Why was the crystal clock frequency for the original Macintosh (128k,Plus, etc.) 15.667 MHz, instead of a multiple of 8 MHz (which was the maximum clock frequency for the speed grade of MC68000 choosen), or a multiple of NTSC color burst frequency, as was the predecessor Apple II/III line of computers.
1I will speculate it was derived from constraints on the video timings regardless of not being NTSC, specifically to get square pixels with a pixel rate that the memory could support. Just a completely unreasoned guess though, I haven't worked it out.– TommyDec 20, 2019 at 3:14
15.667 Mhz is about 2% less than 16 Mhz. So for purposes of "multiple of max. MC68000 frequency", it is close enough. The Mac wasn't competing based on being the fastest machine, so 2% was just not a big deal. And most of the competition was either older 8-bit or Intel family, not MC68000. In fact, the 8088 in the original IBM PC was clocked at 4.77 Mhz, which is about 5% less than the 8088's rated speed of 5 Mhz, so Apple was (sort of) copying IBM in this one aspect.
The main driver seems to be the video. See answers to this other recent question What was the screen refresh rate of the Lisa and original Macintosh? for more details. It gets complicated because Apple was doing some innovative things with video & graphics. Where most other computers of that time were designed (or could be relatively easily adapted) to run on ordinary monitors, Apple based the Mac on using a built-in, Apple-supplied, monitor and nothing else.
3Jup. That's the right answer. With 7.8336 MHz clock frequency video and CPU access could be interleved as 1:1 during a scan line, allowing maximum performance at a given RAM speed.– RaffzahnDec 20, 2019 at 8:53
Sorry, the 2nd part of this answer is wrong. The CRT yoke on the Mac was custom designed (by Alps for George Crow), and could have been specified for almost any nearby (to 16 MHz) scan rate frequency, and still produced square pixels at about the same refresh rate (where higher is usually better).– hotpaw2Dec 21, 2019 at 14:45
@hotpaw2 What you said "custom designed" actually supports my premise - i.e., Apple was not concerned about compatibility with off-the-shelf monitors in the way that many other computer manufacturers were. Dec 21, 2019 at 23:19