Re: AudioQueue and buffer underflows
Re: AudioQueue and buffer underflows
- Subject: Re: AudioQueue and buffer underflows
- From: Jens Alfke <email@hidden>
- Date: Sat, 22 Mar 2008 17:10:01 -0700
I didn't get any replies to my questions, but yesterday I figured out
the answers on my own. What I hadn't realized is that the
AudioQueueOutputCallback really just tells the client "here, I'm done
with this buffer, it's empty now". The client doesn't _have_ to
respond by re-enqueuing that buffer during the callback; it can
enqueue another free buffer, or wait till one's available, and it can
enqueue a buffer at any time.
Once I grasped that, everything made sense. Now I keep my own list of
free buffers. When my callback is invoked, I add that buffer to the
free list. When my data stream gives me new data (usually received
over a socket), I copy it into one or more currently-free buffers and
enqueue them.
If any of this is bogus, please interrupt and correct me =)
1. What's the best way to detect that the queue's stopped playing
because of an underrun? Just counting the number of buffers I fail
to enqueue doesn't seem reliable.
Answer: when my free-buffer list fills up (i.e. its size equals the
number of buffers I allocated.)
2. After an underrun, once I can catch up and buffer some data, how
do I get the queue going again? Do I need to stop, prime and start it?
Answer: The queue's still running, and as soon as I enqueue new
buffers, it'll start playing them.
My next challenge is to deal with synchronization. If I underrun, I
shouldn't just start playing from where I left off, I need to seek
ahead and catch up with where the audio should be at that time. And I
think I'll need to start tagging the enqueued buffers with timestamps,
so they get played at the right instant.
—Jens
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