The C64 VIC-II chip would grab the address bus from the CPU every 8 scan lines on the screen. Some of the early "fast load" cartridges like the Epyx FastLoad cartridge that would accelerate loading games from the floppy drive would blank the entire screen during load so that their async data transfer routines wouldn't get interrupted by the VIC-II chip grabbing the bus. I wrote a similar (better?) cartridge where I would need to use the register on the VIC-II chip that reported the scan line as a sync marker to transfer 3 bytes asynchronously from the 1541 down the clock and data lines of the serial bus. Good times.
In my recollection Epyx Fastload did not blank the screen, though some earlier fast loaders did.
I also remember the software voice synthesizer "SAM" needing to blank the screen to render glitch-free sampled audio. Then along came, what was it "Impossible Mission" ("Another visitor! Stay a while...") doing pretty clean sampled audio with the screen on. Not that the C64 SID chip was even remotely intended to be able to play sampled audio in the first place!
The Amiga was unimaginably powerful by comparison. Even a basic configuration had 8x the memory a C64 had, and it had all those fancy DMA toys to offload the CPU.
> Not that the C64 SID chip was even remotely intended to be able to play sampled audio in the first place!
I don't recall how SAM did it, but sample playing on the C64 SID chip was indeed a nice trick — it was actually done by modulating the main output volume, which made a slight click when changing.
Eventually this got used by some of the C64 musicians / music player libs, so one could play a channel of samples as well as the three regular synth channels on the SID. IIRC, Outrun used this particularly well in its title screen and/or loading music, having some vocal samples "O-O-Outrun!" (and skidding sound effects) as well a sampled drums.
Annoyingly, IIRC, some revisions of the SID chip behaved slightly differently, and had louder or softer sample playback when this hack was used. But still: clever stuff.
... and main output volume had only 16 levels so the samples were 4 bit quantized. It is a wonder that we could get understandable vocal samples with this hack at all. I distinctly remember "Goal!" from Peter Shilton's Fottball and "Accolade presents" from Test Drive [1]. In the examples one can hear the amount of quantization noise that low bit depth caused.
You can actually get about 6-7 bits resolution out of same SID volume register. 4 bits from the volume register, channel 3 disable and 3 filter bits. Requires some setup to get SID in a particular state first.
I was just wondering tonight, while lying awake, if you could get more depth by either using the other voices (probably not) or the filter. Now this paper and the demo are so cool, really made my day.
>The Amiga was unimaginably powerful by comparison.
Interesting that the two systems were only about two years apart in development. The first C64 prototype would have been in 1981 and the Amiga 1983 but of course they were targeting different price points.
