aplay: reduce ALSA buffering

It seems that safari on the iPhone cannot adjust A/V sync when
the audio latency exceeds about 400ms. So to keep the default
delay when using bluealsa-aplay below that critical value, change
the default ALSA hw params to lower values which are still
compatible with the majority of sound cards.

Author: borine

.github/spellcheck-wordlist.txt

@@ -68,6 +68,7 @@ ATRAC
 BlueALSA
 BlueZ
 Fraunhofer
+iPhone
 IWYU
 LDAC
 LHDC

doc/bluealsa-aplay.1.rst

@@ -89,7 +89,7 @@ OPTIONS
 
 --pcm-buffer-time=INT
     Set the playback PCM buffer duration time to *INT* microseconds.
-    The default is 500000. It is recommended to choose a buffer time that is
+    The default is 200000. It is recommended to choose a buffer time that is
     an exact multiple of the period time to avoid potential issues with some
     ALSA plugins (see --pcm-period-time option below).
     ALSA may choose the nearest available alternative if the requested value is
@@ -101,7 +101,7 @@ OPTIONS
 
 --pcm-period-time=INT
     Set the playback PCM period duration time to *INT* microseconds.
-    The default is 100000.
+    The default is 50000.
     ALSA may choose the nearest available alternative if the requested value is
     not supported.
 
@@ -232,12 +232,12 @@ Instead it will choose its own values, which can lead to rounding errors in the
 period size calculation when used with the ALSA `rate` plugin. To avoid this,
 it is recommended to explicitly define the hardware period size and buffer size
 for dmix in your ALSA configuration. For example, suppose we want a period time
-of 100000 µs and a buffer holding 5 periods with an Intel 'PCH' card:
+of 50000 µs and a buffer holding 4 periods with an Intel 'PCH' card:
 
 ::
 
-    defaults.dmix.PCH.period_time 100000
-    defaults.dmix.PCH.periods 5
+    defaults.dmix.PCH.period_time 50000
+    defaults.dmix.PCH.periods 4
 
 Alternatively we can define a PCM with the required setting:
 
@@ -250,8 +250,8 @@ Alternatively we can define a PCM with the required setting:
             ipc_key 12345
             slave {
                 pcm "hw:0,0"
-                period_time 100000
-                periods 5
+                period_time 50000
+                periods 4
             }
         }
     }

utils/aplay/alsa-pcm.c

@@ -89,8 +89,9 @@ static int alsa_pcm_set_sw_params(snd_pcm_t *pcm, snd_pcm_uframes_t buffer_size,
 		goto fail;
 	}
 
-	/* start the transfer when the buffer is full (or almost full) */
-	snd_pcm_uframes_t threshold = (buffer_size / period_size) * period_size;
+	/* start the transfer when the buffer is half full - this allows spare
+	 * capacity to accommodate bursts and short breaks in the bluetooth stream. */
+	snd_pcm_uframes_t threshold = buffer_size / 2;
 	if ((err = snd_pcm_sw_params_set_start_threshold(pcm, params, threshold)) != 0) {
 		snprintf(buf, sizeof(buf), "Set start threshold: %s: %lu", snd_strerror(err), threshold);
 		goto fail;

utils/aplay/aplay.c

@@ -86,8 +86,11 @@ static bool ba_profile_a2dp = true;
 static bool ba_addr_any = false;
 static bdaddr_t *ba_addrs = NULL;
 static size_t ba_addrs_count = 0;
-static unsigned int pcm_buffer_time = 500000;
-static unsigned int pcm_period_time = 100000;
+/* Many devices cannot synchronize A/V with very high audio latency. To keep
+ * the overall latency below 400ms we choose ALSA parameters such that the
+ * ALSA latency is below 200ms. */
+static unsigned int pcm_buffer_time = 200000;
+static unsigned int pcm_period_time = 50000;
 
 /* local PCM muted state for software mute */
 static bool pcm_muted = false;