avformat/pcm: decrease delay when reading PCM streams.

  Thanks for the discussion. Here's the next version, now with /25 and removed
ff_log2().

  The blocksize of the PCM decoder is hard-coded. This creates
unnecessary delay when reading low-rate (<100Hz) streams. This creates
issues when multiplexing multiple streams, since other inputs are only
opened/read after a low-rate input block was completely read.

 This patch decreases the blocksize for low-rate inputs, so
approximately a block is read every 40ms. This decreases the startup
delay when multiplexing inputs with different rates.

Signed-off-by: Philipp M. Scholl <pscholl@bawue.de>
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

1 files changed, 10 insertions, 3 deletions

diff --git a/libavformat/pcm.c b/libavformat/pcm.c
index 806f91b6b1..767bbd045a 100644
--- a/libavformat/pcm.c
+++ b/libavformat/pcm.c
@@ -28,13 +28,20 @@
 
 int ff_pcm_read_packet(AVFormatContext *s, AVPacket *pkt)
 {
+    AVCodecParameters *par = s->streams[0]->codecpar;
     int ret, size;
 
-    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 62ms.
+     * Clamp to RAW_SAMPLES if larger.
+     */
+    size = FFMAX(par->sample_rate/25, 1);
+    size = FFMIN(size, RAW_SAMPLES) * par->block_align;
+
+    ret = av_get_packet(s->pb, pkt, size);
 
     pkt->flags &= ~AV_PKT_FLAG_CORRUPT;
     pkt->stream_index = 0;