MSX1 Overscan

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By Grauw

Ascended (8507)

Grauw's picture

24-08-2019, 21:18

Looks like the TMS9918 properly "supports" overscan, just like the V9938... Very nice findings, now if only we could find a way to trigger it without a special hardware setup requirement!

By mohai

Paladin (841)

mohai's picture

25-08-2019, 20:34

I think that a simple circuit could do the job.
There are a couple of sync generators that could help> LM1882 or 74ACT715
A PIC that generates proper NTSC sync could help too.
Sync level must be pulled up to around 12 Volts. This can be obtained with some kind of transistor, I think.
I am not an electronics expert, so I cannot give details on how to do it.
Maybe there are other ways to generate NTSC sync signals

By mohai

Paladin (841)

mohai's picture

26-08-2019, 21:39

ELM304 could be a suitable IC, because it is very simple, requires very few external components and it outputs a few digital signals that can be converted to analog composite video.
Maybe a similar circuit used in PX-7 for VDP /RESET-SYNC could be used to input sync signal from this chip.

By ARTRAG

Enlighted (6275)

ARTRAG's picture

26-08-2019, 21:55

cloud the circuitry go into a cartridge with a game?
it could be a system for copy protection... without you do not see a part of the screen...

By mohai

Paladin (841)

mohai's picture

26-08-2019, 22:51

ARTRAG wrote:

cloud the circuitry go into a cartridge with a game?
it could be a system for copy protection... without you do not see a part of the screen...

I do not think so, as external sync pulses need to be connected directly to EXT SYNC pin in VDP chip.
This signal is not available in cart slot.

... but this makes me think of register 0, bit 0 (IIRC), EXTERNAL SYNC input enable: What would happen if this bit is set/reset at known intervals? Does VDP loses synchro for a moment and shows overscan?

By thegeps

Champion (278)

thegeps's picture

01-09-2019, 01:29

Register 0 bit 0 Is used to enable/disabile an external VDP. And as I remember this Ext VDP can be in a cartridge

By retrocanada76

Champion (463)

retrocanada76's picture

01-09-2019, 06:45

You cannot inject video on TMS9929A. Only the NTSC 9918 and 9918A support that feature through EXTVDP (pin 35).

For the PAL VDPs extra circuitry is needed as shown on page 3-7:

http://map.grauw.nl/resources/video/texasinstruments_tms9918...

So whatever is doing overscan definitely is not the VDP.

By mohai

Paladin (841)

mohai's picture

02-09-2019, 23:23

retrocanada76 wrote:

You cannot inject video on TMS9929A. Only the NTSC 9918 and 9918A support that feature through EXTVDP (pin 35).

For the PAL VDPs extra circuitry is needed as shown on page 3-7:

http://map.grauw.nl/resources/video/texasinstruments_tms9918...

So whatever is doing overscan definitely is not the VDP.

You are right.
I am not trying to inject Video to a 9929a. I am trying to inject only sync pulses.
I am studying PX-7 service manual and, although a somehow cryptic English texts, I understand that sync can be connected to pin 34, after shifting it to 12 volts.
That point is not clear in the TMS manual. Maybe it is explained in the development documents.

By mohai

Paladin (841)

mohai's picture

03-09-2019, 12:00

thegeps wrote:

Register 0 bit 0 Is used to enable/disabile an external VDP. And as I remember this Ext VDP can be in a cartridge

Yes. You can put a VDP chip in a cartridge, as Goblin / Twin VDP guys did, then you can access to pin 34 (SYNC) in external VDP, but you cannot access to this pin in internal VDP through the cartridge bus.

Well, you would be able to mix the video or the sync from the internal VDP with the external VDP either, as the Twin VDP did.
External VDP could have an extra 60 Hz sync generator to get overscan, either.

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