The Apple II — The First Personal Computer That Came With Color

This patent describes the Apple II's video generation system — how the computer outputs color graphics to a television using minimal hardware. The Apple II used the NTSC color encoding standard, which encodes color as a phase shift in a carrier wave. Wozniak discovered that by carefully timing digital signals from the 6502 CPU, he could generate color pixels simply by manipulating when bits were sent to the composite video output — no separate color graphics chip required. The memory architecture was also designed so that the CPU and video display circuitry could share access to RAM without a dedicated video buffer chip. The result was a color computer that cost a fraction of what dedicated graphics hardware would have required.

The clever bit

Wozniak was obsessed with minimizing chip count — fewer chips meant lower cost, smaller board, and higher reliability. The Apple II used 62 chips when the competing TRS-80 used 40 (but had no color) and the Commodore PET used over 70. Wozniak's color generation trick exploited a quirk of NTSC color encoding: color is determined by the phase relationship of a 3.58 MHz subcarrier. By carefully timing when digital bits were output relative to this subcarrier, different phase relationships — and thus different colors — could be produced purely through software timing, with no color encoding hardware. The trick only worked because Wozniak understood the TV signal standard deeply enough to exploit its mathematical structure. It was widely regarded as one of the most elegant engineering hacks in early computer history.

What it does not cover

• ×The 6502 CPU — designed by MOS Technology, not Apple
• ×The Apple II's sound generation — handled by a separate speaker circuit
• ×Integer BASIC or the DOS — software components are separate IP
• ×High-resolution mode color — the Apple II's HGR mode used different color generation techniques