Re: AudioQueue and buffer underflows
Re: AudioQueue and buffer underflows
- Subject: Re: AudioQueue and buffer underflows
- From: William Stewart <email@hidden>
- Date: Mon, 24 Mar 2008 14:47:13 -0700
On Mar 22, 2008, at 5:10 PM, Jens Alfke wrote:
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.
Yes - in fact its not even "its empty now" - we don't change the
contents of the buffer, so they are the same as what they were when
you enqueued it. Can be useful if you are looping small segments.
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 =)
Nope - that was the response :)
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.
Yes - the queue will try to just add them to the last buffers it had
played (which is the default behaviour that you mostly want).
Bill
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