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Adds some quick arithmetic on the clock speed.

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Thomas Harte 2021-03-20 11:19:44 -04:00
parent 26911a16e8
commit 07a63d62dd
1 changed files with 26 additions and 1 deletions
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27
Machines/Sinclair/ZXSpectrum/ZXSpectrum.cpp
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@ -82,7 +82,32 @@ template<Model model> class ConcreteMachine:
static constexpr unsigned int clock_rate() {
// constexpr unsigned int ClockRate = 3'500'000;
constexpr unsigned int Plus3ClockRate = 3'546'900;
constexpr unsigned int Plus3ClockRate = 3'546'875; // See notes below; this is a guess.
// Notes on timing for the +2a and +3:
//
// Standard PAL produces 283.7516 colour cycles per line, each line being 64µs.
// The oft-quoted 3.5469 Mhz would seem to imply 227.0016 clock cycles per line.
// Since those Spectrums actually produce 228 cycles per line, but software like
// Chromatrons seems to assume a fixed phase relationship, I guess that the real
// clock speed is whatever gives:
//
// 228 / [cycles per line] * 283.7516 = [an integer].
//
// i.e. 228 * 283.7516 = [an integer] * [cycles per line], such that cycles per line ~= 227
// ... which would imply that 'an integer' is probably 285, i.e.
//
// 228 / [cycles per line] * 283.7516 = 285
// => 227.00128 = [cycles per line]
// => clock rate = 3.546895 Mhz?
//
// That is... unless I'm mistaken about the PAL colour subcarrier and it's actually 283.75,
// which would give exactly 227 cycles/line and therefore 3.546875 Mhz.
//
// A real TV would be likely to accept either, I guess. But it does seem like
// the Spectrum is a PAL machine with a fixed colour phase relationship. For
// this emulator's world, that's a first!
return Plus3ClockRate;
}