ATARI VCS / 2600 SYSTEM
• The original version of the VCS was designed to have a game built-in (Combat for the Atari version and Air-Sea Battle for the Tele-Games version). There’s a space on the motherboard for the ROM chip, which does work. A 2K or 4K ROM chip could be installed
here, however, there’s no circuit to switch between the ROM socket and a cart if a 4K ROM is used. The "unicorn" 2600 "JR" variant also was designed to have a game built-in as well, and can handle a 2K or 4K ROM automatically if installed (picture #2).
• The VCS was originally designed to produce sound effects internally via 2 internal speakers (much like Atari's earlier dedicated systems), instead of a TV (via the RF box). The system’s casing has slits on the top-half (picture #3), and brackets on the bottom-half (picture
#4) - these were for the speakers. Jim Heller, who was a former Atari engineer, stated a last-minute production change was made to drop the internal speakers, since it was felt the audio would be better through the TV set. Due to this change, the 2 sound output pins (12 and 13) on the TIA were simply tied together, resulting in mono sound. This also explains why Combat, Air-Sea Battle, Indy 500, and Street Racer are the only 4 games Atari made that were programmed for stereo sound.
• BUG: The black lines that are seen in some
games (on the left side of the screen) are the result of using an HMOVE command trick. If an HMOVE is initiated immediately after HBlank starts (which is the case when HMOVE is used as documented), the [HMOVE] signal is latched and used to delay the end of the HBlank by exactly 8 CLK, or two counts of the HSync Counter. This is achieved in the TIA by resetting the HB (HBlank) latch on the [LRHB] (Late Reset H-Blank) counter decode rather than the normal [RHB] (Reset H-Blank) decode. The extra
HBlank time shifts everything except the Playfield right by 8 pixels, because the position counters will now resume counting 8 CLK later than they would have without the HMOVE. This is also the source of the HMOVE 'comb' effect; the extended HBlank hides the normal playfield output for the first 8 pixels of the line. Some companies (such as Activision) got around this by using the trick on every scanline, which effectively blacked out the screen area where the lines appear. For more information
about the TIA and how it works, check out Andrew Towers TIA Hardware Notes article.
• BUG: Using the Activision work-around
method of avoiding the HMOVE lines doesn’t work on some PAL 2600 JR (all-black) models.
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