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86 lines
3.1 KiB
C++
86 lines
3.1 KiB
C++
//
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// ScanSynchroniser.hpp
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// Clock Signal
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//
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// Created by Thomas Harte on 09/02/2020.
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// Copyright © 2020 Thomas Harte. All rights reserved.
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//
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#pragma once
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#include "../Outputs/ScanTarget.hpp"
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#include <cmath>
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namespace Time {
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/*!
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Where an emulated machine is sufficiently close to a host machine's frame rate that a small nudge in
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its speed multiplier will bring it into frame synchronisation, the ScanSynchroniser provides a sequence of
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speed multipliers designed both to adjust the machine to the proper speed and, in a reasonable amount
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of time, to bring it into phase.
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*/
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class ScanSynchroniser {
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public:
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/*!
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@returns @c true if the emulated machine can be synchronised with the host frame output based on its
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current @c [scan]status and the host machine's @c frame_duration; @c false otherwise.
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*/
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bool can_synchronise(const Outputs::Display::ScanStatus &scan_status, double frame_duration) {
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ratio_ = 1.0;
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if(scan_status.field_duration_gradient < 0.00001) {
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// Check out the machine's current frame time.
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// If it's within 3% of a non-zero integer multiple of the
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// display rate, mark this time window to be split over the sync.
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ratio_ = (frame_duration * base_multiplier_) / scan_status.field_duration;
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const double integer_ratio = round(ratio_);
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if(integer_ratio > 0.0) {
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ratio_ /= integer_ratio;
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return ratio_ <= maximum_rate_adjustment && ratio_ >= 1.0 / maximum_rate_adjustment;
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}
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}
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return false;
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}
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/*!
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@returns The appropriate speed multiplier for the next frame based on the inputs previously supplied to @c can_synchronise.
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Results are undefined if @c can_synchroise returned @c false.
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*/
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double next_speed_multiplier(const Outputs::Display::ScanStatus &scan_status) {
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// The host versus emulated ratio is calculated based on the current perceived frame duration of the machine.
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// Either that number is exactly correct or it's already the result of some sort of low-pass filter. So there's
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// no benefit to second guessing it here — just take it to be correct.
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//
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// ... with one slight caveat, which is that it is desireable to adjust phase here, to align vertical sync points.
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// So the set speed multiplier may be adjusted slightly to aim for that.
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double speed_multiplier = 1.0 / (ratio_ / base_multiplier_);
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if(scan_status.current_position > 0.0) {
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if(scan_status.current_position < 0.5) speed_multiplier /= phase_adjustment_ratio;
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else speed_multiplier *= phase_adjustment_ratio;
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}
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speed_multiplier_ = (speed_multiplier_ * 0.95) + (speed_multiplier * 0.05);
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return speed_multiplier_ * base_multiplier_;
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}
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void set_base_speed_multiplier(double multiplier) {
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base_multiplier_ = multiplier;
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}
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double get_base_speed_multiplier() {
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return base_multiplier_;
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}
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private:
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static constexpr double maximum_rate_adjustment = 1.03;
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static constexpr double phase_adjustment_ratio = 1.005;
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// Managed local state.
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double speed_multiplier_ = 1.0;
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double base_multiplier_ = 1.0;
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// Temporary storage to bridge the can_synchronise -> next_speed_multiplier gap.
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double ratio_ = 1.0;
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};
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}
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