VGA to workstation monitor FAQ

Copyright 1998,1999,2000 Tomi Engdahl Updated last 2005

Connecting other sources than VGA card to PC monitor

Foreword: VGA to workstation monitor FAQ is concentrated connecting a PC to a monitor from a workstation. Quite many people have wendered if other way is possible, so I added also this extra chapter to this FAQ on this topic.

Can I use Unix workstation with my SVGA monitor ?

It is possible to use PC monitor with other coputers than PC, if the SVGA monitor supports the screen refresh rates and resolutions that the workstation outputs. The rest is just interfacing the monitor signals:

First connect the analogue R, G and B output from the workstation to VGA monitor R, G and B pins (connect also the grounds). You need to get the pinouts of VGA monitor and workstation monitor output to figure out the wiring you need.

When you have got the RGB signal wiring figured out, you have to figure out the syncronizing signals:

  • If the UNIX computer gives out separate TTL-level HSYNC and VSYNC signals, then feed them to the SVGA monitor directly.
  • If the computer gives out only composite sync output, then you can try to feed it to the HSYNC pin of the monitor (some brands accept also composite sync signals in HSYNC input). If the monitor can't use composite sync signal, then you must use some custom electronics to separate HSYNC and VSYNC signals from it.
  • If the UNIX box gives out sync on green signals, then you have to make some electronics to separate the syncs from green line to HSYNC and VSYNC singnals (SVGA monitors don't normally support sync on green, byt thre might be exceptions to this rule). Usually the same IC works as in CSYNC separation, but just connected to green line.

    In some cases you might need to add electronics to remove the sync information from green picture component because some monitors don't work correctly with it. I have no tested circuit for this to give. The sync stripper circuit shown at HFA1115 IC datasheet http://www.intersil.com/data/an/an9752.pdf looks promising as it is. But because according to feedback I received through e-mail Intersil has discontinued the HFA1115, so this might not be a working idea anymore.

Can I use VGA monitor with my Commodore Amiga

VGA monitors can be connected to the A4000 and some other Commodore Amiga models. A special cable or adapter is the best way to hook Amiga to VGA monitor.

All this cable really does is buffer the horizontal and vertical sync signals by double-inverting them through a TTL gate. Commodore's DB23-to-HDD15 adapter (supplied with most A4000s) used a 74HCT08 for this, but you can use a 74LS04 or other low-power TTL-level inverters (or other gates wired as inverters or buffers, of course).

A4000 VGA Video Cable from Amiga 4000 Hardware Guide:

  A4000 DB23                                        VGA HDD15

  Red   (Pin 3) ----------------------------------- Red (Pin 1)

  Green (Pin 4) ----------------------------------- Green (Pin 2)

  Blue  (Pin 5) ----------------------------------- Blue (Pin 3)

  Grounds (Pin 16-20) ----------------------------- Grounds (Pin 5-8,10,11)
                                                    and ground for inverter.
                                                    Also use a .01 uF ceramic
                                                    decoupling cap between
                                                    +5V and ground at the
                                                    chip power supply pins.

                               |\     |\
  Horizontal Sync (Pin 11) ----| >o---| >o--------- Horizontal Sync (Pin 13)
                               |/     |/

                               |\     |\
  Vertical Sync (Pin 12) ------| >o---| >o--------- Vertical Sync (Pin 14)
                               |/     |/

  +5V (Pin 23) ------------------------------------ Power supply for
                                                    inverter or buffer chip.

All this cable really does is that it change the connector model and buffers the horizontal and vertical sync signals by double-inverting them through a TTL gate. Commodore's DB23-to-HDD15 adapter (supplied with most A4000s) used a 74HCT08 for this, but you can use a 74LS04 or other low-power TTL-level inverters (or other gates wired as inverters or buffers, of course).

Remember that most VGA monitors won't sync at 15.75 kHz, so you'll have to set the A4000 to use the Double NTSC (or Double PAL) or Multiscan video modes. Even using these modes, the video output may not work with some VGA monitors, since some of the modes use horizontal sync frequencies as low as 23 kHz, and normal VGA starts at 31.5 kHz. Special VGAOnly monitor driver can help the situation.

Can I connect my SVGA monitor to a VCR to view the TV picture on that ?

You can't directly interface SVGA monitor to the VCR in any easy way. To be able to display TV picture directly, you need a SVGA monitor capable of running at 15kHz horizontal frequency. Most SVGA monitors cant do this, their minimum is 31 kHz. Other problem is that VCR uses composite video signal and SVGA monitor wants RGB signals, so those must be converted also.

TV industry on the whole, use a 15-16 Khz horizontal sync, which means they display a line 15000-16000 times a second. VGA monitors use 32 kHz or more, which means they display 32000 or more lines per second. This means that your VGA monitor will try to draw two lines with the picture data from one game line. It simply won't work. Often the monitor will manage to sync horizontally, but the lines will look scrambled and the picture will roll vertically. Sometimes monitor just does not think it can sync properly, so it shows no picture on screen.

So, what about these Multisync monitors? Older mutlisync monitors like the NEC 3D could do both analogue VGA and digital EGA, and as a result they were able to sync down to 15kHz, regardless of whether the picture input was analogue or digital. Since EGA is now dead and buried, modern monitors don't support 15kHz line rate. Modern multisync monitors are capable of syncing *up* to much higher line rates, and down only to about 31.5 kHz. So, no matter how snazzy a monitor you have, if it's new the chances are very slim indeed that it will not work with the signals used in TV broadcast and which comes from your VCR.

There are some solutions to show TV picture on moden VGA monitor, but they are not cheap, because they must do lots of processin to vidoe signal to do the conversion. Commercially available options are:

  • 1. Get a Videocard with TV-in or a TV-card for your PC. This card will take the vidoe signal and put it to the PC graphics card memory. The graphics card the outputs the signl out at whatever rate it is programmed to do that. More information on this kind of products can be found at http://ruel.net/pc/tvtuner.links.htm.
  • 2. Get a Video to VGA converter, a device that stores you video signal and outputs it with the double frequency to match the monitors requirements. There are differen multi-purpose scan converters and scanline doublers for professional video system which can do this conversion.

I haven't seen any electronics project so build the conversion from TV signal to VGA card, because of the complexity of the whole process. When connectying TV signal to computer monitor, you have to do the complicated signal processing (scaling, deinterlacing) and scan rate change (need buffer memory for at least one frame). This kind of device is quite much out of reach of a typical homebuilder, because of it's complexity (ncludes vidoe electronics, fast A/D-converter, fast memory, signal processing and fast D/A converters). If you try to build this kind of device from diskcrete components, it wikk become very complicated, expensive and very hard to get to work well. Commercial onverters typically utilize special ICs for this conversion. Those special ICs are not typically available for homebuilders (you must order large quatities of them), they are typically hard to use (usally lots of programmable features) and some companies even use some custon ICs (ASIC).

Here are some links to get further information related to this topic:

How can I build a circuit which converts composite sync signal to HSYNC and VSYNC ?

Sync separation is a process quite often needed in video technology, but it is a little bit hard to find a chip which especially made for that. What you can find easily is a chip which takes a full composite video signal and outputs separate sync signals. Those chips can be directly used also with composite sync signal because it is just like a composite video signal without picture and has a little bith higher signal level. You can find a good list of such chips and links to their datasheets (which include example circuits) at http://www.epanorma.net/videochips.html.

I have used the following circuit (based on example from datasheet) for separating composite sync signal for monitors which need different kind of sync than what your source gives. The circuit is based on very popular LM1881 sync separation IC.

Sync separator circuit

Features of the circuit:

  • Circuit accepts composite sync signal at any level between 0.3Vpp and 5Vpp
  • Circuit can accept sync from sync on green source if connected to green picture component line which includes sync signal
  • Circuit can accept 1Vpp composte video signal as sync source
  • Circuit works at least at 15kHz to 60kHz HSYNC range (tested at that range, might support higher frequencies)
  • Circuit works on 50 Hz to 75 Hz HSYNC range (tested on that range)
  • Circuit outputs TTL level negative sync signals
  • Circuit consumes less than 100 mA current at +5V
  • Circuit can also be used to to separate sync signal from composite video or sync-on-green signals (connect CSYNC input to composite video or green signal with sync in it)
The use of the circuit is quite easy. You just connect the CSYNC input of the circuit to the sync signal source and the outputs to the VGA monitor. This scheme worked well when I tested it with KFC CB1516SL superVGA monitor and a 75 Hz 1024x768 source which had composite sync output (that signal source was my VGA card signal after conwersion to RGBs format). It might work well with your monitor or not.

The circuit have one serious drawback: The HSYNC output of the circuit is not really a HSYNC signal. To be exact the signal which the circuit outputs is TTL level composite sync signal. Many monitors accept that signal nicely as a HSYNC signal, but not all. For example Nokia 417X refused to show any picture when I connected HSYNC and VSYNC to it, but displayed nice picture when I disconnected VSYNC (that monitor definately can sync to composite sync signal and did it nicely when it received only that). SO if you have roblemss in getting your monitor to sync with both HSYNC and VSYNC connected then try your monitor with only HSYNC connected.

By adding a little bit more electronics it is possible to make a real HSYNC iput by wisely combining composite sync and VSYNC outputs from LM1881 using the following circuit which consits of two gates:

           |--------------|----------------------  Composite (if you need
it)
           |              |    |-----\
           | |-----\      -----|      \
Composite ---|      \          | AND   |---------  Hsync
             | NAND  |o--------|      /
Vsync -------|      /          |-----/
           | |-----/
           |
           |-------------------------------------  Vsync
This circuit provided by John A. Maier eliminates any guess work on the Hsync line and guarantee it will work with any VGA monitor.

If I would need to design a new circuit I would propably design it around some chips which has a real HSYNC output insted of just composite sync. One such IC which comes in mind is EL4583 from Elantec. This chip has a vrey suitable set of outputs: Composite sync, H sync, V sync plus 5 others.

NOTE: Before starting any circuit building projecs check if your monitor can also handle composite sync directly. Some monitors can do that (check your monitor technica data for information on this, some can accept composite sync if it is connected to only HSYNC or VSYNC input). In this case you need only to connect RGB lines directly to monitor and also the composite sync signal to right sync input.

How to connect sync-on-green RGB signal to VGA monitor ?

Sync separation is a process is needed ot get the sync signals out of green picture line. The sync format included in green line has the same format as composite sync, so the LM1881 based circuit described above can be used for this process when it is connected to green line.

The R and B lines can go directly to the monitor. The green line contains normal R signal plus sync signals. To make it exactly normal R signal, those sync signal needs to be removed from R signal. I don't have any good circuit designs to give for that remove the sync signals from R line. You might try to survive without any conversion in R line by just connecting it to the R input in monitor. This direct connection (with sync still in R monitor) should not harm anything in the monitor. Some monitors will show everyhting nicely picture wven though it has those syncs added. Some other monitors have problems in showinbg the colors correctly if they are fed with green signal with syncs (the balance of green picture component gets wrong).

NOTE: Before starting any special building projecss check if your monitor can also handle sync on green green directly. Some monitors can do that (check your monitor technica data for information on this). In this case you need only to connect R, G and B lines directly to monitor.

How can I connect a PAL signal to a VGA monitor ?

You can't connect a PAL signal to VGA monitor directly. (unless your monitor is a special monitor which can accept video signals from horizoanl rate down to around 15 kHz and your video output outputs RGB).

The only way to get those to display PAL signal is to have a conversion electronics which take PAL picture in and outputs VGA picture. Possible equipments for this are TV cards (need to be installed inside PC), video format converter equipments (usually expensve professional devices) or scanline doubler boxes used with some video projectors (also expensive).

How can I convert TTL level RGB signals to analogue RGB signals needed by VGA monitor ?

If your TTL level RGB source HSYNC and VSYNC rates compatible with your VGA monitor (or other display device) then you can quite easily adapt TTL level RGB signals to analogue 0.7Vpp signals with few tricks:

Circuit 1: simplest adaptation

                                ____
TTL level R, G or B in o-------|____|----------o analogue 0.7Vpp signal out
                               470 ohm


         Signal ground o-----------------------o signal ground

This signal will simply convert the signal levels from TTL (0..5V) to the analogue 0.7Vpp signals for 75 ohm loads like normal monitor input. When the circuit put very near to the monitor and the signal source (not long cables) the not perfect impedance does not matter.

Circuit 2: impedance matched

                                ____
TTL level R, G or B in o-------|____|----+-----o analogue 0.6Vpp signal out
                               330 ohm   |
                                        | |
                               100 ohm  | |
                                        |_|
                                         |
         Signal ground o-----------------+-----o signal ground

This circuit generates quite perfect impedance matching for 75 ohm load and 75 ohm cable. The downside of this circuit is that this circuit is more complicated to build and the signal output does not reach the full 0.7V signal value (full brightness). This circuit should be built as near to the signal source as possible. The distance between this circuit output and the monitor can easily be many meters if you use good 75 ohm cable for signal transmission.


Tomi Engdahl <[email protected]>

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