Some time in 2012, the GStreamer team was busy working toward the GStreamer 1.0 major release. Along the way, I did my part and ported the DVD playback components from 0.10. DVD is a pretty complex playback scenario (let’s not talk about Blu-ray)
I gave a talk about it at the GStreamer conference way back in 2010 – video here. Apart from the content of that talk, the thing I liked most was that I used Totem as my presentation tool 🙂
With all the nice changes that GStreamer 1.0 brought, DVD playback worked better than ever. I was able to delete a bunch of hacks and workarounds from the 0.10 days. There have been some bugs, but mostly minor things. Recently though, I became aware of a whole class of DVDs that didn’t work for a very silly reason. The symptom was that particular discs would error out at the start with a cryptic “The stream is in the wrong format” message.
It turns out that these are DVDs that begin with a piece of video that has no sound.
Sometimes, that’s implemented on a disc as a video track with accompanying silence, but in the case that was broken the DVDs have no audio track for that initial section at all. For a normal file, GStreamer would handle that by not creating any audio decoder chain or audio sink output element and just decode and play video. For DVD though, there are very few discs that are entirely without audio – so we’re going to need the audio decoder chain sooner or later. There’s no point creating and destroying when the audio track appears and disappears.
Accordingly, we create an audio output pad, and GStreamer plugs in a suitable audio output sink, and then nothing happens because the pipeline can’t get to the Playing state – the pipeline is stuck in the Paused state. Before a pipeline can start playing, it has to progress through Ready and Paused and then to Playing state. The key to getting from Paused to Playing is that each output element (video sink and audio sink) in our case, has to receive some data and be ready to output it. A process called Pre-roll. Pre-rolling the pipeline avoids stuttering at the start, because otherwise the decoders would have to race to try and deliver something in time for it to get on screen.
With no audio track, there’s no actual audio packets to deliver, and the audio sink can’t Pre-roll. The solution in GStreamer 1.0 is a GAP event, sent to indicate that there is a space in the data, and elements should do whatever they need to to skip or fill it. In the audio sink’s case it should handle it by considering itself Pre-rolled and allowing the pipeline to go to Playing, starting the ring buffer and the audio clock – from which the rest of the pipeline will be timed.
Everything up to that point was working OK – the sink received the GAP event… and then errored out. It expects to be told what format the audio samples it’s receiving are so it knows how to fill in the gap… when there’s no audio track and no audio data, it was never being told.
In the end, the fix was to make the dummy place-holder audio decoder choose an audio sample format if it gets a GAP event and hasn’t received any data yet – any format, it doesn’t really matter as long as it’s reasonable. It’ll be discarded and a new format selected and propagated when some audio data really is encountered later in playback.
That fix is #c24a12 – later fixed up a bit by thiagoss to add the ‘sensible’ part to format selection. The initial commit liked to choose a samplerate of 1Hz 🙂
If you have any further bugs in your GStreamer DVD playback, please let us know!
5 thoughts on “DVD playback in GStreamer 1.0”
There is a similar problem with subtitles, at least for BluRays. I was able to work around this by making the preroll buffers insanely huge.
Happened on 0.10 and 1.0, dont know if this is fixed with 1.2
I’d be interested in hearing more about your experience with blu-ray. What are you using to play those with GStreamer? Or are you talking about playing dumps directly?
I’ve been unable to watch some DVDs on Fedora 20 using Totem, but they work fine with VLC. I suspect VLC still uses gstreamer 0.10 (but haven’t checked), and I’ve installed All Of The Things from rpmfusion.
I’m hoping that this is the bug I’m seeing. Do you know when this fix will land in a stable Fedora?
It’s sounds likely that your problem with totem playing some DVDs is this bug. The fix will be in GStreamer 1.2.3, which will be Fedora 21, or in a backport repo before then.
VLC doesn’t use GStreamer – it is its own thing entirely, and hence has its own different set of bugs 🙂
I just reported https://bugzilla.gnome.org/show_bug.cgi?id=724467.
This is about seemlessly selecting audio and subtitle streams in totem during DVD playback. I’m not sure if there are any parts in gstreamer which require modification for this to be supported tough…