[dvblast-devel] PATCH: Avoid jitter in RTP timestamps

[Marian Ďurkovič]

Hi,

  after sucessfully setting up dvblast in our environment, I noticed some
unexpected jitter in RTP timestamps. Closer investigation shows, that this
is caused by the fact, that kernel feeds us TS packets from DVB cards in
chunks once per approx. 12 msec. If the packet with PCR happens to be at
the end of a chunk, next TS packets will only arrive after 12 msec, which
will make the RTP timestamp inacurate.

   The attached patch fixes this problem by taking reference wallclock
samples during output phase. This way RTP timestamp always matches PCR
contained in transmitted IP packet.

      With kind regards,

           M. 

--------------------------------------------------------------------------
----                                                                  ----
----   Marian Ďurkovič                       network  manager         ----
----                                                                  ----
----   Slovak Technical University           Tel: +421 2 571 041 81   ----
----   Computer Centre, Nám. Slobody 17      Fax: +421 2 524 94 351   ----
----   812 43 Bratislava, Slovak Republic    E-mail/sip: md at bts.sk    ----
----                                                                  ----
--------------------------------------------------------------------------
-------------- next part --------------
Index: output.c
===================================================================
--- output.c	(revision 14)
+++ output.c	(working copy)
@@ -95,7 +95,7 @@
     p_output->p_pmt_section = NULL;
     p_output->p_sdt_section = NULL;
     p_output->i_ref_timestamp = 0;
-    p_output->i_ref_wallclock = mdate();
+    p_output->i_ref_wallclock = 0;
     p_output->i_eit_last_table_id = 0;
 
     p_output->i_maddr = i_maddr;
@@ -245,9 +245,19 @@
 
 static void rtp_SetHdr( output_t *p_output, uint8_t *p_hdr )
 {
-    mtime_t i_timestamp = p_output->i_ref_timestamp
-                           + (mdate() - p_output->i_ref_wallclock) * 9 / 100;
+    mtime_t i_timestamp;
 
+    if (!p_output->i_ref_wallclock)
+    {
+        i_timestamp = p_output->i_ref_timestamp;
+        p_output->i_ref_wallclock = mdate();
+    }
+    else
+    {
+        i_timestamp = p_output->i_ref_timestamp
+                    + (mdate() - p_output->i_ref_wallclock) * 9 / 100;
+    }
+
     p_hdr[0] = 0x80;
     p_hdr[1] = 33;
     p_hdr[2] = p_output->i_cc >> 8;
Index: demux.c
===================================================================
--- demux.c	(revision 14)
+++ demux.c	(working copy)
@@ -225,7 +225,7 @@
                     if ( pp_outputs[j]->i_sid == i_sid )
                     {
                         pp_outputs[j]->i_ref_timestamp = i_timestamp;
-                        pp_outputs[j]->i_ref_wallclock = i_wallclock;
+                        pp_outputs[j]->i_ref_wallclock = 0;
                     }
                 }
             }