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Why the original Macintosh had a screen resolution of 512×324

512pixels.net

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Re: Why the original Macintosh had a screen resolution of 512×324

#41

Post by nyanpasu64 »

perching_aix wrote: post #32
bluGill wrote: post #28

Earlier quoted context omitted.

Have you ever seen something running at 30 Hz? Or even 15? The difference in flicker between 30 and 60 is much much larger than the difference between 60 and 120! Yeah 60 isn't flicker free, any finite number is not (there is probably quantum limits), but realistically you reach a point where you can't really tell. For most purposes 60Hz is close enough, though you can still tell.

I don't remember frankly. For what it's worth, TV sets would always be 50 Hz here (PAL) (unless they did some tomfoolery I'm not aware of and ran at 100 Hz "in secret" or something) and evidently I could watch those on end without too many holdups for years and years, so clearly it wasn't a dealbreaker. But on monitors, yeah, I just wouldn't tolerate it, whereas 85 Hz felt perfect (no discernible flicker for me that…

- I hear that some later digital PAL TVs stored an image in a framebuffer and scanned it out twice at 100 Hz, which retro gamers today avoid because it increases latency relative to direct scanout.

- I've heard mixed reports over whether CRT monitors had faster-decaying phosphors than televisions. Maybe part of it is a computer has a white image, which causes more noticeable flicker than a dark background with white text (or darker TV scenes).

Re: Why the original Macintosh had a screen resolution of 512×324

#42

Post by analog31 »

pavon wrote: post #29

The article didn't nail down an exact reason. Here is my guess. The quote from Andy Hertzfeld suggests the limiting factor was the memory bandwidth not the memory volume: > The most important decision was admitting that the software would never fit into 64K of memory and going with a full 16-bit memory bus, requiring 16 RAM chips instead of 8. The extra memory bandwidth allowed him to double the display resolution, g…

A lot of those old machines had clock speeds and video pixel rates that meshed together. On some color machines the system clock was an integer multiple of the standard colorburst frequency.

The Timex Sinclair did all of its computation during the blanking interval which is why it was so dog slow.

Re: Why the original Macintosh had a screen resolution of 512×324

#44

Post by msgodel »

perching_aix wrote: post #32
bluGill wrote: post #28

Earlier quoted context omitted.

Have you ever seen something running at 30 Hz? Or even 15? The difference in flicker between 30 and 60 is much much larger than the difference between 60 and 120! Yeah 60 isn't flicker free, any finite number is not (there is probably quantum limits), but realistically you reach a point where you can't really tell. For most purposes 60Hz is close enough, though you can still tell.

I don't remember frankly. For what it's worth, TV sets would always be 50 Hz here (PAL) (unless they did some tomfoolery I'm not aware of and ran at 100 Hz "in secret" or something) and evidently I could watch those on end without too many holdups for years and years, so clearly it wasn't a dealbreaker. But on monitors, yeah, I just wouldn't tolerate it, whereas 85 Hz felt perfect (no discernible flicker for me that…

That's interesting. 60hz TVs always gave me headaches but my 75 hz computer monitor didn't.

I think it was actually the interlacing and not the refresh rate that did it.

Re: Why the original Macintosh had a screen resolution of 512×324

#45

Post by stefan_ »

pavon wrote: post #29

The article didn't nail down an exact reason. Here is my guess. The quote from Andy Hertzfeld suggests the limiting factor was the memory bandwidth not the memory volume: > The most important decision was admitting that the software would never fit into 64K of memory and going with a full 16-bit memory bus, requiring 16 RAM chips instead of 8. The extra memory bandwidth allowed him to double the display resolution, g…

Displays are still bandwidth killers today, we kept scaling them up with everything else. Today you might have a 4k 30bpp 144hz display and just keeping that fed takes 33Gbit/s purely for scanout, not even composing it.

Re: Why the original Macintosh had a screen resolution of 512×324

#46

Post by pragma_x »

pavon wrote: post #29

The article didn't nail down an exact reason. Here is my guess. The quote from Andy Hertzfeld suggests the limiting factor was the memory bandwidth not the memory volume: > The most important decision was admitting that the software would never fit into 64K of memory and going with a full 16-bit memory bus, requiring 16 RAM chips instead of 8. The extra memory bandwidth allowed him to double the display resolution, g…

It's also interesting to look at other architectures at the time to get an idea of how fiendish a problem this is. At this time, Commodore, Nintendo, and some others, had dedicated silicon for video rendering. This frees the CPU from having to generate a video signal directly, using a fraction of those cycles to talk to the video subsystem instead. The major drawback with a video chip of some kind is of course cost (custom fabrication, part count), which clearly the Macintosh team was trying to keep as low as possible.

Re: Why the original Macintosh had a screen resolution of 512×324

#47

Post by meatmanek »

bluGill wrote: post #31
Suppafly wrote: post #26

Earlier quoted context omitted.

>Whenever I saw a CRT running at 60 Hz, I'd be immediately be able to tell. Always used at minimum 75 Hz but preferably 85 Hz at home (early 2000s, Windows). Same, I remember installing some program that would let you quickly change the display settings on basically every computer I ever interacted with. It was especially bad if the crt was in a room with fluorescent lighting.

If your lighting and display have flicker at mathematical ratio you will notice unless the frequency is extremely high. 1:1 is most likely because it is easy to sync lights and the CRT to the AC line frequency which is 60Hz in the US (50Hz in Europe). 1:2 (used to be somewhat common) or 4:5 ratios would also cause issues. Though now that I think of it, the CRT should be syncing with the signal and there is no reason…

Wikipedia for NTSC alludes to a couple reasons why you'd want your refresh rate to be based on your power line frequency:

> Matching the field refresh rate to the power source avoided intermodulation (also called beating), which produces rolling bars on the screen. Synchronization of the refresh rate to the power incidentally helped kinescope cameras record early live television broadcasts, as it was very simple to synchronize a film camera to capture one frame of video on each film frame by using the alternating current frequency to set the speed of the synchronous AC motor-drive camera.

(I suspect shows that were pre-recorded and telecined for broadcast would've also been filmed at 30fps using a synchronous AC motor.)

> In early TV systems, a master voltage-controlled oscillator was run at twice the horizontal line frequency, and this frequency was divided down by the number of lines used (in this case 525) to give the field frequency (60 Hz in this case). This frequency was then compared with the 60 Hz power-line frequency and any discrepancy corrected by adjusting the frequency of the master oscillator.

I think later TVs would've just synchronized to the received signal.

https://en.wikipedia.org/wiki/NTSC#Resolution_and_refresh_ra...

Re: Why the original Macintosh had a screen resolution of 512×324

#48

Post by meatmanek »

ajross wrote: post #40
pavon wrote: post #29

The article didn't nail down an exact reason. Here is my guess. The quote from Andy Hertzfeld suggests the limiting factor was the memory bandwidth not the memory volume: > The most important decision was admitting that the software would never fit into 64K of memory and going with a full 16-bit memory bus, requiring 16 RAM chips instead of 8. The extra memory bandwidth allowed him to double the display resolution, g…

Exactly. Like the Apple ][, the original Mac framebuffer was set up with alternating accesses, relying on the framebuffer reads to manage DRAM refresh. It looks like DRAM was set up on a 6-CPU-cycle period, as 512 bits (32 16-bit bus accesses) x 342 lines x 60 Hz x 6 cycles x 2 gives 7.87968 MHz, which is just slightly faster than the nominal 7.83 MHz, the remaining .6% presumably being spent during vblank.

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.

Re: Why the original Macintosh had a screen resolution of 512×324

#50

Post by kristianp »

The folklore link they reference: https://www.folklore.org/Five_Different_Macs.html

The 1st edition of macworld, notably the first page is an advert for microsoft's products, multiplan spreadsheet, word, etc. https://archive.org/details/MacWorld_8404_April_1984_premier...

The original floppy used on the mac was a single-sided 400KB disk. I imagine that was another set of trade-offs. https://folklore.org/Disk_Swappers_Elbow.html

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