Dot clock rates

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The following frequencies are the dot clock rates of digital picture generators outputting 240p low-definition video signals that are compatible with NTSC monitors or otherwise at near-System M scan rates (15.7 kHz horizontal, 60 Hz vertical). The pixel aspect ratio that each rate produces, assuming the 480-line picture and 52.148 microsecond active scanline period of Rec. 601, is also given.

A few things may cause this ratio to differ in a particular environment:

  • Some coin-operated machines use a different active scanline period, and the arcade operator is expected to adjust the monitor's picture size to match a test pattern displayed in a particular game's service mode. An arcade-to-television adapter would produce the pixel aspect ratio listed below.
  • Some devices also support 480i (interlaced) video, where even scanlines are drawn in one field and odd scanlines are drawn in the other field. Interlaced video makes pixels half as tall as in 240p, doubling the pixel aspect ratio, and ratios assuming interlace are marked as such.
  • Sets of the 240p era were built for 4:3 display aspect ratio, as opposed to the 16:9 aspect ratio that became common between the late 1990s and mid-2000s; multiply the pixel aspect ratios by 4/3 if stretching the picture on a widescreen TV.
Frequency Exact frequency PAR Exact PAR Devices
1.02 MHz 315/308 MHz 1.50 3:2 Apple II (GR; each line repeated four times)
3.58 MHz 315/88 MHz 1.71 12:7 Devices with 1 pixel per NTSC subcarrier period: Atari 2600, Atari 7800, Intellivision
5.00 MHz 5/1 MHz 1.22 27:22 Atari Missile Command[1] and Crystal Castles hardware[2], Stern Berzerk/Frenzy hardware[3] (all 256px)
5.37 MHz 945/176 MHz 1.14 8:7 The 256px display of the TMS9918 (TI-99/4A, ColecoVision, MSX, and SG-1000), its descendants (MSX2, Master System, Genesis), and other VDPs inspired by it (NES, TurboGrafx-16,[4] Super NES), and 256px mode of the original PlayStation[5]
5.59 MHz 16777216/3 Hz 1.10 6328125:5767168 Nintendo DS (256px) is the ~33.5 MHz (apparently 225 Hz) master clock divided by 6[6]
6.00 MHz 6/1 MHz 1.02 45:44 Data East Lock'n Chase/BurgerTime family (240 or 256px)[7], DECO Cassette System (256px)[8], and BreakThru family (240px)[9]
Technos Double Dragon hardware,[10] SNK pre-Neo Geo 68000-based hardware,[11] Taito Arkanoid[12] and Bubble Bobble[13] hardware, Jaleco Mega System 1-A[14] (all 256px)
Konami GX[15] and pre-GX (Mystic Warriors) family[16] (288px)
Neo Geo MVS (320px)[17]
6.04 MHz 6048/1001 MHz 1.02 65:64 Neo Geo AES (320px)[18]
6.09 MHz 1071/176 MHz 1.01 120:119 Nintendo 64 (320px)[19][20][21]
6.14 MHz 135/22 MHz 1.00 1:1 Rec. 601 defines the NTSC picture such that this is square on 4:3 monitors
6.14 MHz 768/125 MHz 1.00 5625:5632 48000 kHz (DAT and DVD audio rate) * 128[22] approximates square pixels to within 1 part in 800
Irem M52 (Moon Patrol) (240px)[23]
Namco Pac-Man[24]/Galaxian[25] hardware and derivatives such as Pengo (288px)[26]
Namco Super Pac-Man family,[27] System 86,[28] 1,[29] 2,[30] and ND-1[31] (all 288px)
6.25 MHz 25/4 MHz 0.98 54:55 Sega X-Board (320px)[32]
6.29 MHz 900/143 MHz 0.975 39:40 Sega Hang-On/Space Harrier[33], OutRun[34], and System 16B hardware[35] (all 320px) divide a VGA crystal by 4.
Mode X (320x240) on VGA has twice the dot rate and repeats each line twice, giving it the same PAR.
6.71 MHz 4725/704 MHz 0.91 32:35 Sega Genesis and PlayStation 320px mode,[36][5] very close to SIF
6.75 MHz 27/4 MHz 0.91 10:11 MPEG-1 NTSC SIF (352 or 360px): CD-i, Video CD. Equal to Rec. 601 chroma rate or half Rec. 601 luma rate.
Toaplan/Raizing 2nd-generation arcade boards (320px)[37]
6.75 MHz 27/4 MHz 1.82 (i) 20:11 (i) Low-resolution DVD-Video, such as that produced by DVD recorders in long play mode
7.00 MHz 7/1 MHz 0.88 135:154 Toaplan/Taito Wardner hardware,[38] Toaplan "1st generation" hardware,[39] Technos Double Dragon 3 family (all 320px)[40]
7.16 MHz 315/44 MHz 0.86 6:7 Apple II (40-column text, HGR 280px); TG16 (mid-res, up to 352px); Atari 7800 (hi-res, 320px); Amiga, IBM CGA (40-column or 320px)[22] and EGA in 200-line mode (40-column or 320px)[41]; Atari 800[42]; Tandy Color Computer (COCO) and other computers using Motorola 6847 video chip[43]; Atari Tetris and Klax arcade games (336px)[44]
7.50 MHz 15/2 MHz 0.82 9:11 Capcom CPS3 (384px)[45]
7.67 MHz 4725/616 MHz 0.8 4:5 PlayStation (366px)[5]
8.00 MHz 8/1 MHz 0.77 135:176 Williams 6809-based arcade system (276, 292, or 316px)[46]
Capcom CPS1 and CPS2 (384px)[47]
8.19 MHz 630/77 MHz 0.75 3:4 Commodore 64 and Apple IIGS 4bpp super hi-res[48] (both 320px). Square on widescreen monitors.
10.74 MHz 945/88 MHz 0.57 4:7 Twice the TMS9918 pixel rate produces 512px on Super NES, TG16, and PlayStation[5]
12.27 MHz 135/11 MHz 1.00 (i) 1:1 (i) Square interlaced pixels
13.42 MHz 4725/352 MHz 0.46 16:35 PlayStation (640px)[5]
13.50 MHz 27/2 MHz 0.91 (i) 10:11 (i) Rec. 601 luma rate as used in D1 tape and DVD Video (704px)

Nintendo GameCube (640px), based on (nominally?) 27 and 54 MHz clock signals in digital AV out[49][50]

14.32 MHz 315/22 MHz 0.43 3:7 Apple II (80-column text, double hi-res); Amiga; IBM CGA (80-column)[22]
16.36 MHz 1260/77 MHz 0.375 3:8 Apple IIGS 2bpp super hi-res (640px)[48] though Apple has claimed 5:12 instead[51]

For comparison, here are some enhanced-definition (480p) rates, with a horizontal scan frequency close to 31.5 kHz:

Frequency Exact frequency PAR Exact PAR Devices
24.58 MHz 3072/125 MHz 1.00 5625:5632 Namco System 22 family (640x480)[52]
25.17 MHz 3600/143 MHz 0.975 39:40 VGA mode 12h (640x480). The horizontal scan rate is twice NTSC, and the dot clock is 800 times that: 640 dots of picture and 160 of blanking.[53]
27.00 MHz 27/1 MHz 0.91 10:11 Twice Rec. 601 luma rate, as produced by progressive scan DVD players (704px)

Nintendo GameCube[49][50] and Wii progressive component video (640px)

References

  1. missile.cpp from MAME
  2. ccastles.cpp from MAME
  3. berzerk.cpp from MAME
  4. HuC6260 - Archaic Pixels
  5. 5.0 5.1 5.2 5.3 5.4 The PS1 divides a 15 times NTSC (53.69 MHz) crystal by 10, 8, 7, 5, or 4 Post by Ryphecha on Mednafen forum
  6. GBATEK by Martin Korth
  7. btime.cpp from MAME
  8. decocass.cpp from MAME
  9. brkthru.cpp from MAME
  10. ddragon.cpp from MAME
  11. snk68.cpp from MAME
  12. arkanoid.cpp from MAME
  13. bublbobl.cpp from MAME
  14. megasys1.cpp from MAME. The same pixel clock is possible but unverified for the other Mega System 1 variants.
  15. konamigx.cpp from MAME
  16. mystwarr.cpp from MAME
  17. video/neogeo_spr.h from MAME. Many Neo Geo games are designed around a safe area approximately 304 pixels wide.
  18. 384 pixels per standard 227.5-cycle scanline period. See NESdev posts by TmEE and psycopathicteen. TODO: Verify whether the timing differences from MVS are noted anywhere in the MAME source code.
  19. assemblergames forum post
  20. MX8350 clock generator
  21. includes/n64.h from MAME. Note the mention of quarter-pixel scanline length in n64.cpp; divide the given DACRATE_NTSC by 8 to obtain the result here.
  22. 22.0 22.1 22.2 Wikipedia:Crystal oscillator frequencies
  23. m52.cpp from MAME
  24. pacman.cpp from MAME
  25. video/galaxian.cpp from MAME. The "6MHz" mentioned in some comments is only an approximation.
  26. pengo.cpp from MAME
  27. mappy.cpp from MAME
  28. namcos86.cpp from MAME
  29. namcos1.cpp from MAME
  30. namcos2.cpp from MAME
  31. namcond1.cpp from MAME
  32. segaxbd.cpp from MAME
  33. MAME segahang.cpp
  34. MAME segaorun.cpp
  35. MAME segas16b.cpp. Look carefully at how MASTER_CLOCK_25MHz is defined. System 16A contains the same crystal according to comments in xtal.h, but its use is not yet noted in segas16a.cpp.
  36. kyuusaku reasons from the 15*CB oscillator
  37. toaplan2.cpp from MAME
  38. wardner.cpp from MAME
  39. toaplan1.cpp from MAME
  40. ddragon3.cpp from MAME
  41. reenigne. "VGA 15kHz Output". 2014-01-18. Accessed 2016-01-26.
  42. phaeron's post
  43. The 6847 is clocked at the NTSC color burst frequency. Its datasheet lists the left and right border widths as 29.5 and 28 cycles on p. 5, and subtracting those from the 186+2/3 cycles of a standard scanline leaves just over 131 cycles of active picture. In addition, tDOT on p. 7 appears to be half a color burst cycle. Furthermore, the width of low and high phases of the clock "controls the width of alternate dots on the television screen" (p. 13).
  44. atetris.cpp and klax.cpp from MAME
  45. cps3.cpp from MAME
  46. williams.cpp from MAME
  47. includes/cps1.h from MAME. For comparison, Irem's M72 family (m72.cpp) and Konami's Xexex (xexex.cpp) also use a 384px-wide clean aperture and an 8MHz dot clock, but they have slower-than-NTSC frame rates of about 54–55 Hz.
  48. 48.0 48.1 Post by Michael J. Mahon to comp.sys.apple2
  49. 50.0 50.1 "Nintendo GameCube Digital AV Connector". NFG Games + GameSX. Accessed 2016-04-02.
  50. Apple IIGS Tech Note #41: Font Family Numbers
  51. namcos22.cpp from MAME
  52. Wikipedia:Video Graphics Array. Accessed 2013-09-24.