libavfilter/af_atempo: Avoid round-off error build-up, ticket #2484

Current method for constraining fragment position drift suffers from
round-off error build up.

Instead of calculating cumulative drift as a sum of input fragment
position corrections, it is more accurate to calculate drift as the
difference between current fragment position and the ideal position
specified by the tempo scale factor.

Signed-off-by: Pavel Koshevoy <pkoshevoy@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavfilter/af_atempo.c

@@ -123,8 +123,9 @@ typedef struct {
     // tempo scaling factor:
     double tempo;
 
-    // cumulative alignment drift:
+    // a snapshot of previous fragment input and output position values
-    int drift;
+    // captured when the tempo scale factor was set most recently:
+    int64_t origin[2];
 
     // current/previous fragment ring-buffer:
     AudioFragment frag[2];
@@ -159,6 +160,16 @@ static const AVOption atempo_options[] = {
 
 AVFILTER_DEFINE_CLASS(atempo);
 
+inline static AudioFragment *yae_curr_frag(ATempoContext *atempo)
+{
+    return &atempo->frag[atempo->nfrag % 2];
+}
+
+inline static AudioFragment *yae_prev_frag(ATempoContext *atempo)
+{
+    return &atempo->frag[(atempo->nfrag + 1) % 2];
+}
+
 /**
  * Reset filter to initial state, do not deallocate existing local buffers.
  */
@@ -168,13 +179,15 @@ static void yae_clear(ATempoContext *atempo)
     atempo->head = 0;
     atempo->tail = 0;
 
-    atempo->drift = 0;
     atempo->nfrag = 0;
     atempo->state = YAE_LOAD_FRAGMENT;
 
     atempo->position[0] = 0;
     atempo->position[1] = 0;
 
+    atempo->origin[0] = 0;
+    atempo->origin[1] = 0;
+
     atempo->frag[0].position[0] = 0;
     atempo->frag[0].position[1] = 0;
     atempo->frag[0].nsamples    = 0;
@@ -308,6 +321,7 @@ static int yae_reset(ATempoContext *atempo,
 
 static int yae_set_tempo(AVFilterContext *ctx, const char *arg_tempo)
 {
+    const AudioFragment *prev;
     ATempoContext *atempo = ctx->priv;
     char   *tail = NULL;
     double tempo = av_strtod(arg_tempo, &tail);
@@ -323,20 +337,13 @@ static int yae_set_tempo(AVFilterContext *ctx, const char *arg_tempo)
         return AVERROR(EINVAL);
     }
 
+    prev = yae_prev_frag(atempo);
+    atempo->origin[0] = prev->position[0] + atempo->window / 2;
+    atempo->origin[1] = prev->position[1] + atempo->window / 2;
     atempo->tempo = tempo;
     return 0;
 }
 
-inline static AudioFragment *yae_curr_frag(ATempoContext *atempo)
-{
-    return &atempo->frag[atempo->nfrag % 2];
-}
-
-inline static AudioFragment *yae_prev_frag(ATempoContext *atempo)
-{
-    return &atempo->frag[(atempo->nfrag + 1) % 2];
-}
-
 /**
  * A helper macro for initializing complex data buffer with scalar data
  * of a given type.
@@ -689,12 +696,21 @@ static int yae_adjust_position(ATempoContext *atempo)
     const AudioFragment *prev = yae_prev_frag(atempo);
     AudioFragment       *frag = yae_curr_frag(atempo);
 
+    const double prev_output_position =
+        (double)(prev->position[1] - atempo->origin[1] + atempo->window / 2);
+
+    const double ideal_output_position =
+        (double)(prev->position[0] - atempo->origin[0] + atempo->window / 2) /
+        atempo->tempo;
+
+    const int drift = (int)(prev_output_position - ideal_output_position);
+
     const int delta_max  = atempo->window / 2;
     const int correction = yae_align(frag,
                                      prev,
                                      atempo->window,
                                      delta_max,
-                                     atempo->drift,
+                                     drift,
                                      atempo->correlation,
                                      atempo->complex_to_real);
 
@@ -704,9 +720,6 @@ static int yae_adjust_position(ATempoContext *atempo)
 
         // clear so that the fragment can be reloaded:
         frag->nsamples = 0;
-
-        // update cumulative correction drift counter:
-        atempo->drift += correction;
     }
 
     return correction;