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All models of the Commodore PET line had built-in monitors, and at least the U.S. models had 60 Hz refresh. For the European models there was no need for compatibility with European television standards as far as I'm aware; did they also use 60 Hz refresh? Were there any situations in which a PET monitor did not use 60 Hz refresh?

If they all did, was the same true of the CBM-II hi-profile models with their built-in monitors?

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  • While they didn't have to use European broadcast TV standards like PAL, they did have to run off 50 Hz European power. Mains is usually the source of the 50 or 60 Hz vertical blank in a cheap CRT.
    – RETRAC
    Dec 12, 2019 at 18:01
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    @RETRAC: Mains isn't usable as a source of vertical blanking, since there's no fixed relationship between line phase and broadcast frames. On displays without good high-voltage filtering, image geometry will be affected by the phase relationship between line phase and video frames, which would create annoying shuddering if the rates weren't reasonably close, but a typical computer's video output won't be close enough to US mains frequency to prevent such effects in the absence of good HV filtering.
    – supercat
    Dec 12, 2019 at 19:51

2 Answers 2

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TL;DR:

Non-CRTC PETs (Original PET, early 30xx) are fixed to 60 Hz.

CRTC (6545) based PETs (40xx/80xx/8x96/9000 - *1) use screen refresh rates set by its ROM (*2). 50 Hz for European machines, 60 Hz for US/Canadian units.

Regarding the details:

For the European models there was no need for compatibility with European television standards as far as I'm aware;

You're right. As self contained system, there is no need to synchronize to any external source. Why Commodore switched to 50 Hz seams strange. There is no technical need nor any legal to do so.

did they also use 60 Hz refresh? Were there any situations in which a PET monitor did not use 60 Hz refresh?

Early did. Later not. The majority of European units did use 50 Hz.

was the same true of the CBM-II hi-profile models with their built-in monitors

Basically yes (as well as for external screens). So 'regular' 600/700 units for Sweden and Germany (distinction due keyboard) were using a 50 Hz refresh. Except 'regular' is a wide array here, as there is a wide variety of ROMs for the CBM-II series, all of them at best beta versions. So yes, they did use 50 Hz, but which did or did not is hard to tell.

*1 - As a generalization this means all BASIC 4.0 machines.

*2 - The ROMs differ as well within the editor ROM for keyboard layout.

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  • Presumably the 60 and 50Hz outputs otherwise match standard video timings? A 4:3 display, 63.55µs or 64µs scans, ~263 or ~312 lines per field, etc? If I took the screen out of a PET, I could probably route suitably-decoded standard definition video to it?
    – Tommy
    Dec 12, 2019 at 18:32
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    Matching CRT refresh rate to the local grid AC rate makes for a better display because it minimizes bad interactions with AC rate flicker of lights.
    – George Phillips
    Dec 12, 2019 at 18:58
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    @Tommy yes. After all, the original PET screens were pretty standard TV tubes :))
    – Raffzahn
    Dec 12, 2019 at 19:07
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    @GeorgePhillips There's always a flicker, as they are never really synchronized.
    – Raffzahn
    Dec 12, 2019 at 19:07
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    @Raffzahn: I don't think the issue is so much flicker as line-phase-dependent screen geometry. Feed a VIC-20's output into a TV with bad high-voltage filtering and the screen geometry will "breathe" somewhat annoyingly, in a way that can be reduced by enabling interlacing (which adjusts the frame rate closer to 60Hz).
    – supercat
    Dec 12, 2019 at 19:53
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The early PET models (all 2001, early 30xx) with discrete video circuitry used 60 Hz in all models worldwide.

The later PET models (later 30xx, 40xx/80xx/8x96/9000) used a 6545 CRTC chip where the ROM could set the refresh rate. (These can usually be identified by the ROM providing BASIC 4.0.) Candian/U.S. ROMs set a 60 Hz refresh rate; European ROMs set a 50 Hz refresh rate. There was indeed no technical reason to use different refresh rates; it's not known why they did this.

The CBM-II series also used different refresh rates between country models. Certainly many of the series 600/700 models in Sweden and Germany were using 50 Hz refresh. But perhaps not all: there were a wide variety of ROMs for the CBM-II systems, none fully developed, so whether any particular unit, regardless of country, used 60 Hz or 50 Hz can be hard to tell.

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  • Not really sure why you think it's a good idea to just copy information form another question. But when doing so, you should at least not twist it - like I didn't mention any difference between 600 and 700 or them being send to different countries. Also, it's not many not fully developed, but all - the CBM-II series development was never finished at all. Each and every version known got some bugs. Only he after market CBUG development is kind of a standard.
    – Raffzahn
    Dec 13, 2019 at 1:13
  • @Raffzahn I rewrote your answer because I found it hard to understand many details: a perfect example of that, as you (perhaps inadvterantly) pointed out in your comment is that the 600/700 thing.. Can you explain what I got wrong there?
    – cjs
    Dec 13, 2019 at 1:18
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    I have no idea what went wrong. For some reason you assign 600 units to Sweden and 700 to Germany. Why?
    – Raffzahn
    Dec 13, 2019 at 1:22
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    @Raffzahn Because that's exactly what you do in your answer: "600/700 units for Sweden and Germany." So what are these "600 units" and how are they relevant to the question?
    – cjs
    Dec 13, 2019 at 2:24
  • 600/700 are the CBM-II models. What else? And there are Swedish and German versions of both. And it's relevant, as they use 50 Hz refresh. And they are to be distinct from the 500s, as these are even less finalized. Here almost every unit differs.
    – Raffzahn
    Dec 13, 2019 at 14:43

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