[mythtv-users] Playback problem -- random short pauses

[Michael T. Dean]

On 05/23/2011 02:16 PM, Don Lewis wrote:
> On 23 May, Michael T. Dean wrote:
>> On 05/23/2011 12:07 PM, Paul Gardiner wrote:
>>> On 23/05/2011 16:42, Michael T. Dean wrote:
>>>> If you want to prevent all the "random short pauses" that occur during
>>>> playback, simply change your Playback Profile group such that you never
>>>> use VDPAU decoding.  Use ffmpeg decode with VDPAU rendering, and all
>>>> works well.  The only pauses you'll see is the at-recording-start/stop
>>>> pauses (which is a completely separate and unrelated problem).
>>> Confused! I thought it was the at-recording-start/stop pauses we
>>> were talking about. I don't believe I see any other type of
>>> pause on my system, and I'm using VDPAU decode, so I guess
>>> I must be lucky.
>> There are 3 types of pauses that occur during playback with current
>> MythTV on a properly-configured system.  These are distinct and
>> unrelated issues.  This thread has various users talking about the 3
>> different issues and providing information that seems contradictory, but
>> that is well explained by the fact that they're discussing very
>> different things.  The 3 issues are:
>>
>> 1) Short pauses that occur when the UI thread is blocked during
>> qapp-processEvents() call.  This is the at-recording-start/stop pauses I
>> mention.  These will occur regardless of which decoder/renderer you use.
>>
>> 2) Short pauses that occur due to video buffer starvation due to a
>> combination of ffmpeg demux (reading data from the file to pass to the
>> decoder) and vdpau decode.  These pauses occur at "random" intervals
>> during playback--even when nothing is happening on the backend or the
>> frontend.  These can be worked around by not using VDPAU decode--instead
>> use ffmpeg decode.
>>
>> 3) Short pauses that occur at Live TV program boundaries.  These may or
>> may not be related to #1, and I know nothing at all about them because I
>> don't use Live TV.
> 4) Short pauses while watching a previously recorded program that occur
> when mythbackend starts and/or stops recording.
>
> I have problems #1 and #4.

Ummm...  #1 is #4.  This pause occurs whether watching recordings in 
progress or previously recorded.

>    I don't see #2 and I don't use Live TV.
>
> There have been times when I've had as many as four recordings in
> progress, and is long as recordings are not starting or stopping,
> playback is flawless.  A single recording starting or stopping (I don't
> happen to know which) on an otherwise idle system causes playback
> pauses.

Yeah, that's the same as #1, the "at-recording-start/stop pauses".

>> So, if you use ffmpeg decode (rather than VDPAU decode), you'll only see
>> #1 and #3.  Issue #1 only occurs when the backend starts or stops a
>> recording (so generally happens on the top of or half past the hour, and
>> only while you're actually recording shows while watching other
>> recordings).  Issue #3 only occurs in Live TV, as it's a Live TV
>> specific thing, and only occurs on program boundary/when the backend
>> stops and starts the Live TV recordings.
>>
>> Also, it's possible that specifying videobuffersize=32 in the filter
>> section of the VDPAU-decode playback profile may work around the VDPAU
>> video buffer starvation issue (#2), but that may "undo" all the changes
>> that have allowed use of VDPAU on low-memory video cards (like 256MB
>> cards)--you'll likely need a 512MB or greater card.  We're still testing
>> this, and would appreciate users providing feedback about whether it
>> helps or not.  This may be why you're not seeing the VDPAU issues, Paul
>> (so please let us know if you have specified videobuffersize or other
>> filters).
> I'll try experimenting with VDPAU, but I think I was still using the
> Nvidia 7050 onboard video, which is not VPDAU capable, when I upgraded
> from 0.23 to 0.24 and started seeing the problem.  I don't think I
> dropped in the GT220 until later, when I got my HD-PVR.  It's been so
> long ago that I'm kind of fuzzy on the timeline at this point.

Yes, #1 is not affected by decoder choice.  Only the VDPAU-specific 
pauses in #2 are avoided by using ffmpeg decoding.

Mike