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The Motorola MC6847 Video Display Generator (VDG) generates video output with NTSC timings. (It does not do NTSC colour, however; it gives luma, R-Y and B-Y signals that need an external NTSC colour encoder.)

According to Wikipedia: "The Dragon has additional circuitry to make the MC6847 VDG compatible with European 625-line television standards, rather than the US 525-line NTSC standard." This would presumably involve not only adding the extra horizontal lines, but also somehow converting the 60 Hz frame rate to 50 Hz. (PAL colour is presumably easily dealt with by simply substituting whatever NTSC colour encoder that would normally be used with a PAL one instead.)

How exactly did it do this?

There are some schematics available here that may be a useful reference.

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I used the redrawn schematics here on Github as it is difficult to read part numbers from the scans, and also it documents some detail quite well.

You are right that it does expand the line count to be compatible with 50 Hz timing. I will not go much into details yet but will give an overview, as it takes time to figure out the exact detals.

There is a circuit made of logic chips that adds 50 lines per field, by triggering on the VSYNC output. A multiplexer switches where the video output is taken, from the chip, or from the empty line generator circuit, which generates a TV line with proper length and sync pulse (and burst?) timings based on the VCLK pixel clock.

During the time of generating the extra lines, the clock to the MC6847 is stopped and resumed after the extra lines are generated.

So in short, a circuit generates empty lines while the MC6847 is stopped to add in the lines needed to bring down the 60Hz VSYNC rate to 50Hz. So the VSYNC rate does not need a separate conversion, just adding more lines between VSYNCs already does that as the HSYNC line rate stays constant.

The YUV (YPbPr) output generated is then simply modulated with PAL colour encoding for composite output, but in this case the phase alternation (PAL switch) is also done manually by flipping the polarity of one of the colour difference signals for each line by using the HSYNC pulse.

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    And the almost 1% difference in line rate (31.469kHz for color NTSC vs. 31.250kHz for PAL) can be mostly ignored, although later Dragons apparently replaced the SAM's usual 14.318MHz crystal with a 14.218MHz one, which happens to bring the line rate almost perfectly into spec.
    – hobbs
    Aug 10, 2023 at 13:19
  • @hobbs Where is the crystal 14.218 MHz, in the reverse-engineered schematics? I think they just rounded 14.31818 MHz to 14.32 MHz in the scanned schematics I saw, so I was not aware it was changed. Yes the NTSC vs PAL line rate difference is usually irrelevant, but the line rates are exactly half than what you say, 15.734 and 15.625 kHz.
    – Justme
    Aug 10, 2023 at 19:34
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    XL2, in the upper-left, just to the left of IC15 (the SAM). Corroborated by some other sources if you search on "14.218 MHz".
    – hobbs
    Aug 10, 2023 at 20:25
  • (and yes, I was off by x2)
    – hobbs
    Aug 10, 2023 at 20:26

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