audio: change how filters are inserted on playback speed changes

Use a pseudo-filter when changing speed with resampling, instead of
somehow changing a samplerate somewhere. This uses the same underlying
mechanism, but is a bit more structured and cleaner. It also makes some
of the following changes easier.

Since we now always use filters to change audio speed, move most of the
work set_playback_speed() does to recreate_audio_filters().

audio/decode/dec_audio.c

@@ -227,7 +227,6 @@ static int filter_n_bytes(struct dec_audio *da, struct mp_audio_buffer *outbuf,
     // Filter
     struct mp_audio filter_data;
     mp_audio_buffer_peek(da->decode_buffer, &filter_data);
-    filter_data.rate = da->afilter->input.rate; // due to playback speed change
     len = MPMIN(filter_data.samples, len);
     filter_data.samples = len;
     bool eof = error == AD_EOF && filter_data.samples == 0;

audio/filter/af.c

@@ -53,6 +53,7 @@ extern const struct af_info af_info_center;
 extern const struct af_info af_info_sinesuppress;
 extern const struct af_info af_info_karaoke;
 extern const struct af_info af_info_scaletempo;
+extern const struct af_info af_info_forcespeed;
 extern const struct af_info af_info_bs2b;
 extern const struct af_info af_info_lavfi;
 extern const struct af_info af_info_convert24;
@@ -83,6 +84,7 @@ static const struct af_info *const filter_list[] = {
     &af_info_center,
     &af_info_sinesuppress,
     &af_info_karaoke,
+    &af_info_forcespeed,
     &af_info_scaletempo,
 #if HAVE_LIBBS2B
     &af_info_bs2b,

audio/filter/af.h

@@ -120,6 +120,7 @@ enum af_control {
     AF_CONTROL_SET_PAN_BALANCE,
     AF_CONTROL_GET_PAN_BALANCE,
     AF_CONTROL_SET_PLAYBACK_SPEED,
+    AF_CONTROL_SET_PLAYBACK_SPEED_RESAMPLE,
 };
 
 // Argument for AF_CONTROL_SET_PAN_LEVEL

audio/filter/af_forcespeed.c

@@ -0,0 +1,69 @@
+/*
+ * This file is part of mpv.
+ *
+ * mpv is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * mpv is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with mpv.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include "af.h"
+
+struct priv {
+    double speed;
+};
+
+static int control(struct af_instance *af, int cmd, void *arg)
+{
+    struct priv *priv = af->priv;
+
+    switch (cmd) {
+    case AF_CONTROL_REINIT: {
+        struct mp_audio *in = arg;
+        struct mp_audio orig_in = *in;
+        struct mp_audio *out = af->data;
+
+        mp_audio_copy_config(out, in);
+        out->rate = in->rate * priv->speed;
+
+        return mp_audio_config_equals(in, &orig_in) ? AF_OK : AF_FALSE;
+    }
+    case AF_CONTROL_SET_PLAYBACK_SPEED_RESAMPLE: {
+        priv->speed = *(double *)arg;
+        return AF_OK;
+    }
+    }
+    return AF_UNKNOWN;
+}
+
+static int filter(struct af_instance *af, struct mp_audio *data, int flags)
+{
+    mp_audio_copy_config(data, af->data);
+    return 0;
+}
+
+static int af_open(struct af_instance *af)
+{
+    struct priv *priv = af->priv;
+    af->control = control;
+    af->filter = filter;
+    priv->speed = 1.0;
+    return AF_OK;
+}
+
+#define OPT_BASE_STRUCT struct priv
+
+const struct af_info af_info_forcespeed = {
+    .info = "Force audio speed",
+    .name = "forcespeed",
+    .open = af_open,
+    .priv_size = sizeof(struct priv),
+};

old-makefile

@@ -127,6 +127,7 @@ SOURCES = audio/audio.c \
           audio/filter/af_equalizer.c \
           audio/filter/af_export.c \
           audio/filter/af_extrastereo.c \
+          audio/filter/af_forcespeed.c \
           audio/filter/af_format.c \
           audio/filter/af_hrtf.c \
           audio/filter/af_karaoke.c \

player/audio.c

@@ -43,25 +43,61 @@
 #include "core.h"
 #include "command.h"
 
+static int try_filter(struct MPContext *mpctx,
+                      char *name, char *label, char **args)
+{
+    struct dec_audio *d_audio = mpctx->d_audio;
+
+    if (af_find_by_label(d_audio->afilter, label))
+        return 0;
+
+    struct af_instance *af = af_add(d_audio->afilter, name, args);
+    if (!af)
+        return -1;
+
+    af->label = talloc_strdup(af, label);
+
+    return 1;
+}
+
 static int recreate_audio_filters(struct MPContext *mpctx)
 {
     assert(mpctx->d_audio);
 
-    struct af_stream *afs = mpctx->d_audio->afilter;
     struct MPOpts *opts = mpctx->opts;
+    struct af_stream *afs = mpctx->d_audio->afilter;
 
-    struct mp_audio in_format;
-    mp_audio_buffer_get_format(mpctx->d_audio->decode_buffer, &in_format);
-    int new_srate = in_format.rate;
+    double speed = opts->playback_speed;
 
-    if (!af_control_any_rev(afs, AF_CONTROL_SET_PLAYBACK_SPEED,
-                            &opts->playback_speed))
-    {
-        new_srate = in_format.rate * opts->playback_speed;
-        if (new_srate != afs->output.rate)
-            opts->playback_speed = new_srate / (double)in_format.rate;
+    if (speed != 1.0) {
+        int method = AF_CONTROL_SET_PLAYBACK_SPEED_RESAMPLE;
+        if (speed > 1.0 && opts->pitch_correction)
+            method = AF_CONTROL_SET_PLAYBACK_SPEED;
+        if (!af_control_any_rev(afs, method, &speed)) {
+            if (af_remove_by_label(afs, "playback-speed") < 0)
+                return -1;
+
+            // Compatibility: if the user uses --af=scaletempo, always use
+            // this filter to change speed. Don't insert a second "scaletempo"
+            // filter either.
+            if (!af_control_any_rev(afs, AF_CONTROL_SET_PLAYBACK_SPEED, &speed))
+            {
+                char *filter = method == AF_CONTROL_SET_PLAYBACK_SPEED
+                             ? "scaletempo" : "forcespeed";
+                if (try_filter(mpctx, filter, "playback-speed", NULL) < 0)
+                    return -1;
+                // Try again.
+                if (!af_control_any_rev(afs, method, &speed))
+                    return -1;
+            }
+        }
+    } else {
+        if (af_remove_by_label(afs, "playback-speed") < 0)
+            return -1;
+        // The filters could be inserted by the user (we don't remove them).
+        af_control_any_rev(afs, AF_CONTROL_SET_PLAYBACK_SPEED, &speed);
+        af_control_any_rev(afs, AF_CONTROL_SET_PLAYBACK_SPEED_RESAMPLE, &speed);
     }
-    afs->input.rate = new_srate;
 
     if (af_init(afs) < 0) {
         MP_ERR(mpctx, "Couldn't find matching filter/ao format!\n");
@@ -88,23 +124,6 @@ int reinit_audio_filters(struct MPContext *mpctx)
     return 1;
 }
 
-static int try_filter(struct MPContext *mpctx,
-                      char *name, char *label, char **args)
-{
-    struct dec_audio *d_audio = mpctx->d_audio;
-
-    if (af_find_by_label(d_audio->afilter, label))
-        return 0;
-
-    struct af_instance *af = af_add(d_audio->afilter, name, args);
-    if (!af)
-        return -1;
-
-    af->label = talloc_strdup(af, label);
-
-    return 1;
-}
-
 void set_playback_speed(struct MPContext *mpctx, double new_speed)
 {
     struct MPOpts *opts = mpctx->opts;
@@ -117,19 +136,6 @@ void set_playback_speed(struct MPContext *mpctx, double new_speed)
     if (!mpctx->d_audio)
         return;
 
-    if (new_speed > 1.0 && opts->pitch_correction) {
-        if (!af_control_any_rev(mpctx->d_audio->afilter,
-                                AF_CONTROL_SET_PLAYBACK_SPEED,
-                                &new_speed))
-        {
-            if (try_filter(mpctx, "scaletempo", "playback-speed", NULL) < 0)
-                return;
-        }
-    } else {
-        if (af_remove_by_label(mpctx->d_audio->afilter, "playback-speed") < 0)
-            return;
-    }
-
     recreate_audio_filters(mpctx);
 }
 

wscript_build.py

@@ -114,6 +114,7 @@ def build(ctx):
         ( "audio/filter/af_equalizer.c" ),
         ( "audio/filter/af_export.c",            "sys-mman-h" ),
         ( "audio/filter/af_extrastereo.c" ),
+        ( "audio/filter/af_forcespeed.c" ),
         ( "audio/filter/af_format.c" ),
         ( "audio/filter/af_hrtf.c" ),
         ( "audio/filter/af_karaoke.c" ),