[Bug 730586] New: gstalsasrc/gstaudiobasesrc clock

[GStreamer (bugzilla.gnome.org)]

https://bugzilla.gnome.org/show_bug.cgi?id=730586
  GStreamer | gst-plugins-base | unspecified

           Summary: gstalsasrc/gstaudiobasesrc clock
    Classification: Platform
           Product: GStreamer
           Version: unspecified
        OS/Version: Windows
            Status: UNCONFIRMED
          Severity: normal
          Priority: Normal
         Component: gst-plugins-base
        AssignedTo: gstreamer-bugs at lists.freedesktop.org
        ReportedBy: tom.deseyn at gmail.com
         QAContact: gstreamer-bugs at lists.freedesktop.org
     GNOME version: ---


In a pipeline where gstalsasrc provides the pipeline clock, time spent in
buffer overruns is missing.

In such a pipline, the pipeline clock is provided by
gst_audio_base_src_get_time which does not increase during overruns. The buffer
timestamps are set in gst_audio_base_src_create and do not take into account
buffer overruns.

This causes problems in pipelines which have live sinks where this is detected
as clock skew (https://bugzilla.gnome.org/show_bug.cgi?id=692953).

When working around this by setting provide-clock=false on alsasink this
triggers alsasrc to add buffer timestamps.
(http://cgit.freedesktop.org/gstreamer/gst-plugins-base/commit/?id=a049b102da7c1b68a31c49717a4b37b351ea4b34).
In this case, overruns are visible in the buffer timestamps but the difference
in clock (alsa vs pipeline) is not compensated.

So either buffer overruns are not visible (alsasrc provides clock) or buffer
timestamps are not matched against the pipeline clock (alsasrc doesn't provide
the clock).

Do do it properly the alsasrc should provide a clock which takes into account
buffer overrun and use that clock to timestamp the buffers.
If the pipeline uses another (monotonic) clock, the alsa timestamps should be
reconverted to that clock.

Some simple changes can be made:
Perhaps alsasrc should not provide a clock by default?
For most if not all audio cards, the current code works fine when the pipeline
clock is the monotonic OS clock. This is because snd_pcm_status_get_htstamp
equals the clock time. If the call to the first in gstalsasrc was replaced by a
call to the latter, it will even work fine for all monotonic system clocks.

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