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New Atari Lynx demo released - Jitter by Desire


Heaven/TQA
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Some technical informations:

 

It loads parts from cart

Sprite blitter is used for the polygons

Most of time rendered into sprite SCB and then put on screen with Suzy

Samples are 8bit signed. 6000 KHz.

3D Engine (matrix multiplications etc) are using maths copro.

Engine can handle 256 faces and 256 vertices per object.

Concave objects, sorting faces done by CPU.

 

 

Main issue developing for lynx is that the emulators are nowhere accurate to real HW esp when using the custom chips.

 

But even the 65c02 emulation in Handy seeks "off" as playback of sample is not done by timer IRQs but with CPU loops and real HW is off to emulator leading to pitched samples.

 

Maths calculations are faster in handy than real hw and I suspect rendering with Suzy, too.

 

Overall took and drove me nuts in the end. Ok, I am using STeem, WinUAE, Altirra and Vice... so would assume better emulators for Lynx then... ;)

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I burned the demo on a cart and did a byte-by-byte check that every byte is correct.

 

The demo runs until the 4 boxes have rotated for a while. Then the demo freezes and the music turns to an engine sound - brrrrr. But only on Lynx 2. I tried with two units.

 

On Lynx I the error comes just when the apes are appearing. The screen goes black. But the music continues. At the credits part the screen comes on again and the demo plays nicely to the end. I also tried this on two Lynx I's with identical outcome.

 

This Lynx I error is something I have seen before while testing Championship Rally. The reason was that Lynx I is very picky about when you can change the palette.

 

On Mednafen (Ubuntu 64bit 16.04) the demo runs fine.

Edited by karri
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I don't remember exactly what solved the issue in Championship Rally. But when we changed the order of accessing the registers in Suzy the problem went away. It would be interesting to see if Lynx I could display the cubes correctly by removing the fade before the apes.

 

On the real Lynx I cannot see any problems with colours. But I have one McWill screen and on that one the top part of the cubes start having small colour errors at the top of the screen like the palette changes would be late. Perhaps 1/3 top area of the screen. The effect grows bigger until the demo crashes.

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I see the palette problem (i would rather guess, doublebuffer not switched on VBL?) in the cube part ony my unmodded lynx 2. But didnt want to get picky on it. but now that you mention it...

And the credits part went with completley black screen.

 

mednafen and handy use the same codebase, but there are slight differences. esp in the IRQ part (sound). But its not so clear what is the correct behaviour.

Proboem with the sound is, that changes here break compatibility for other games. we now the something is fishy but have to idea how to fix it.

Edited by sage
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