Video Overlay..looking for schematics ?

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By MSX Crazy

Supporter (7)

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11-12-2015, 11:51

Hi.

I've recently been messing around with some old MSX hardware and got it into my head to design a Video overlay circuit. so I can have two VDP on my old MSX test machine. What I'm trying to do is inject a V9938/V9958 VDP signal into a TMS9918 (as it has an external Video in Pin) has anyone already done this ?? Is it easy , I've had mixed results to say the least !

hopefully someone has already done this and can show me a working schematic ??

Thanks for reading !! oO

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By RetroTechie

Paragon (1563)

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11-12-2015, 19:05

Welcome to this forum, MSX Crazy!

Afaik many MSX1 machines have a TMS9928 or TMS9929, which doesn't have external video input pin.

What you're trying to do, would require 2 (of the same type!) VDP's that share the same clock signal, are reset at the exact same time, and don't do anything (like execute VDP commands, or switch 50/60 Hz) that would cause them to go out of sync.

For the TMS9918 (and -28/29), the TMS9918 databook has a few examples for this setup ("External VDP operation"). With 2 VDP9938/58's, I doubt this would work. And when combining V9938/58 with an MSX1 VDP, there would be no synchronisation between the chips.

In the latter case (and in general with any external video input), you need a circuit that's called a genlock. Which is very much NOT a trivial circuit! Just see Sony HB-G900 or Philips NMS8280 circuit diagrams what's used to synchronise VDP with the external video signal. In short: lots of components...

And then there's still the switching between those video sources (with VDP's other than TMS9918), each VDP needing its own VRAM etc.

So better forget this. Sad Or buy an MSX with genlock built in.

By Wild_Penguin

Hero (541)

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11-12-2015, 19:10

Hi MSX Crazy, and Welcome!

I've never done anything like what you are doing, but if I were, I'd look into some MSX service manual for a model which already has superimposing capabiities, like the Philips NMS 8280 (I think Sony had one too and probably others, but I don't know about SM availability online, but the 8280 SM certainly is available).

EDIT:Oh, also listen to RetroTechie! Smile

By MSX Crazy

Supporter (7)

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12-12-2015, 11:35

I was thinking of making my own video board...just for the hell of it ! One idea was to use a V9938 and connect it's Composite video output to a TMS9918 EXTVID input thus giving a superimposed image & this looks like the easiest to do. The two clocks have to be synchronised and both VDPs RESET/initialised together but it looks a lot simpler than other methods. The 21.477MHz Master Clock being divided by 2 to give 10.7MHz for the TMS9918, thus both VDPs use the same Clock.

Other Alternatives (though much more complex)

Use a V9958 with a CXA1645 (PAL/NTSC encoder giving composite output) and superimpose the TMS9918 video on top.
or
A V9990 giving again composite output with TMS9918 superimposed...?

At the moment it's all 'Pie in the Sky' but it should be achievable, with the end goal making my own Homebrew MSX compatible microcomputer !

By anonymous

incognito ergo sum (109)

anonymous's picture

12-12-2015, 18:43

Why not trying to make a clon of the Twin VDP cartridge (aka Goblin)? IMHO this would be a very nice development.

By madscient

Master (179)

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14-12-2015, 11:24

I was considering twin VDP system with V9958 and 315-5124(a.k.a. SMS VDP).

Here is my superimpose circuit diagram:
https://twitter.com/madscient/status/653552892691898370

This circuit has no external genlock since it was based on prerequisite that both of VDPs share the same clock.
Video encoder should be added after RGB output.
Note that the circuit have not been implemented nor tested yet.

By MSX Crazy

Supporter (7)

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14-12-2015, 13:12

I think I'll keep things as simple as possible to start with.. I'm using an old Toshiba MSX that I bought as faulty and repaired the ON/OFF switch. The motherboard from this will be my 'TESTBED computer' The video card will be a V9938+TMS9918A.

V9938 , 192K DRAM composite output.
TMS9918A instead of DRAMs I'll probably use 128K or 512K SRAm with a CPLD used to interface the multiplexed address lines from the TMS9918 to the SRAM. This will allow 8 (128k SRAM) or 32 (512K SRAM) 16K pages. This will obviously only give Composite NTSC 60Hz output, but as most modern TV can easily switch between PAL and NTSC this is no problem.

I could Map the V9938 to MSX I/O port and the TMS9918 to Colecovision thus allowing MSX 2 or even Colecovision software ?

Things are obviously still very much in the design/theory phase !

By MSX Crazy

Supporter (7)

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14-12-2015, 13:22

I can't seem to find much information on the GOBLIN Twin VDP cartridge... did it ever go into production or was it just a Myth ??

By MSX Crazy

Supporter (7)

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14-12-2015, 13:29

My Twin VDP board specification...

First VDP:
V9938 with 6pcs (D41464-10 DRAMS) giving 192K and composite NTSC video output
Second VDP:
TMS9918 with 512K SRAM :VDP/SRAM interface using (probably) a EPM7032/EPM7064 allowing 32 x 16K pages of VRAM

How it would work (Operation)
The V9938's composite signal would be fed straight into the TMS9918 EXTVID input , the TMS9918 has an internal BIT which allows an external composite signal to be input and the TMS9918 video would be 'Superimposed' on-top of the V9938s video signal. As long as the two signal are synchronised from the same clock, this should work. This design should be the simplest and hopefully the cheapest to implement. The control signal would be from my old MSX test-bed computer.

By l_oliveira

Hero (520)

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14-12-2015, 19:05

Take 21Mhz from V9938 clock output pin (XO pin), amplify with a 74Fxx buffer chip (74F07 maybe?) then divide it by 2 with 74F74?

That should make the clock for the TMS9918 synchronous with the V9938. Smile

By RetroTechie

Paragon (1563)

RetroTechie's picture

14-12-2015, 19:36

madscient wrote:

I was considering twin VDP system with V9958 and 315-5124(a.k.a. SMS VDP).

Here is my superimpose circuit diagram:
https://twitter.com/madscient/status/653552892691898370

This circuit has no external genlock since it was based on prerequisite that both of VDPs share the same clock.

MSX Crazy wrote:

The V9938's composite signal would be fed straight into the TMS9918 EXTVID input , the TMS9918 has an internal BIT which allows an external composite signal to be input and the TMS9918 video would be 'Superimposed' on-top of the V9938s video signal. As long as the two signal are synchronised from the same clock, this should work.

There seems to be some confusion here. Running from the same master clock != synchronized. This just means: running at the exact same clock speed.

But depending on VDP's used, one may produce a few more screen lines. One may spend 603 clock cycles on a screen line, while the other uses 607 clock cycles for the same (just a random example). TMS9918A/9928A produce 60 frames/sec with 262 lines/frame, for TMS9929A this is 50 frames/sec with 313 lines/frame. So even running from the same master clock, they would start frames at different times, and would start screen lines at different times. Same thing with TMS9918A + V9938 or similar combo's.

The way a genlock circuit solves this, is by varying the VDP clock (+/- a few %) such that the VDP can 'catch up' with the external video source (or slow down until external video source has caught up with the VDP). And perhaps (if possible) use input signals that tell the VDP something like "start a new frame NOW!". As said, all of that is very much non-trivial. Hence why Philips 8280, Sony HB-G900 etc have such complex video circuitry.

Bottom line: you need 2 EXACT same type VDP's. And feed them the same master clock. And pay extra attention to things like the reset circuitry. And even then it may not work (depending on what type VDP you started with). OR you need a genlock circuit with all the complexity that comes with it.
Switching between video signals is NOT the hard part. Getting those video sources synchronized, is. And simply starting with the same master clock won't do the trick. That's where a genlock circuit comes in.

One other thing: 192K VRAM on a V9938/58 isn't much use, the extra 64K can't be used as additional display pages. Whatever goes in there, has to be copied back & forth to the regular 64 or 128K VRAM in order to appear on screen. So it's just a block of 64K RAM that happens to be controlled by the VDP.
This is why practically no software uses it. Just a gimmick, really.

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