than anything else,
and the
magnitude of the
reflection from a full-swing digital signal is much
more likely to show
up than
what you'd get form a 1-volt p-p analog signal.
Moreover, since you've
got much
smaller transitions on the analog video signal, and
since you've got an
automatic gain control in the monitor to offset the losses, the line
losses
aren't as much a factor as they'd be without
it. Digital signaling was
OK back
in the text-only and monochrome era, but once things
went to
full-spectrum
color, it was necessary to present them in analog. The
EGA had 16 (?)
colors
(maybe that was 16 shades of each of three colors),
which already
required some
digital=>analog processing at some stage in the
process, and, from what
I've
observed, it had to be on the adapter board, since the
signal from there
to the
monitor was already a small-swing analog signal.
Gosting is another problem that's never plagued me in this arena.
Normally it
can be dealt with by proper termination of the video
signal. Some
monitors rely
on a 150-ohm resistor in the line and only provide that
same amount
themselves,
while other have a 75-ohm to ground/return. To
accompany this some
adapters
have a divider rather than simply a series resistor to
the output. It
depends
on what's in the combination, how well it will
work, but I've not been
sufficiently bothered by any mismatch to warrant hacking either circuit.
Dick
----- Original Message -----
From: "Iggy Drougge" <optimus(a)canit.se>
To: "Richard Erlacher" <classiccmp(a)classiccmp.org>
Sent: Thursday, May 10, 2001 5:26 PM
Subject: Re: Converting TTL monitor to Analog
> Richard Erlacher skrev:
>
> >What sort of digital interface would such devices use? They'd have
to
have
> >lots of wires, e.g. 26 or so pairs, if the
thing is using 24-bit
color.
> >Perish the thought someone would use more
color depth than that!
It's hard
> >to imagine they'd do that. The number of
wires that have to
transport
> >high-frequency signal is the reason the analog
is still so difficult
to
> >surpass.
>
> >After all, a 26-pair shielded cable with an appropriate connector,
made in
> >the US, would immediately become the most
costly component in a
computer
> >system. I doubt a computer maker would go for
that, as the 20"
monitor
> >(@~$300-$400) is presently the most costly
component. That would
essentially
> >make the 20" display and cable more than
2/3 the cost of the
computer.
>
> But isn't the digital signal less dependent on absolute signal
integrity?
> After all, if there can only be two levels, making
out the difference
can't be
> all that difficult. This also seems to hold true
WRT my successful use
of
> long, thin, unshielded cables for EGA and CGA
monitors without any
ghosting.
--
En ligne avec Thor 2.6a.
I dunno, I dream in Perl sometimes...
--Larry Wall in <8538(a)jpl-devvax.JPL.NASA.GOV>