[vcf-midatlantic] Original Macintosh Architecture Questions

David Riley fraveydank at gmail.com
Thu Jun 24 15:21:22 UTC 2021

On Jun 23, 2021, at 4:24 PM, John Heritage via vcf-midatlantic <vcf-midatlantic at lists.vcfed.org> wrote:
> Hey folks --
> Two items I'm curious about on the classic (and very original) Apple Macs..
> 1.  Mac (128kb ,512kb, etc) audio -- was there a custom or off the shelf
> sound chip for the audio on the original Mac?  and was it 1 channel / and
> was it a custom chip by Apple?  (how did it achieve speech synthesis in
> 1984)

It used some extra gates on the PALs in the timing and/or memory controllers to do a PWM which was then filtered down by an RC circuit to make a crude (but effective) DAC, similar to how delta-sigma DACs work.  A similar circuit did the speed control for the floppy drive, which slowed down as it approached the outer tracks.  Both took the PWM width data from unused bytes at the end of the video rows as the video scanning circuit went through horizontal refresh, so there weren't any real wasted cycles.

> 2.  The Macintosh Classic released in 1990 "was about 25% faster than the
> original Macs".  This is in line with the Atari ST often being quoted as
> faster than a Mac when emulating a Mac via Magic Sac or similar.  What
> changed in the architecture to make the Mac Classic (or an ST in emulation)
> faster than the original Mac?  Was it faster memory that gave more
> availability for CPU usage?  were only certain instructions sped up or was
> everything faster?

It was actually improvements in the bus timing PALs that made the cycles a little tighter, allowing the CPU to run faster because it wasn't waiting for RAM as long.  Jecel Assumpciao has/had some really good stuff on the memory timing generator (including the sound/floppy PWM) here: http://archive.retro.co.za/mirrors/homepage.ntlworld.com/kryten_droid/AppleMac/apple_mac_logic.htm

In any case, it was still an 8 MHz 68000, but the improved memory timings did yield a performance improvement.

- Dave

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