clock_noslope

app/meson.build

@@ -277,10 +277,6 @@ if get_option('buildtype') == 'debug'
             'src/util/strbuf.c',
             'src/util/term.c',
         ]],
-        ['test_clock', [
-            'tests/test_clock.c',
-            'src/clock.c',
-        ]],
         ['test_control_msg_serialize', [
             'tests/test_control_msg_serialize.c',
             'src/control_msg.c',

app/src/clock.c

@@ -1,116 +1,34 @@
 #include "clock.h"
 
+#include <assert.h>
+
 #include "util/log.h"
 
 #define SC_CLOCK_NDEBUG // comment to debug
 
 void
 sc_clock_init(struct sc_clock *clock) {
-    clock->count = 0;
-    clock->head = 0;
-    clock->left_sum.system = 0;
-    clock->left_sum.stream = 0;
-    clock->right_sum.system = 0;
-    clock->right_sum.stream = 0;
-}
-
-// Estimate the affine function f(stream) = slope * stream + offset
-static void
-sc_clock_estimate(struct sc_clock *clock,
-                  double *out_slope, sc_tick *out_offset) {
-    assert(clock->count);
-
-    if (clock->count == 1) {
-        // If there is only 1 point, we can't compute a slope. Assume it is 1.
-        struct sc_clock_point *single_point = &clock->right_sum;
-        *out_slope = 1;
-        *out_offset = single_point->system - single_point->stream;
-        return;
-    }
-
-    struct sc_clock_point left_avg = {
-        .system = clock->left_sum.system / (clock->count / 2),
-        .stream = clock->left_sum.stream / (clock->count / 2),
-    };
-    struct sc_clock_point right_avg = {
-        .system = clock->right_sum.system / ((clock->count + 1) / 2),
-        .stream = clock->right_sum.stream / ((clock->count + 1) / 2),
-    };
-
-    double slope = (double) (right_avg.system - left_avg.system)
-                 / (right_avg.stream - left_avg.stream);
-
-    if (clock->count < SC_CLOCK_RANGE) {
-        /* The first frames are typically received and decoded with more delay
-         * than the others, causing a wrong slope estimation on start. To
-         * compensate, assume an initial slope of 1, then progressively use the
-         * estimated slope. */
-        slope = (clock->count * slope + (SC_CLOCK_RANGE - clock->count))
-              / SC_CLOCK_RANGE;
-    }
-
-    struct sc_clock_point global_avg = {
-        .system = (clock->left_sum.system + clock->right_sum.system)
-                 / clock->count,
-        .stream = (clock->left_sum.stream + clock->right_sum.stream)
-                 / clock->count,
-    };
-
-    sc_tick offset = global_avg.system - (sc_tick) (global_avg.stream * slope);
-
-    *out_slope = slope;
-    *out_offset = offset;
+    clock->range = 0;
+    clock->offset = 0;
 }
 
 void
 sc_clock_update(struct sc_clock *clock, sc_tick system, sc_tick stream) {
-    struct sc_clock_point *point = &clock->points[clock->head];
-
-    if (clock->count == SC_CLOCK_RANGE || clock->count & 1) {
-        // One point passes from the right sum to the left sum
-
-        unsigned mid;
-        if (clock->count == SC_CLOCK_RANGE) {
-            mid = (clock->head + SC_CLOCK_RANGE / 2) % SC_CLOCK_RANGE;
-        } else {
-            // Only for the first frames
-            mid = clock->count / 2;
-        }
-
-        struct sc_clock_point *mid_point = &clock->points[mid];
-        clock->left_sum.system += mid_point->system;
-        clock->left_sum.stream += mid_point->stream;
-        clock->right_sum.system -= mid_point->system;
-        clock->right_sum.stream -= mid_point->stream;
+    if (clock->range < SC_CLOCK_RANGE) {
+        ++clock->range;
     }
 
-    if (clock->count == SC_CLOCK_RANGE) {
-        // The current point overwrites the previous value in the circular
-        // array, update the left sum accordingly
-        clock->left_sum.system -= point->system;
-        clock->left_sum.stream -= point->stream;
-    } else {
-        ++clock->count;
-    }
-
-    point->system = system;
-    point->stream = stream;
-
-    clock->right_sum.system += system;
-    clock->right_sum.stream += stream;
-
-    clock->head = (clock->head + 1) % SC_CLOCK_RANGE;
-
-    // Update estimation
-    sc_clock_estimate(clock, &clock->slope, &clock->offset);
+    sc_tick offset = system - stream;
+    clock->offset = ((clock->range - 1) * clock->offset + offset)
+                  / clock->range;
 
 #ifndef SC_CLOCK_NDEBUG
-    LOGD("Clock estimation: %f * pts + %" PRItick, clock->slope, clock->offset);
+    LOGD("Clock estimation: pts + %" PRItick, clock->offset);
 #endif
 }
 
 sc_tick
 sc_clock_to_system_time(struct sc_clock *clock, sc_tick stream) {
-    assert(clock->count); // sc_clock_update() must have been called
-    return (sc_tick) (stream * clock->slope) + clock->offset;
+    assert(clock->range); // sc_clock_update() must have been called
+    return stream + clock->offset;
 }

app/src/clock.h

@@ -3,12 +3,9 @@
 
 #include "common.h"
 
-#include <assert.h>
-
 #include "util/tick.h"
 
 #define SC_CLOCK_RANGE 32
-static_assert(!(SC_CLOCK_RANGE & 1), "SC_CLOCK_RANGE must be even");
 
 struct sc_clock_point {
     sc_tick system;
@@ -21,40 +18,18 @@ struct sc_clock_point {
  *
  *     f(stream) = slope * stream + offset
  *
- * To that end, it stores the SC_CLOCK_RANGE last clock points (the timestamps
- * of a frame expressed both in stream time and system time) in a circular
- * array.
- *
- * To estimate the slope, it splits the last SC_CLOCK_RANGE points into two
- * sets of SC_CLOCK_RANGE/2 points, and computes their centroid ("average
- * point"). The slope of the estimated affine function is that of the line
- * passing through these two points.
+ * Theoretically, the slope encodes the drift between the device clock and the
+ * computer clock. It is expected to be very close to 1.
  *
- * To estimate the offset, it computes the centroid of all the SC_CLOCK_RANGE
- * points. The resulting affine function passes by this centroid.
+ * Since the clock is used to estimate very close points in the future (which
+ * are reestimated on every clock update, see delay_buffer), the error caused
+ * by clock drift is totally negligible, so it is better to assume that the
+ * slope is 1 than to estimate it (the estimation error would be larger).
  *
- * With a circular array, the rolling sums (and average) are quick to compute.
- * In practice, the estimation is stable and the evolution is smooth.
+ * Therefore, only the offset is estimated.
  */
 struct sc_clock {
-    // Circular array
-    struct sc_clock_point points[SC_CLOCK_RANGE];
-
-    // Number of points in the array (count <= SC_CLOCK_RANGE)
-    unsigned count;
-
-    // Index of the next point to write
-    unsigned head;
-
-    // Sum of the first count/2 points
-    struct sc_clock_point left_sum;
-
-    // Sum of the last (count+1)/2 points
-    struct sc_clock_point right_sum;
-
-    // Estimated slope and offset
-    // (computed on sc_clock_update(), used by sc_clock_to_system_time())
-    double slope;
+    unsigned range;
     sc_tick offset;
 };
 

app/src/delay_buffer.c

@@ -194,7 +194,7 @@ sc_delay_buffer_frame_sink_push(struct sc_frame_sink *sink,
     sc_clock_update(&db->clock, sc_tick_now(), pts);
     sc_cond_signal(&db->wait_cond);
 
-    if (db->first_frame_asap && db->clock.count == 1) {
+    if (db->first_frame_asap && db->clock.range == 1) {
         sc_mutex_unlock(&db->mutex);
         return sc_frame_source_sinks_push(&db->frame_source, frame);
     }

app/tests/test_clock.c

@@ -1,79 +0,0 @@
-#include "common.h"
-
-#include <assert.h>
-
-#include "clock.h"
-
-void test_small_rolling_sum(void) {
-    struct sc_clock clock;
-    sc_clock_init(&clock);
-
-    assert(clock.count == 0);
-    assert(clock.left_sum.system == 0);
-    assert(clock.left_sum.stream == 0);
-    assert(clock.right_sum.system == 0);
-    assert(clock.right_sum.stream == 0);
-
-    sc_clock_update(&clock, 2, 3);
-    assert(clock.count == 1);
-    assert(clock.left_sum.system == 0);
-    assert(clock.left_sum.stream == 0);
-    assert(clock.right_sum.system == 2);
-    assert(clock.right_sum.stream == 3);
-
-    sc_clock_update(&clock, 10, 20);
-    assert(clock.count == 2);
-    assert(clock.left_sum.system == 2);
-    assert(clock.left_sum.stream == 3);
-    assert(clock.right_sum.system == 10);
-    assert(clock.right_sum.stream == 20);
-
-    sc_clock_update(&clock, 40, 80);
-    assert(clock.count == 3);
-    assert(clock.left_sum.system == 2);
-    assert(clock.left_sum.stream == 3);
-    assert(clock.right_sum.system == 50);
-    assert(clock.right_sum.stream == 100);
-
-    sc_clock_update(&clock, 400, 800);
-    assert(clock.count == 4);
-    assert(clock.left_sum.system == 12);
-    assert(clock.left_sum.stream == 23);
-    assert(clock.right_sum.system == 440);
-    assert(clock.right_sum.stream == 880);
-}
-
-void test_large_rolling_sum(void) {
-    const unsigned half_range = SC_CLOCK_RANGE / 2;
-
-    struct sc_clock clock1;
-    sc_clock_init(&clock1);
-    for (unsigned i = 0; i < 5 * half_range; ++i) {
-        sc_clock_update(&clock1, i, 2 * i + 1);
-    }
-
-    struct sc_clock clock2;
-    sc_clock_init(&clock2);
-    for (unsigned i = 3 * half_range; i < 5 * half_range; ++i) {
-        sc_clock_update(&clock2, i, 2 * i + 1);
-    }
-
-    assert(clock1.count == SC_CLOCK_RANGE);
-    assert(clock2.count == SC_CLOCK_RANGE);
-
-    // The values before the last SC_CLOCK_RANGE points in clock1 should have
-    // no impact
-    assert(clock1.left_sum.system == clock2.left_sum.system);
-    assert(clock1.left_sum.stream == clock2.left_sum.stream);
-    assert(clock1.right_sum.system == clock2.right_sum.system);
-    assert(clock1.right_sum.stream == clock2.right_sum.stream);
-}
-
-int main(int argc, char *argv[]) {
-    (void) argc;
-    (void) argv;
-
-    test_small_rolling_sum();
-    test_large_rolling_sum();
-    return 0;
-};