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CLK/Machines/Apple/Macintosh/DriveSpeedAccumulator.cpp
Thomas Harte bac0461f7f Switches the drive-speed accumulator to the delegate pattern.
This allows the Macintosh to ensure that the IWM is kept up just-in-time with drive speed changes.
2019-08-07 21:39:23 -04:00

57 lines
2.0 KiB
C++

//
// DriveSpeedAccumulator.cpp
// Clock Signal
//
// Created by Thomas Harte on 01/06/2019.
// Copyright © 2019 Thomas Harte. All rights reserved.
//
#include "DriveSpeedAccumulator.hpp"
using namespace Apple::Macintosh;
void DriveSpeedAccumulator::post_sample(uint8_t sample) {
if(!delegate_) return;
// An Euler-esque approximation is used here: just collect all
// the samples until there is a certain small quantity of them,
// then produce a new estimate of rotation speed and start the
// buffer afresh.
samples_[sample_pointer_] = sample;
++sample_pointer_;
if(sample_pointer_ == samples_.size()) {
sample_pointer_ = 0;
// The below fits for a function like `a + bc`; it encapsultes the following
// beliefs:
//
// (i) motor speed is proportional to voltage supplied;
// (ii) with pulse-width modulation it's therefore proportional to the duty cycle;
// (iii) the Mac pulse-width modulates whatever it reads from the disk speed buffer;
// (iv) ... subject to software pulse-width modulation of that pulse-width modulation.
//
// So, I believe current motor speed is proportional to a low-pass filtering of
// the speed buffer. Which I've implemented very coarsely via 'large' bucketed-averages,
// noting also that exact disk motor speed is always a little approximate.
// Sum all samples.
// TODO: if the above is the correct test, do it on sample receipt rather than
// bothering with an intermediate buffer.
int sum = 0;
for(auto s: samples_) {
sum += s;
}
// The formula below was derived from observing values the Mac wrote into its
// disk-speed buffer. Given that it runs a calibration loop before doing so,
// I cannot guarantee the accuracy of these numbers beyond being within the
// range that the computer would accept.
const float normalised_sum = float(sum) / float(samples_.size());
const float rotation_speed = (normalised_sum * 27.08f) - 259.0f;
delegate_->drive_speed_accumulator_set_drive_speed(this, rotation_speed);
}
}