Clock rate

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. Furthermore, the PAL CPU's master clock could have been divided by 15 to preserve the ratio between CPU and PPU speeds, but Nintendo chose to keep the Johnson counter structure, which always has an even period, and divide by 16 instead. So the main differences between the NTSC and PAL PPUs are depicted in the below chart:

Property	NTSC (2C02)	PAL (2C07)	Dendy	RGB (2C03)	RGB (2C04)	RGB (2C05)
Master clock speed	21.477272 MHz ± 40 Hz 236.25 MHz ÷ 11 by definition	26601712 Hz ± ? 26601712.5 Hz by definition	Like PAL	Like NTSC		Like NTSC
PPU clock speed	21.477272 MHz ÷ 4	26.601712 MHz ÷ 5	Like PAL	Like NTSC		Like NTSC
Corresponding CPU clock speed	21.47 MHz ÷ 12 = 1.789773 MHz 3 dots per CPU cycle Same as NTSC Amiga clock ÷ 4	26.60 MHz ÷ 16 = 1.662607 MHz 3⅕ dots per CPU cycle	26.60 MHz ÷ 15 = 1.773448 MHz 3 dots per CPU cycle Same as PAL Amiga clock ÷ 4	Like NTSC		Like NTSC
Height of picture	240 scanlines			Like NTSC		Like NTSC
Nominal visible picture height (see Overscan)	224 scanlines	268 scanlines	Like PAL	Like NTSC		Like NTSC
"Post-render" blanking lines between end of picture and NMI	1 scanline	1 scanline	51 scanlines	Like NTSC		Like NTSC
Length of vertical blanking after NMI	20 scanlines	70 scanlines	20 scanlines	Like NTSC		Like NTSC
Time during which OAM can be written	Vertical or forced blanking	Only during first 20 scanlines after NMI	Like NTSC	Like NTSC		Like NTSC
"Pre-render" lines between vertical blanking and next picture	1 scanline			Like NTSC		Like NTSC
Total number of dots per frame	341 × 261 + 340.5 = 89341.5 (pre-render line is one dot shorter in every other frame)	341 × 312 = 106392	Like PAL
Vertical scan rate	60.0988 Hz	50.0070 Hz	Like PAL
Color of top and bottom borders	N/A	Always black ($1D)	Like PAL
Side borders	Palette entry at $3F00	Always black ($1D), intruding on left and right 2 pixels of picture	Like PAL
Color emphasis (with correlating bit in PPUMASK)	Blue (D7), green (D6), red (D5)	Blue (D7), red (D6), green (D5)	Like PAL	Blue, green, red (full scale)		Blue, green, red (full scale)
Other quirks					Permutated palette	PPU registers $2000 and $2001 are swapped; revision ID in $2002

The 2C03, 2C04, and 2C05 PPUs were all found in Nintendo's PlayChoice-10 (a.k.a. PC10 or PC-10) arcade systems. Famicom Titler, Famicom TVs, and RGB-modded NES consoles would use either the 2C03 or a 2C05 with glue logic to unswap $2000 and $2001. (Later RGB mods used a 2C02 in output mode and faked out all palette logic.)

The color emphasis bits on the PAL NES have their red and bits http://wiki.nesdev.org/w/index.php/PPU_registers#Mask_.28.242001.29_.3E_write

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 using NTSC frequencies but PAL's color modulation.

Dendy is a clone of the Famicom distributed by Steepler and sold in Russia. 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. 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). This explains the faster CPU divider and longer post-render period vs. the authentic PAL NES.

To compensate for these differences, you can detect the TV system at power-on.