plz see embedded remarks below.
Dick
-----Original Message-----
From: Tony Duell <ard_at_p850ug1.demon.co.uk>
To: Discussion re-collecting of classic computers
<classiccmp_at_u.washington.edu>
Date: Wednesday, July 21, 1999 2:00 PM
Subject: Re: Sun Monitor (UK) (2)
>> >No normal VGA card (I am not talking about the special ones designed to
>> >work with sync-on-green monitors) has the hardware for sync-on-green. So
>> >programming the card, however you do it, won't produce a sync-on-green
>> >output. That is what your resistor mod is for - to stick sync pulses (I
>> >wouldn't have thought they met the specs either...) on the green signal.
>> >
>> I seem to recall that several of the Brooktree DAC's, almost excusively
<snip>
>> the DAC pins, had the ability to impose negative sync on the green video
as
>> well.
>
>Since monitor video inputs are almost always AC coupled (at least on real
>monitors), you don't need to be able to generate a -ve output.
>
>If you want to use the DAC for this, then either you need to use up part
>of the range of the DAC for the sync pulse (which will reduce the number
>of colours you can get), and find a way of loading the right values into
>the DAC during the sync pulse time, or you need to fiddle the bias
>votlages at that time. Either way it's a pain.
>
I've got a couple of Philips workstation monitors and one from DEC which
aren't AC-coupled, or at least don't tolerate the resistor bias trick I
mentioned before.
>
>But adding an external sync mixer is a lot easier. Using separate syncs
>is easier still.
>
The advantage to using the resistor from composite blanking is that the
frequencies, albeit not the pulse widths, are correct.
>
>> Multiple encounters with this particular task have taught me that this
>> entails a >1K-hour involvement for a specialist who has experience and
all
>> the precise specifications at his disposal, including the unpublished
ones.
>> I have no idea how long it would take someone who knows little about
>> graphics and less about the undocumented features of the display
controller
>> LSI.
>
>Never underestimate what hobbyists can and will do. It may suprise you,
>but often a good hobbyist can easily outperform many so-called
professionals.
>
That's true, but in this case, we're looking for a solution, not an amusing
problem.
>
>I am not suggesting that reprogramming an SVGA card is a good idea if you
>know _nothing_ about video - although you will surely know a lot about
>video by the time you've done it. Just that it's certainly possible.
>
>> >At VHF you may well have problems if you try to make it on stripboard.
>> >Dead-bugging would work. A proper double-sided PCB with a ground plane
>> >would be even better. Decoupling is going to be very important.
>> >
>> >Compare that to inverting a 64kHz signal (less than 1 thousandth of the
>> >frequency) with a 74LS04. You can stick that on stripboard, tag a 0.1uF
>> >capacitor across the power lines for decoupling and expect it to work
>> >first time.
>> >
>> With orderly and precise assembly techniques, the little circuit I
recommend
>> and use will work every time. The transistors are spec'd to 500 MHz and
the
>
>I don't dispute that at all. But RF (and this most certainly is RF)
>circuit construction _is_ an art that not everybody has experience of.
>There is a a lot more to RF construction than components that happen to
>have suitable specs.
>
>There is a well known myth in electronics. It goes like this : Resistors
>have resistance, capacitors have capacitance, inductors have inductance.
>It reality, all components have all 3 properties. So do pieces of wire.
>And while I'll believe in wires without resistance, to have them without
>inductance or capacitance would involve major changes to the laws of
>physics.
>
>So, in fact, you might find that your nice little electrolytic capacitor
>has a very high impedance at 100MHz due to its self-inductance. Not what
>a beginner would expect at all.
>
>Now, all this is 'old hat' to anyone who's done a lot of RF work (high
>speed digital electronics is most certainly RF, BTW). But I've seen
>plenty of so-called designers who have made a mess of it.
>
>I'd have no worries in building this (I've worked considerably faster,
>many, many times).
>
>> >But if the card already supports something near the right rates, it's
>> >worth giving it a go. Maybe the card supports 1024 lines at 60Hz
>> >vertical. Your monitor uses 52Hz vertical (I've seen monitors that have
>> >that, for some odd reason), also at 1024 lines displayed. You probably
>> >could reprogram the card to do that.
>> >
>> I keep forgetting that a major part of the world doesn't use 60 Hz.
!!! )
>
>It's not that at all (FWIW a lot of machines over here do use 60Hz
>vertical rates, or whatever is used in the rest of the world). The reason
>some workstation monitors used strange scan rates was precisely the
>reason you mentioned earlier - the DACs weren't fast enough, so they had
>to slow everything down to get the number of pixels/line that they wanted.
>
>> This means that you have to do your own arithmetic. I've designed and
>
>Which is not hard, provided you remember that #displayed line != #total
>lines and #horizontal displayed pixels != #equivalent pixels/line. There
>are blnking intervals to consider...
>
>> >At these sorts of frequencies it's worth taking care with the layout,
>> >decoupling, etc. After all, ghosting on green (only) is going to look
>> >terrible. I am not saying it can't work. It can. I would also claim that
>> >you could have problems with it.
>> >
>> You'll see it's dirt simple to build one of these that works fine because
>> the switching speed is relatively leisurely, though the Brooktree folks
>> spec'd a 1/2 GHz transistor array. The one I used is spec'd faster than
the
>> CA3227. Like I wrote above, it's likely a 100 KHz version would work
since
>> all it switches is the sync.
>
>Sure.. But none-the-less you are working with a 100MHz video signal, even
>if only to resistively mix it with the sync pulses. And that's where
>problems _could_ start.
>
The combiner is only active well into the blanking cycle. Before the
blanking cycle, and I'm referring to my 5-transistor/2-diode green+sync
combiner, the circuit draws its constant current from ground through one
side of the dif-amp. When sync goes active, it switches to the other side,
which draws current from green video and sinks it into a mirror-controlled
constant current sink. That's why the negative voltage is needed. That
way, the circuit is essentially passive except during blanking. The only
connection to GREEN is through the collector of one of the transistors in
the dif-amp, which is in cutoff when sync is inactive. I admit it may have
imperfections, but it's served many of us well for many years. What's more,
it's easy to build, since the transistor array is a package which serves as
a platform for the diodes and resistors.
>
>> >You seem to think of everything in terms of money. You are totally
>> >forgetting that (a) you'll learn a lot from doing this (or are you in
>> >favour of knowing nothing and letting everybody else do the work). (b)
>> >that some people enjoy doing this sort of thing. (c) that the 20"
monitor
>> >from the PC shop is most likely a cheap/poor design which gives a
>> >marginal picture even when new. That Sony looks like a good design from
>> >the schematics.
>> >
>> . . . and you're assuming that, perhaps like you when you started this
>> stuff, one knows nothing about this stuff. The third time you do this
job
>
>I am not sure I follow you. Of course I knew nothing when I started, but
>I learnt it pretty quickly. And I've used knowledge gained from doing
>things like this to solve problems elsewhere.
>
That wasn't my point at all. I know from your previous writings that you
are a devoted hobbyist and you enjoy very much tinkering with things that
perhaps don't work quite as you'd like in order to learn how to make them do
what you do like. The guy who started this thread, however, at least in my
perception, was concerned about how to get some utility from a very heavy
and difficult to ship monitor he already owns and is unlikely to move
without considerable expense/effort on his part. What I tried to do is find
him a quick and easy, perhaps quick-and-dirty, solution to his particular
difficulty which probably involved a tradeoff between utility, economic
conservation, and space. My solution may not be THE solution, but it's
worked for me and others, so I thought it might work for him as well.
>
>The sure way to learn something is to do it. You can read all the books
>you like, and you _won't_ get the full story.
>
>
>> I remember the monitors we bought for $30K each back in the mid-'80's,
which
>> are comparable in the most superficial way to this GDM1950. The 20"
>> monitors down at the discount have everything superior in almost every
way
>> to the SONY except for the tube. The ones with a SONY tube cost $500
>
>Have you sat down with the schematics and compared them? Looked at build
>quality? Looked at the specs they're supposed to meet for things like
>corner convergence? You'd be amazed.
>
>I've got a PC monitor on my bench at the moment. The fault is an
>open-circuit horizontal driver transformer (not the flyback). Now this
>transformer handles almost no power. There is no other fault that would
>cause it to burn out. No, the reason it failed is that the wire was
>damaged when it was wound (I have unwound it, you see, as I intend to
>rewind it). That's the sort of thing you get on cheap monitors.
>
>> >You seem to be of the opinion that it's not worth learning how to do
>> >something if somebody else (doesn't matter who or where) can do it for
>> >you. This is a strange attitude for a hobby. It also probably explains
>> >the state the computer industry has got into.
>> >
>> Well, if it's an industry, it's not a hobby to everyone, and the state
it's
>> gotten into is PROFITABLE, which means it will be around a while longer.
>
>Profitable != it will be around, alas. Not in this case. What has
>happened is that companies are making a lot of money selling the same old
>stuff. Just finding ways to cheapen it. There is next to no inovation
>going on any more, at least not in the UK.
>
>Oh, there'll still be companies selling computers/software. Whether
>anybody will still be designing them is another matter...
>
>> >Sometime in the future I intend to make a mechanical clock. The
necessary
>> >tools are certainly not cheap. And it's going to cost me around \pounds
>> >100 for the metal, etc to do it. The result will be less accurate than a
>> >\pounds 5.00 quartz clock. So what!
>> >
>> Well, some of us already know enough about how a mechanical clock works
and
>
>Have you made one?
>
>Sure it's trivial to 'count a train' (work out the number of teeth on the
>gears in a clock). Any schoolkid could do that. What is a lot less
>trivial is to make an escapement that runs, carries on running, and keeps
>good time.
>
>There are other things that seem wrong until you've done them, as well.
>Most people think that the holes in clock plates are cylindrical.
>They're not. They're conical - the pivots run essentially on an edge.
>There is a lot of 'slop' when you put a pivot into just one of the
>plates. That's how it should be. A clock with cylindrical holes will
>almost certainly stop at once.
>
>> even how to build one. Of course that's not everybody's goal, but . . .
>
>I don't beleive anyone can claim they can do something until they've
>actually done it. In this case, by taking a piece of sheet brass and a
>length of steel rod and actually making a clock.
>
>-tony
>
Received on Wed Jul 21 1999 - 16:00:25 BST