% YM2151-style Envelope Generator with proper gate handling (AR, D1R, D2R, RR)
% 2025-07-16 mchris

clear; close all; clc;

% Sampling and timing
Fs = 44100;
T_total = 3.0;
t = linspace(0, T_total, Fs*T_total);

% Envelope parameters
AR  = 0.02;   % Attack time (seconds)
D1R = 0.3;    % Decay 1 time (seconds)
SL  = 0.5;    % Sustain level (0–1)
D2R = 5.0;    % Decay 2 time (seconds) — optional decay below SL
RR  = 0.1;    % Release time (seconds)
gateTime = 2.0;  % When gate goes low (key released)

% Sample indices
attack_end = round(AR * Fs);
decay1_end = attack_end + round(D1R * Fs);
gate_idx = round(gateTime * Fs);

env = zeros(size(t));
release_start_val = 0;
released = false;

for i = 1:length(t)
    if i <= attack_end
        % Attack
        env(i) = (i - 1) / attack_end;

    elseif i <= decay1_end
        % Decay 1: from 1 → SL
        env(i) = 1 - ((i - attack_end) / (decay1_end - attack_end)) * (1 - SL);

    elseif t(i) <= gateTime
        % Decay 2: from SL → 0 (or holds if D2R = 0)
        if D2R > 0
            decay2_progress = (t(i) - t(decay1_end)) / D2R;
            env(i) = SL * (1 - decay2_progress);
            env(i) = max(env(i), 0);  % don't go below zero
        else
            env(i) = SL;  % sustain level is held
        end
        release_start_val = env(i);  % continuously update release start value

    else
        % Release phase begins after gate low
        if ~released
            released = true;
            release_start_idx = i;
        end
        release_progress = (i - release_start_idx) / (RR * Fs);
        env(i) = release_start_val * (1 - release_progress);
        env(i) = max(env(i), 0);
    end
end

% Plot
plot(t, env, 'LineWidth', 2);
xlabel('Time (s)');
ylabel('Amplitude');
title('YM2151 Envelope Generator: AR → D1R → SL → D2R → RR');
grid on;
ylim([-0.1, 1.1]);