Converting TTL monitor to Analog

From: ajp166 <ajp166_at_bellatlantic.net>
Date: Fri May 11 21:11:25 2001

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_at_idcomm.com>
To: classiccmp_at_classiccmp.org <classiccmp_at_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_at_canit.se>
>To: "Richard Erlacher" <classiccmp_at_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
>> >(_at_~$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_at_jpl-devvax.JPL.NASA.GOV>
>>
>>
>
Received on Fri May 11 2001 - 21:11:25 BST