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Implements latest advocated MC3470 behaviour.

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This commit is contained in:
Thomas Harte 2020-07-15 19:34:05 -04:00
parent d065d6d98f
commit aed61f6251
1 changed files with 18 additions and 8 deletions
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26
Storage/Disk/Drive.cpp
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@ -250,6 +250,15 @@ void Drive::run_for(const Cycles cycles) {
// MARK: - Track timed event loop
void Drive::get_next_event(float duration_already_passed) {
/*
Quick word on random-bit generation logic below; it seeks to obey the following logic:
if there is a gap of 15µs between recorded bits, start generating flux transitions
at random intervals thereafter, unless and until one is within 5µs of the next real transition.
This behaviour is based on John Morris' observations of an MC3470, as described in his WOZ
file format documentation — https://applesaucefdc.com/woz/reference2/
*/
if(!disk_) {
current_event_.type = Track::Event::IndexHole;
current_event_.length = 1.0f;
@ -268,17 +277,18 @@ void Drive::get_next_event(float duration_already_passed) {
// If gain has now been turned up so as to generate noise, generate some noise.
if(random_interval_ > 0.0f) {
current_event_.type = Track::Event::FluxTransition;
current_event_.length = float(2 + (random_source_&1)) / 1000000.0f;
current_event_.length = float(2 + (random_source_&1)) / 1'000'000.0f;
random_source_ = (random_source_ >> 1) | (random_source_ << 63);
if(random_interval_ < current_event_.length) {
current_event_.length = random_interval_;
// If this random transition is closer than 5µs to the next real bit,
// discard it.
if(random_interval_ < current_event_.length - 5.0f / 1'000'000.f) {
random_interval_ = 0.0f;
} else {
random_interval_ -= current_event_.length;
set_next_event_time_interval(current_event_.length);
return;
}
set_next_event_time_interval(current_event_.length);
return;
}
if(track_) {
@ -299,9 +309,9 @@ void Drive::get_next_event(float duration_already_passed) {
// convert it into revolutions per second; this is achieved by multiplying by rotational_multiplier_
float interval = std::max((current_event_.length - duration_already_passed) * rotational_multiplier_, 0.0f);
// An interval greater than 15ms => adjust gain up the point where noise starts happening.
// Seed that up and leave a 15ms gap until it starts.
constexpr float safe_gain_period = 15.0f / 1000000.0f;
// An interval greater than 15µs => adjust gain up the point where noise starts happening.
// Seed that up and leave a 15µs gap until it starts.
constexpr float safe_gain_period = 15.0f / 1'000'000.0f;
if(interval >= safe_gain_period) {
random_interval_ = interval - safe_gain_period;
interval = safe_gain_period;