Hacker News Reader

Comment by analog31 | Hacker News Reader

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.

View on HN

There’s an interesting blog post about how a far simpler machine generates its video signal, if people are curious about the signals involved:

http://blog.tynemouthsoftware.co.uk/2023/10/how-the-zx80-gen...

“The CPU then only produces a TV picture when BASIC is waiting for input (or paused). At other times it does not bother to produce a video picture, so the CPU can run the program at full speed.”

The Commodore Amigas had their 68k clock speed differ based on region due to carrier frequency difference (more specifically, 2x freq for NTSC, 1.6x for PAL, which resulted in almost the same, but not quite, clock speed).

It's interesting how the differing vertical resolutions between these two (200p /400i vs 256p /512i) also had some secondary effects on software design, it was always easy to tell if a game was made in NTSC regions or with global releases in mind because the bottom 20% of the screen was black in PAL.

To save the curious a search: the Timex Sinclair is the American variant of the ZX Spectrum.

The ZX Spectrum had (primitive) video hardware. The GP commenter means the ZX80 and ZX81 which used the Z80 CPU to generate the display and so really were unable to both "think" and generate the display at the same time. On the ZX81 there were two modes, SLOW mode and FAST mode. In FAST mode the Z80 CPU prioritized computations over generating the display, so the display would go fuzzy grey while programs were running, then would reappear when the program ended or it was waiting for keyboard input.

You're right! I was thinking of the Timex Sinclair 2068. It was preceded by the 1000 and 1500 (ZX80 & ZX81 respectively as you say).

There was an adventure game that showed “The Mists of Time” for 30 seconds during initial map generation - it was a very creative way to describe the analogue tv noise caused by missing video signal.

Reply on news.ycombinator.com