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3.9 Television Standards
The NES connects to a television to display the game to user. As a result different versions
of the system were created for the two television formats, NTSC and PAL. NTSC (National
Television Standards Committee) is the standard used in North America, most of South
America and parts of Asia [34]. PAL (Phase Alternating Line) is the standard used in Europe,
much of Asia, Africa and Australasia [35]. Table 3-1 shows the differences between NTSC
and PAL versions of the NES.
Frames per second
Time per frame (milliseconds)
Scanlines per frame (of which is V-Blank)
CPU cycles per scanline
Resolution
CPU speed
Table 3-1. Comparison of NTSC and PAL NES systems.
Images are displayed on a television screen by a stream of high speed electrons which
moves across the screen, from left to right, drawing each pixel. A single line of pixels is
referred to as a scanline. At the end of a scanline the electron beam must move to the next
line and return to the left before it can proceed. The time it takes to do this is known as the
Horizontal Blank period (H-Blank).
After drawing the screen once, the electron beam must return to the top left corner, ready to
start the next frame. The time it takes to do this is known as the Vertical Blank period (V-
Blank). When entering the V-Blank period, the PPU indicates this by setting bit 7 of I/O
register $2002. This bit is reset when the CPU next reads from $2002.
On an NTSC version of the NES, there are 240 scanlines on the screen (although the top
and bottom eight lines are cut off) and it takes an additional 3 scanlines worth of CPU cycles
to enter V-Blank. The V-Blank period takes a further 20 scanlines worth before the next
frame can be drawn.
NTSC
60
16.67
262 (20)
113.33
256 x 224
1.79 MHz
25
PAL
50
20
312 (70)
106.56
256 x 240
1.66 MHz
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