Add new 96ppq sequencer tick timer

1 files changed, 34 insertions, 9 deletions

diff --git a/src/sequencer.c b/src/sequencer.c
index 632a777..11c0f5a 100644
--- a/src/sequencer.c
+++ b/src/sequencer.c
@@ -346,21 +346,46 @@ play_step(void) {
     step_counter = (step_counter + 1) % 16;
 }
 
+static nseq_ticks = 0;
+void
+sequencer_tick(void) {
+    if (nseq_ticks++ < 24) {
+        return;
+    }
+    nseq_ticks = 0;
+
+    // NOTE: ACK interrupt before play_step???
+    play_step();
+}
+
 void
 set_time(int bpm) {
     if (settings.sync == SYNC_IN_LINK) {
         return;
     }
-    // The number of ticks of a 1024 cycle clock in a step based on the BPM can
-    // be calculated as:
-    //     X bpm -> 60000 / 4 / bpm = Y ms = Ye-3 s
-    //     Y ms  -> Ye-3 / 59.99e-9 / 1024 = Z ticks
-    // We have to operate on integer values, so the numbers have been
-    // precalculated to `n_ticks = 244181 / bmp`
-    int n_ticks = -244181 / bpm;
-    irs_set(IRQ_TIMER_2, play_step);
+    // We use a high resolution 96 PPQ clock for sequencing. With the following
+    // formula we can calculate the time (in seconds) based on the PPQ and BPM:
+    //
+    //      t = 60 / (PPQ * BPM)
+    //
+    // To translate this timer to a GBA timer running at 64 CPU cycles per tick
+    // we need to divide this by the constant Q = 3.97329943e-06. In theory, the
+    // period of this timer running a a frequency of 262.21kHz should be
+    // Q = 3.815us, but using this number resulted in a less accurate sequencer
+    // compared to the metronome of a DAW (Bitwig). Tuning Q using a japanese
+    // GBA-SP and an Analogue Pocket resulted in the previously mentioned value.
+    //
+    // If we factor out the BPM we can obtain a fixed constant:
+    //
+    //      C = 60 / (96 * 3.97329943e-06) = 157300
+    //
+    // Allowing us to calculate the clock timer as -C/BPM. Note that this is
+    // integer division and we will be losing the fractional part, resulting in
+    // some minor precision differences, but it seems to work well in practice.
+    int n_ticks = -157300 / bpm;
+    irs_set(IRQ_TIMER_2, sequencer_tick);
     TIMER_DATA_2 = n_ticks;
-    TIMER_CTRL_2 = TIMER_CTRL_IRQ | TIMER_CTRL_ENABLE | TIMER_CTRL_FREQ_3;
+    TIMER_CTRL_2 = TIMER_CTRL_IRQ | TIMER_CTRL_ENABLE | TIMER_CTRL_FREQ_1;
 }
 
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