Hi this is a email reply I got about connecting a msx to vga screen. IT IS
NOT ENCOURAGING. It looks like connecting a msx (any rgb) to vga is gona be
expensive and probably not a diy home electronics project!
>
> JP GROBLER
> [EMAIL PROTECTED]
> ----- Original Message -----
> From: "Koen van Hartingsveldt" <[EMAIL PROTECTED]>
> To: <[EMAIL PROTECTED]>
> Sent: Thursday, August 03, 2000 5:19 PM
> Subject: LM1881N
>
>
> > Hi
> >
> > I received an email from you via Peter Burkhard (Switzerland). He asked
me
> > if I could help you.
> > First I'll repeat the tekst of your email to Peter B:
> >
> > > This subject again! I'll get my ln1881 this week and start playing
> (hopefully!)
> > >
> > > This circuit is also of interest to people who follow this topic.
> > > ftp.ibilce.unesp.br/pub/msx/txt/hdw2sync.arj
> > >
> > > I wonder does anybody know who maintains this site and if we could get
> more info on this
> > > circuit. Maybe the more electronic minded guys with time on there
hands
> could help - a d.i.y.
> > > rgb_to_vga_cable would be very welcome.
> >
> > Now, having read your email, and looked at the .GIF files in the
> > hdw2sync.arj file on that ftp site, I understood that you're trying to
> > connect a VGA monitor (probably a PC monitor) to an RGB source (an MSX
> > computer?). Because VGA monitors use separate H- and V-sync signals,
> you're
> > trying to make a sync separator with the LM1881N from National
> > Semiconductors.
> >
> > As you may already know, the LM1881N can be used in two ways:
> >
> > - input a CVBS signal (0.5-2 Vp-p composite video signal). The LM1881N
> > first extracts the C-sync portion of the CVBS signal, and outputs it as
a
> > TTL level C-sync signal. In addition, this TTL C-sync signal is also
used
> > internally to generate the TTL level V-sync signal, which is also
> > outputted.
> >
> > - input a C-sync signal (0.5-4V). The LM1881N will output the same
C-sync
> > signal, but now at TTL levels. And again, the C-sync signal is used to
> > extract the V-sync signal.
> >
> > Normally, both CVBS signals and C-sync signals, generated by video
> > equipment should be approximately 1Vp-p.
> >
> >
> > Still, you don't have extracted a H-sync signal with the LM1881N. I
expect
> > monitors not to have problems with a C-sync signal on their H-sync
input,
> > but correct operation is not guaranteed, generally. Fortunately, it's
> > rather easy to convert the C-sync signal into a H-sync signal, by using
a
> > so-called monostable multivibrator (MMV). Another option is to use the
> > EL4583 from Elantec (France), which has the same functionality as the
> > LM1881, but also provides a H-sync signal and a Video Detect output.
> >
> > Note that VGA monitors may request positive polarized H- and V-sync
> > signals, instead of the negative signals, generated by the LM1881. With
> two
> > simple TTL inverters, this can be solved.
> >
> > So far so good. But now the real problem, which I hope you are aware of.
> >
> > Most VGA monitors cannot handle the video timing as generated by MSX
> > computers, televisions, VCRs, camcorders, etc. The timing of standard
> video
> > signals, as used by all this equipment is as follows:
> >
> > Vertical frequency (frame/field frequency) = 50Hz or 60Hz
> > Horizontal frequency (line frequency) = 15.625kHz or 15.734kHz
> >
> > The vertical frequency is no problem for most PC monitors. However, the
> > line frequency of about 15.7kHz cannot be handled by most VGA monoitors.
> > The horizontal frequency range of these monitors starts at about 30kHz
(a
> > standard VGA signal has a line frequency of 31.5kHz), and it cannot work
> > with line frequencies below that value. 15.7kHz is far too low for these
> > monitors.
> >
> > Not that they will be destroyed, but you won't get a synchronized image,
> > but only crap (you will hardly see an image at all).
> >
> > There exist special VGA monitors that cover the complete range from
15kHz
> > to say 80kHz or even more. NEC has produced these kind of special
> monitors,
> > but they were really expensive. If you have such a monitor, then you can
> > indeed use the LM1881 (or else the EL4583) to connect your MSX (or any
> > other TV standard RGB signal) to that monitor. If you don't have such a
> > monitor, you'll need a lot more electronics to display the MSX image
> > correctly on the PC monitor.
> >
> > What you need is a so-called scan-doubler. This is a circuit that
converts
> > the 15.7kHz RGB+Sync signals into 31.5kHz RGB+Sync signals. And that's
not
> > an easy task. Because we're dealing here with retiming (scanrate
> > conversion), the RGB signals coming from the MSX computer, have to be
> > digitized by a triple analog-to-digital converter (tripe ADC), then
stored
> > in momory, read from memory at the double clock rate, and finally
> converted
> > back into analog RGB signals again, with a triple digital-to-analog
> > converter (triple DAC).
> >
> > The sampling rate of the ADC and DAC must be high enough, to get a
> > sufficiently high horizontal resolution. The ADC should have a word size
> of
> > 6 to 8 bits per ADC, and for the DAC holds the same, both to get enough
> > colours. The momory has to be large enough to store at least two video
> > lines (which is definately less than 16kB, so the memory size itself is
> not
> > a problem). Usually, special line memories are used, which don't have to
> be
> > addressed all the time, but which just have an input clock and an output
> > clock terminal.
> >
> > The triple ADCs and triple DACs, nor the line memories are cheap. The
> price
> > of a triple ADC may rise up to Euro 30.- if you buy 50 of them, for
> > example. Of course, three seperate ADCs and DAC may be used as well.
> >
> >
> > There are companies that sell these scan-doublers. We (Stichting
Sunrise)
> > have one such a scan-doubler bought from a German company for about
> DM200.-
> > However, that device doesn't have an RGB input, but only a CVBS and
S-VHS
> > input (which reduces the image quality significantly). Moreover, its
> > horizontal resolution is too low, and there's quite some interference
> > visible on the screen of the VGA monitor.
> >
> > We also have a scan-doubler for the amiga, but also that thing had a
> rather
> > poor image quality (low resolution, and disturbance), although it has an
> > RGB input.
> >
> > As the quality of these two scan-doublers is too bad, we're busy with
> > making a scan-doubler ourselves. But it will not only double the line
> > frequency. It will also convert interlaced images into non-interlaced
> > (progressive scan) images. This means that pictures and text can be used
> in
> > the interlaced modes (double vertical resolution), without line flicker
> > effects. Moreover, it will be able to handle resolutions up to 2048 dots
> > per line. However, this kit will be rather expensive (Euro 250.- would
not
> > be surprising).
> >
> > The following two sites can be visited, to read about commercially
> availale
> > scan-doublers:
> >
> >
>
http://www.videolabs.com/html/product_lines/accessories/scan_converter.html
> > http://www.projector-central.com/dvdo_specifications.htm
> >
> >
> > Resuming: just connecting a normal VGA monitor to a TV standard RGB
video
> > source (like the MSX computer) by inserting a C-sync -> H/V-sync
> converter,
> > won't work. A real multi-sync monitor that can handle 15.7kHz H-sync
> > signals, may be a solution, though expensive probably. A scan-doubler is
> > another solution, but may result in poor quality (depending on the type
of
> > scan-doubler), and may still be too expensive. A third solution is to
buy
> a
> > TV-card for your PC, and connect the MSX (or whatever) to the CVBS or
> S-VHS
> > input of the TV-card. Unfortunately, these cards do not always support
RGB
> > signals.
> >
> >
> > If you do have a VGA monitor that can handle 15.7kHz line frequencies,
and
> > if you need help with the LM1881N and/or V-, C- or H-sync extraction
> > circuitry, please ask me. I've made these kind of circuits myself
already.
> > The schematics are available.
> >
> >
> > Greetings
> >
> > Koen van Hartingsveldt
> >
> > Stichting Sunrise
> > P.O. Box 61054
> > 2506 AB Den Haag
> > The Netherlands
> >
>
>
****
Problems? contact [EMAIL PROTECTED] See also http://www.faq.msxnet.org/
****
