[pulseaudio-discuss] Problem using the Simple API

eric ekilfoil at gmail.com
Fri Oct 30 18:36:21 PDT 2009

On Fri, Oct 30, 2009 at 6:23 PM, Lennart Poettering
<lennart at poettering.net>wrote:

> Returning immediately *instead* of giving you 640 bytes of PCM?
> I mean, are you suggesting that pa_simple_read() is in fact *not*
> returning 640 bytes?

Or is your confusion simply because you expect that at 44khz you
> expect that calling pa_simple_read() for 640 bytes will block for
> exactly 3.6 ms? That's not how things work. If it did you'd get
> dropouts in the time between two _read() calls.

I definitely expected 640 bytes of PCM (~0.01s i thought) to be returned.  I
assumed that pa_simple_read() would block until I had that.  This was the
behavior in 0.9.14.  Of course, it would be buffered, so I would expect from
the time I opened the stream i would not have any data.  In other words:

1. Open stream
2. sleep for x seconds
3. 1000000 bytes of data are in a buffer
4. i read 640 bytes and get 640 bytes (this takes microseconds)
5. Eventually my read catches up with the buffer

The above situation is not what I am dealing with.  This is not my situation
at all. I would be fine if I eventually caught up with the buffer, but I
never do, and as you pointed out, it's most likely because of latency.  I
need to get data *at least* every .1s.

> if you want to do time synchronization, then query for the *time*,
> don't count samples. The time will be independant of buffering and so
> on. In the simple API there is pa_simple_get_latency() which you can
> query before you read something and which will then tell you how much
> earlier what you will read next was actually recorded.

I don't really need time sync.  If I drop packets, it's not *that* critical.
 But under normal operation i need to get audio every .1s.

> If you use the complex API then you can use pa_stream_get_time() to
> get a stream time, relative to the beginning of the stream.
I'm trying to avoid the complex API because.... frankly.. it's really
complex.  Is there a way to set the latency using the simple API?

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