Much software on the 8-bit Atari is highly interactive and timing-critical. For example, many games are highly responsive to input from the game player via a game controller (such as a joystick). The Atari software programmer can reasonably assume that the video display device can reflect changes as output by the Atari with no perceivable additional delay. Software timing considerations are normally limited to the capabilities and constraints of the Atari hardware itself.
However, in some cases a television or video monitor may introduce a noticeable lag time between the video signal as input by the Atari, and the video signal as actually displayed by the device. While any such effect is truly negligible with CRT televisions or monitors (the standard of the time of the Atari), the modern high-definition television (HDTV) has emerged as an important exception. An HDTV typically dedicates considerable processing efforts (hardware+software) toward presenting the most attractive picture possible. Since few HDTV applications require precise interactive timing, this video processing is generally not optimized for time efficiency. When used with an older but timing-precise device such as an 8-bit Atari computer, the resulting delay can be quite noticeable, making the HDTV essentially unusable for "serious" action gaming.
Fortunately, many HDTVs offer a "game mode" that circumvents much of the video processing otherwise performed, thereby eliminating most of the display lag otherwise introduced by the television. If an HDTV is to be used with the Atari for gaming, a "game mode" on the HDTV is essential.
Wikipedia on the topic: http://en.wikipedia.org/wiki/Display_lag
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