Faster Chipmem bus in PAL mode

Yeah. That's right. You can get chipmem bus running slightly faster in PAL mode than normally in a bigger part of screen. As you probably know chipmem bus in NTSC mode runs at 7.16 MHz and in PAL it runs at 7.09 Mhz. The trick is very simple, you have to turn on NTSC (bit 5 in BEAMCON0 cleared) at the vertical blank and switch it to PAL (bit 5 in BEAMCON0 set) in 261th scanline. Actually you can do it in 262th, but display area below will get totally blank (will have darker color than 000000 - seems like no power is supplied on video output then). Of course the most convinient way to make this trick is to use COPPER.

The example copperlist:

Copper: dc.l 01dc0000           ; go NTSC,
        ; what-so-ever
        dc.l ffdffffe,05d1fffe ; let's wait for line 261,
        dc.l 01dc0020           ; go PAL,
        dc.l fffffffe           ; let's wait for NIL.

It should work on ECS to AGA chips.

The main drawback is that when switching from PAL to NTSC (or viceversa) display distorts a bit, so you'd better set display area up in between lines 0 and 261.

Interesting links: