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Fungus
Registered: Sep 2002
Posts: 629 |
PAL/NTSC Half Variance Stable Raster
I needed such a routine, and this is based on JackAssers, which also looks very very similar to crossbows. So I whipped this up real quick.
Hope someone can add it to codebase.
ldx #$38
cpx $d012
bne *-3
ldy #$09
dey
bne *-1
cmp #$ea ;ntsc with nop nop
bit $ea
nop
inx
cpx $d012
beq branch1
nop
nop
branch1
ldy #$09
dey
bne *-1
cmp #$ea ;ntsc with nop nop
bit $ea
nop
inx
cpx $d012
beq branch2
bit $ea
branch2
ldy #$09
dey
bne *-1
cmp #$ea ;ntsc with nop nop
nop
nop
inx
cpx $d012
bne branch3
branch3
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JackAsser
Registered: Jun 2002
Posts: 1994 |
I got "mine" from Krill. |
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Fungus
Registered: Sep 2002
Posts: 629 |
Hah cool, the first time I saw it was from crossbow in one of his demos. I was aware of it, but didn't get how it worked until I read your article. |
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chatGPZ
Registered: Dec 2001
Posts: 11148 |
BTW - "Crossbows" IIRC is actually Hannes Sommer's - published in 64er many moons ago (He also published the lightpen-trigger stable raster thing there, and the $D011 "wanker" i believe)
I think i learned it from Marko's article that explained stable raster using an auxiliary timer. Maybe. :) |
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Krill
Registered: Apr 2002
Posts: 2855 |
And because it's not as widely known as it should be: i'm using Quiss+Copyfault's ingenious 8-byte sync method whenever i can. =) |
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chatGPZ
Registered: Dec 2001
Posts: 11148 |
beware of broken emulators :) |
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Krill
Registered: Apr 2002
Posts: 2855 |
Quoting chatGPZbeware of broken emulators :) The other way around. Broken eumlators, beware! =) |
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chatGPZ
Registered: Dec 2001
Posts: 11148 |
you make Ian cry :( |
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Oswald
Registered: Apr 2002
Posts: 5029 |
btw never occured to me until now, but wtf is half variance |
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chatGPZ
Registered: Dec 2001
Posts: 11148 |
each "step" in the above code halves the jitter (variance), until it gone |
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mankeli
Registered: Oct 2010
Posts: 110 |
Fixed link to Quiss+Copyfault's ingenious 8-byte sync method |
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Fungus
Registered: Sep 2002
Posts: 629 |
Well apparently it was off by 1 cycle for each stage, thanks to Jedi for using it and fixing it.
ldx #$38
cpx $d012
bne *-3
ldy #$09
dey
bne *-1
fix1
cmp #$ea ;ntsc with nop nop
nop
nop
inx
cpx $d012
beq branch1
nop
nop
branch1
ldy #$09
dey
bne *-1
fix2
cmp #$ea ;ntsc with nop nop
nop
nop
inx
cpx $d012
beq branch2
bit $ea
branch2
ldy #$0a
dey
bne *-1
fix3
cmp #$ea ;ntsc with nop nop
inx
cpx $d012
bne branch3
branch3
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Oswald
Registered: Apr 2002
Posts: 5029 |
Quote: each "step" in the above code halves the jitter (variance), until it gone
thanks, I didnt know that I thought only 1 cycle flicker can be eliminated per go. |
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JackAsser
Registered: Jun 2002
Posts: 1994 |
Quote: thanks, I didnt know that I thought only 1 cycle flicker can be eliminated per go.
If you have X cycle of jitter and make sure the middle of this jitter is on a raster line boundary you effectively half the jitter. |
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Krill
Registered: Apr 2002
Posts: 2855 |
Quoting JackAsserIf you have X cycle of jitter and make sure the middle of this jitter is on a raster line boundary you effectively half the jitter. Sort of like a binary search of the original raster position, which you compensate for as you go along. =) |
