Difference between revisions of "Clock rate"

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The '''clock rate''' of various components in the NES differs between consoles in the USA and Europe due to the different television standards used (NTSC M vs. PAL B). The color encoding method used by the NES (see [[NTSC video]]) requires that the master clock frequency be six times that of the color subcarrier, but this frequency is about 24% higher on PAL than on NTSC. In addition, PAL has more scanlines per field and fewer fields per second than NTSC.
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#REDIRECT [[Cycle reference chart]]
Furthermore, Nintendo inexplicably divided the PAL CPU's master clock by 16 instead of 15, resulting in a significantly slower CPU and a different ratio between CPU and PPU speeds.
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So the main differences between the NTSC and PAL PPUs are as follows:
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{| class="tabular"
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! Property || NTSC || PAL || Dendy
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|-
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| Master clock speed || 21.477272 MHz &plusmn; 40 Hz<br>236.25 MHz / 11 by definition || 26601712 Hz &plusmn; ?<br>26601712.5 Hz by definition || Like PAL
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|-
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| PPU clock speed || 21.477272 MHz / 4 || 26.601712 MHz / 5 || Like PAL
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|-
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| Corresponding CPU clock speed || 21.47MHz / 12 = 1.78977267 MHz<br>3 dots per CPU cycle || 26.601712 MHz / 16 = 1.662607 MHz<br>3 1/5 dots per CPU cycle || 26.601712 MHz / 15 = 1.773448 MHz<br>3 dots per CPU cycle
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|-
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| Height of picture || 240 scanlines || 240 scanlines || Like PAL
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|-
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| Nominal visible picture height<br>(see [[Overscan]]) || 224 scanlines || 268 scanlines || Like PAL
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|-
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| "Post-render" blanking lines between end of picture and NMI || 1 scanline || 1 scanline || 51 scanlines
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|-
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| Length of vertical blanking after NMI || 20 scanlines || 70 scanlines || 20 scanlines
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|-
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| "Pre-render" lines between vertical blanking and next picture || 1 scanline || 1 scanline || Like PAL
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|-
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| Total number of dots per frame || 341*261 + 340.5 = 89341.5<br>(pre-render line is one dot shorter in every other frame) || 341*312 = 106392 || Like PAL
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|-
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| Vertical scan rate || 60.0988 Hz || 50.0070 Hz || Like PAL
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|}
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Other than these differences and color encoding, the NTSC and PAL PPUs function exactly the same.
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The authentic NES sold in Brazil is an NTSC NES with an adapter board to turn the NTSC video into PAL/M video, a variant of PAL with frequencies much closer to those of NTSC.
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[[wikipedia:Dendy (console)|Dendy]] is a clone of the Famicom distributed by Steepler and sold in Russia.
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Because not many people in the English-speaking NESdev community have a Dendy, its precise differences from the authentic Nintendo hardware are not completely understood, and the values above are partly conjecture.
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But it is known that the chipset in Dendy and several other PAL famiclones is designed for compatibility with Famicom games, including games with CPU cycle counting mappers (e.g. [[VRC4]]) and games that use a cycle-timed NMI handler (e.g. ''Balloon Fight'').
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This explains the faster CPU divider and longer post-render period vs. the authentic PAL NES.
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To compensate for these speed differences, you can [[detect TV system|detect the TV system]] at power-on.
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Latest revision as of 23:39, 19 November 2018