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Thomas BERNARD edited this page Mar 31, 2019 · 1 revision

The DreamCast video chip is able to output either 15KHz or 31Khz signal.

  • 15KHz is meant for output on a standard PAL or NTSC television. The output is either composite, Y/C S-Video or RGB.
  • 31KHz for VGA screens (RGB).

The standard resolution is 640x480 60Hz, which is progressive using 31KHz or interlaced in 15KHz mode. My intro are not using the PowerVR chip but are writing directly to the VRAM frame buffer. Unfortunately access to VRAM from the SH-4 is slow :( Also I wanted to give an Amiga look to my intros, so I wanted to use a 320x240 mode.

It is possible to half the vertical resolution to 320 pixels. You then disable the interlace to get a 320x240 progressive mode on your 15KHz TV or commodore 1084S monitor as I was using. But in 31KHz, 480 lines modes are already non-interlaced so you get a 320x480 60Hz mode. When forcing 240 lines in 31KHz, you get 320x240 120Hz !

120Hz in VGA mode !

VGA compatible PC video cards achieve 200 or 240 lines mode in 70Hz and 60Hz by Scan Doubling, ie displaying each line twice. So in 320x200 MCGA mode, the frame buffer contains 200 lines but 400 lines are displayed. I don't know if the DreamCast hardware support Scan Doubling.

see the dc_init_video320 code in https://github.com/miniupnp/DreamCastIntros/blob/master/myintros/dc.s

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