[vlc-devel] VLC audio output FIFO questions ...
j.zorko at att.net
j.zorko at att.net
Wed May 14 20:42:08 CEST 2003
Hello, all ...
In my explorations of the VLC source, as well as adding functionality and observing how things
work, i've now come to the audio output stuff. There is some behavior i'm seeing that i'm trying to
determine the cause of, and while i've a theory (and started looking at the audio output source), I
wanted to ask the people here, as well as perhaps get some additional pointers on where to look.
The behavior I see is this: I've added functionality to VLC to make it do fast-forward and rewind,
and I implemented this by seeking forward or backward in the file (either purely on a temporal
basis, or looking for the nth occurence of a sequence header / MP3 frame and setting the new file
position there). This works well with the audio and video sources i've tested (MP3, MPEG2 VOBs).
The idea is that, when VLC is acting as a server to other VLCs, this is sort of like RTSP i.e. a control
protocol -- some external process can send the ff / rewind commands to the server VLC, and the
clients receiving the stream don't need to know anything.
However, after several ff / rewind commands, what i'm observing is that, with MP3 audio, the
audio output 'wobbles' i.e. gets faster, then slower, then finally settling in. Since i'm not altering
the number of packets the input thread sends to the FIFO (only where those packets are i.e.
seeking forward or backward in the file before reading the packets), one can infer from this that
some FIFO is based on time in the buffer, not # of packets in the buffer. However, so far i've not
found this code, and i'm frankly a bit confused (the VLC code seems to have FIFOs everywhere, i'm
not sure which one is the one that can result in wobbly audio).
So -- can someone give me some pointers, where to look, whether this mystical, magical FIFO is
indeed based on the spacing of packets in time vs. the quantity of packets in it? Which FIFO is
this? Where is it?
John, VLC hacker (in training) <smile>
Falling You - exploring the beauty
of voice and sound
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