Comment by meatmanek
12 days ago
But why 342 and tune the clock speed down instead of keeping the clock speed at 8MHz and having floor(8e6/2/6/60/32) = 347 lines?
I suspect kmill is right: https://news.ycombinator.com/item?id=44110611 -- 512x342 is very close to 3:2 aspect ratio, whereas 347 would give you an awkward 1.476:1 aspect ratio.
That doesn't sound right. The tube the mac was displaying on was much closer to a TV-style 4:3 ratio anyway, there were significant blank spaces at the top and bottom.
If I was placing bets, it was another hardware limitation. Maybe 342 put them right at some particular DRAM timing limit for the chips they were signing contracts for. Or maybe more likely, the ~21.5 kHz scan rate was a hard limit from the tube supplier (that was much faster than TVs could do) and they had a firm 60 Hz requirement from Jobs or whoever.
You guys are ignoring the horizontal and vertical blanking periods, which were usually 25~35% of the active horizontal period and 8~10% of the active vertical period. I could find these two links that explain that the actual signal sent to the CRT was around 704x370 pixels:
https://nerdhut.de/2016/06/26/macintosh-classic-crt-1/
https://bobparadiso.com/wp-content/uploads/2014/09/timing.pn...
You could reduce the gain on the horizontal deflection drive coil by 2% to get back to 3:2. In fact, I doubt that it was precise to within 2%.