diff --git a/libavformat/pcm.c b/libavformat/pcm.c
index 806f91b6b..1ea15a9e8 100644
--- a/libavformat/pcm.c
+++ b/libavformat/pcm.c
@@ -28,13 +28,21 @@
 
 int ff_pcm_read_packet(AVFormatContext *s, AVPacket *pkt)
 {
-    int ret, size;
+    int ret, size = INT_MAX;
+    AVCodecParameters *par = s->streams[0]->codecpar;
 
-    size= RAW_SAMPLES*s->streams[0]->codecpar->block_align;
-    if (size <= 0)
+    if (par->block_align <= 0)
         return AVERROR(EINVAL);
 
-    ret= av_get_packet(s->pb, pkt, size);
+    /*
+     * Compute read size to complete a read every 40ms.  Clamp to RAW_SAMPLES if
+     * larger. Use power of two as readsize for I/O efficiency.
+     */
+    size = FFMAX(par->sample_rate/25, 1);
+    size = FFMIN(size, RAW_SAMPLES) * par->block_align;
+    size = 1 << ff_log2(size);
+
+    ret = av_get_packet(s->pb, pkt, size);
 
     pkt->flags &= ~AV_PKT_FLAG_CORRUPT;
     pkt->stream_index = 0;
diff --git a/tests/ref/seek/lavf-alaw b/tests/ref/seek/lavf-alaw
index 4b1f8fbc0..b61e687a4 100644
--- a/tests/ref/seek/lavf-alaw
+++ b/tests/ref/seek/lavf-alaw
@@ -1,53 +1,53 @@
-ret: 0         st: 0 flags:1 dts: 0.000000 pts: 0.000000 pos:      0 size:  1024
+ret: 0         st: 0 flags:1 dts: 0.000000 pts: 0.000000 pos:      0 size:   512
 ret: 0         st:-1 flags:0  ts:-1.000000
-ret: 0         st: 0 flags:1 dts: 0.000000 pts: 0.000000 pos:      0 size:  1024
+ret: 0         st: 0 flags:1 dts: 0.000000 pts: 0.000000 pos:      0 size:   512
 ret: 0         st:-1 flags:1  ts: 1.894167
-ret: 0         st: 0 flags:1 dts: 1.894150 pts: 1.894150 pos:  41766 size:  1024
+ret: 0         st: 0 flags:1 dts: 1.894150 pts: 1.894150 pos:  41766 size:   512
 ret: 0         st: 0 flags:0  ts: 0.788345
-ret: 0         st: 0 flags:1 dts: 0.788345 pts: 0.788345 pos:  17383 size:  1024
+ret: 0         st: 0 flags:1 dts: 0.788345 pts: 0.788345 pos:  17383 size:   512
 ret: 0         st: 0 flags:1  ts:-0.317506
-ret: 0         st: 0 flags:1 dts: 0.000000 pts: 0.000000 pos:      0 size:  1024
+ret: 0         st: 0 flags:1 dts: 0.000000 pts: 0.000000 pos:      0 size:   512
 ret: 0         st:-1 flags:0  ts: 2.576668
 ret:-EOF
 ret: 0         st:-1 flags:1  ts: 1.470835
-ret: 0         st: 0 flags:1 dts: 1.470839 pts: 1.470839 pos:  32432 size:  1024
+ret: 0         st: 0 flags:1 dts: 1.470839 pts: 1.470839 pos:  32432 size:   512
 ret: 0         st: 0 flags:0  ts: 0.364989
-ret: 0         st: 0 flags:1 dts: 0.364989 pts: 0.364989 pos:   8048 size:  1024
+ret: 0         st: 0 flags:1 dts: 0.364989 pts: 0.364989 pos:   8048 size:   512
 ret: 0         st: 0 flags:1  ts:-0.740816
-ret: 0         st: 0 flags:1 dts: 0.000000 pts: 0.000000 pos:      0 size:  1024
+ret: 0         st: 0 flags:1 dts: 0.000000 pts: 0.000000 pos:      0 size:   512
 ret: 0         st:-1 flags:0  ts: 2.153336
 ret:-EOF
 ret: 0         st:-1 flags:1  ts: 1.047503
-ret: 0         st: 0 flags:1 dts: 1.047483 pts: 1.047483 pos:  23097 size:  1024
+ret: 0         st: 0 flags:1 dts: 1.047483 pts: 1.047483 pos:  23097 size:   512
 ret: 0         st: 0 flags:0  ts:-0.058322
-ret: 0         st: 0 flags:1 dts: 0.000000 pts: 0.000000 pos:      0 size:  1024
+ret: 0         st: 0 flags:1 dts: 0.000000 pts: 0.000000 pos:      0 size:   512
 ret: 0         st: 0 flags:1  ts: 2.835828
 ret:-EOF
 ret: 0         st:-1 flags:0  ts: 1.730004
-ret: 0         st: 0 flags:1 dts: 1.730023 pts: 1.730023 pos:  38147 size:  1024
+ret: 0         st: 0 flags:1 dts: 1.730023 pts: 1.730023 pos:  38147 size:   512
 ret: 0         st:-1 flags:1  ts: 0.624171
-ret: 0         st: 0 flags:1 dts: 0.624172 pts: 0.624172 pos:  13763 size:  1024
+ret: 0         st: 0 flags:1 dts: 0.624172 pts: 0.624172 pos:  13763 size:   512
 ret: 0         st: 0 flags:0  ts:-0.481678
-ret: 0         st: 0 flags:1 dts: 0.000000 pts: 0.000000 pos:      0 size:  1024
+ret: 0         st: 0 flags:1 dts: 0.000000 pts: 0.000000 pos:      0 size:   512
 ret: 0         st: 0 flags:1  ts: 2.412517
 ret:-EOF
 ret: 0         st:-1 flags:0  ts: 1.306672
-ret: 0         st: 0 flags:1 dts: 1.306667 pts: 1.306667 pos:  28812 size:  1024
+ret: 0         st: 0 flags:1 dts: 1.306667 pts: 1.306667 pos:  28812 size:   512
 ret: 0         st:-1 flags:1  ts: 0.200839
-ret: 0         st: 0 flags:1 dts: 0.200816 pts: 0.200816 pos:   4428 size:  1024
+ret: 0         st: 0 flags:1 dts: 0.200816 pts: 0.200816 pos:   4428 size:   512
 ret: 0         st: 0 flags:0  ts:-0.904989
-ret: 0         st: 0 flags:1 dts: 0.000000 pts: 0.000000 pos:      0 size:  1024
+ret: 0         st: 0 flags:1 dts: 0.000000 pts: 0.000000 pos:      0 size:   512
 ret: 0         st: 0 flags:1  ts: 1.989161
 ret: 0         st: 0 flags:1 dts: 1.989161 pts: 1.989161 pos:  43861 size:   239
 ret: 0         st:-1 flags:0  ts: 0.883340
-ret: 0         st: 0 flags:1 dts: 0.883356 pts: 0.883356 pos:  19478 size:  1024
+ret: 0         st: 0 flags:1 dts: 0.883356 pts: 0.883356 pos:  19478 size:   512
 ret: 0         st:-1 flags:1  ts:-0.222493
-ret: 0         st: 0 flags:1 dts: 0.000000 pts: 0.000000 pos:      0 size:  1024
+ret: 0         st: 0 flags:1 dts: 0.000000 pts: 0.000000 pos:      0 size:   512
 ret: 0         st: 0 flags:0  ts: 2.671655
 ret:-EOF
 ret: 0         st: 0 flags:1  ts: 1.565850
-ret: 0         st: 0 flags:1 dts: 1.565850 pts: 1.565850 pos:  34527 size:  1024
+ret: 0         st: 0 flags:1 dts: 1.565850 pts: 1.565850 pos:  34527 size:   512
 ret: 0         st:-1 flags:0  ts: 0.460008
-ret: 0         st: 0 flags:1 dts: 0.460000 pts: 0.460000 pos:  10143 size:  1024
+ret: 0         st: 0 flags:1 dts: 0.460000 pts: 0.460000 pos:  10143 size:   512
 ret: 0         st:-1 flags:1  ts:-0.645825
-ret: 0         st: 0 flags:1 dts: 0.000000 pts: 0.000000 pos:      0 size:  1024
+ret: 0         st: 0 flags:1 dts: 0.000000 pts: 0.000000 pos:      0 size:   512
diff --git a/tests/ref/seek/lavf-mulaw b/tests/ref/seek/lavf-mulaw
index 4b1f8fbc0..b61e687a4 100644
--- a/tests/ref/seek/lavf-mulaw
+++ b/tests/ref/seek/lavf-mulaw
@@ -1,53 +1,53 @@
-ret: 0         st: 0 flags:1 dts: 0.000000 pts: 0.000000 pos:      0 size:  1024
+ret: 0         st: 0 flags:1 dts: 0.000000 pts: 0.000000 pos:      0 size:   512
 ret: 0         st:-1 flags:0  ts:-1.000000
-ret: 0         st: 0 flags:1 dts: 0.000000 pts: 0.000000 pos:      0 size:  1024
+ret: 0         st: 0 flags:1 dts: 0.000000 pts: 0.000000 pos:      0 size:   512
 ret: 0         st:-1 flags:1  ts: 1.894167
-ret: 0         st: 0 flags:1 dts: 1.894150 pts: 1.894150 pos:  41766 size:  1024
+ret: 0         st: 0 flags:1 dts: 1.894150 pts: 1.894150 pos:  41766 size:   512
 ret: 0         st: 0 flags:0  ts: 0.788345
-ret: 0         st: 0 flags:1 dts: 0.788345 pts: 0.788345 pos:  17383 size:  1024
+ret: 0         st: 0 flags:1 dts: 0.788345 pts: 0.788345 pos:  17383 size:   512
 ret: 0         st: 0 flags:1  ts:-0.317506
-ret: 0         st: 0 flags:1 dts: 0.000000 pts: 0.000000 pos:      0 size:  1024
+ret: 0         st: 0 flags:1 dts: 0.000000 pts: 0.000000 pos:      0 size:   512
 ret: 0         st:-1 flags:0  ts: 2.576668
 ret:-EOF
 ret: 0         st:-1 flags:1  ts: 1.470835
-ret: 0         st: 0 flags:1 dts: 1.470839 pts: 1.470839 pos:  32432 size:  1024
+ret: 0         st: 0 flags:1 dts: 1.470839 pts: 1.470839 pos:  32432 size:   512
 ret: 0         st: 0 flags:0  ts: 0.364989
-ret: 0         st: 0 flags:1 dts: 0.364989 pts: 0.364989 pos:   8048 size:  1024
+ret: 0         st: 0 flags:1 dts: 0.364989 pts: 0.364989 pos:   8048 size:   512
 ret: 0         st: 0 flags:1  ts:-0.740816
-ret: 0         st: 0 flags:1 dts: 0.000000 pts: 0.000000 pos:      0 size:  1024
+ret: 0         st: 0 flags:1 dts: 0.000000 pts: 0.000000 pos:      0 size:   512
 ret: 0         st:-1 flags:0  ts: 2.153336
 ret:-EOF
 ret: 0         st:-1 flags:1  ts: 1.047503
-ret: 0         st: 0 flags:1 dts: 1.047483 pts: 1.047483 pos:  23097 size:  1024
+ret: 0         st: 0 flags:1 dts: 1.047483 pts: 1.047483 pos:  23097 size:   512
 ret: 0         st: 0 flags:0  ts:-0.058322
-ret: 0         st: 0 flags:1 dts: 0.000000 pts: 0.000000 pos:      0 size:  1024
+ret: 0         st: 0 flags:1 dts: 0.000000 pts: 0.000000 pos:      0 size:   512
 ret: 0         st: 0 flags:1  ts: 2.835828
 ret:-EOF
 ret: 0         st:-1 flags:0  ts: 1.730004
-ret: 0         st: 0 flags:1 dts: 1.730023 pts: 1.730023 pos:  38147 size:  1024
+ret: 0         st: 0 flags:1 dts: 1.730023 pts: 1.730023 pos:  38147 size:   512
 ret: 0         st:-1 flags:1  ts: 0.624171
-ret: 0         st: 0 flags:1 dts: 0.624172 pts: 0.624172 pos:  13763 size:  1024
+ret: 0         st: 0 flags:1 dts: 0.624172 pts: 0.624172 pos:  13763 size:   512
 ret: 0         st: 0 flags:0  ts:-0.481678
-ret: 0         st: 0 flags:1 dts: 0.000000 pts: 0.000000 pos:      0 size:  1024
+ret: 0         st: 0 flags:1 dts: 0.000000 pts: 0.000000 pos:      0 size:   512
 ret: 0         st: 0 flags:1  ts: 2.412517
 ret:-EOF
 ret: 0         st:-1 flags:0  ts: 1.306672
-ret: 0         st: 0 flags:1 dts: 1.306667 pts: 1.306667 pos:  28812 size:  1024
+ret: 0         st: 0 flags:1 dts: 1.306667 pts: 1.306667 pos:  28812 size:   512
 ret: 0         st:-1 flags:1  ts: 0.200839
-ret: 0         st: 0 flags:1 dts: 0.200816 pts: 0.200816 pos:   4428 size:  1024
+ret: 0         st: 0 flags:1 dts: 0.200816 pts: 0.200816 pos:   4428 size:   512
 ret: 0         st: 0 flags:0  ts:-0.904989
-ret: 0         st: 0 flags:1 dts: 0.000000 pts: 0.000000 pos:      0 size:  1024
+ret: 0         st: 0 flags:1 dts: 0.000000 pts: 0.000000 pos:      0 size:   512
 ret: 0         st: 0 flags:1  ts: 1.989161
 ret: 0         st: 0 flags:1 dts: 1.989161 pts: 1.989161 pos:  43861 size:   239
 ret: 0         st:-1 flags:0  ts: 0.883340
-ret: 0         st: 0 flags:1 dts: 0.883356 pts: 0.883356 pos:  19478 size:  1024
+ret: 0         st: 0 flags:1 dts: 0.883356 pts: 0.883356 pos:  19478 size:   512
 ret: 0         st:-1 flags:1  ts:-0.222493
-ret: 0         st: 0 flags:1 dts: 0.000000 pts: 0.000000 pos:      0 size:  1024
+ret: 0         st: 0 flags:1 dts: 0.000000 pts: 0.000000 pos:      0 size:   512
 ret: 0         st: 0 flags:0  ts: 2.671655
 ret:-EOF
 ret: 0         st: 0 flags:1  ts: 1.565850
-ret: 0         st: 0 flags:1 dts: 1.565850 pts: 1.565850 pos:  34527 size:  1024
+ret: 0         st: 0 flags:1 dts: 1.565850 pts: 1.565850 pos:  34527 size:   512
 ret: 0         st:-1 flags:0  ts: 0.460008
-ret: 0         st: 0 flags:1 dts: 0.460000 pts: 0.460000 pos:  10143 size:  1024
+ret: 0         st: 0 flags:1 dts: 0.460000 pts: 0.460000 pos:  10143 size:   512
 ret: 0         st:-1 flags:1  ts:-0.645825
-ret: 0         st: 0 flags:1 dts: 0.000000 pts: 0.000000 pos:      0 size:  1024
+ret: 0         st: 0 flags:1 dts: 0.000000 pts: 0.000000 pos:      0 size:   512
