IEEE Consumer Electronics Magazine - July 2015 - 63

There was a more difficult problem: the hardware was
designed for easily moving objects, but, to save a lot of circuit area, they were based on polynomial counters with simple shift register cells, rather than on binary counters with
binary comparators. Positioning the moving object horizontally was complicated; there were offsets and reset signals
for each of five horizontal position systems: P0, P1, M0, M1,
and the "Ball." I wrote a utility called Compute Horizontal
ReSeT (CHRST) that accepted a binary value for the horizontal position and issued the hardware commands to place the
object there. This was an example of a device driver hiding
the hardware complexity from the game designers. CHRST
was widely used by game designers.

FIGURE 6. The Atari VCS and four sets of game controllers. (Photo
courtesy of Deb Freng.)

sounD I/o
Games need sound as well as video, so we decided to include a pair
of sound circuits. Each sound circuit had three components: a frequency divider, a noise generator, and a multiplying 4-bit A/D converter. The frequency divider starts with the 3.579-MHz color clock
divided by 114 to result in about 31.4 kHz. A 5-bit number is used
to divide that down one to 32 times. The noise generator contains

a 5-bit shift register and a 4-bit shift register; depending on feedback, they can generate 9-bit "white noise" (e.g., jets flying), 4-bit
clunky noises (e.g., tanks rolling), and other combinations. Each
channel allows a 4-bit number to set the output volume.
The combined two-channel sound was multiplexed in
with the video to form a standard composite NTSC signal
before it was RF modulated to TV channels 3 or 4.

FIGURE 7. A block diagram of the Stella system from Decuir's engineering notebook.

july 2015

^

IEEE Consumer Electronics Magazine

63



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