Subject: Re: [Bluez-devel] yes, a2dp timing is whacked
From: Marcel Holtmann <marcel@holtmann.org>
To: bluez-devel@lists.sourceforge.net
In-Reply-To: <1115429201.9031.5.camel@pegasus>
References: <42792979.90109@xmission.com>
	 <20050507031314.548afb03.henryk@ploetzli.ch>
	 <1115429201.9031.5.camel@pegasus>
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Date: Sat, 07 May 2005 21:27:33 +0200

Hi Henryk,

> > > Maybe for convenience, libsbc should compute the timeslice for the
> > > frame.
> > 
> > It does now. I added a function sbc_time(sbc_t *sbc) which computes the
> > microseconds for one frame. I then replaced the fixed value in a2play's
> > usleep() call with the time the whole packet should take to play (time
> > for one frame times number of frames in the packet) multiplied by
> > (arbitrarily chosen) 0.8 to make up for the time lost with computing and
> > sending.
> 
> the time that is eaten up by encoding or decoding could be measured with
> gettimeofday() and then you can subtract this from the whole time.
> 
> Since it is a linear operation we can make the SBC library do it all for
> us. Let sbc_encode() and sbc_decode() calculate the processing time and
> then store a sleep value for us inside sbc_t.

forget about this stupid idea. I tried it and it is not working out. So
I only moved the time calculation into sbc->duration and added the
gettimeofday() magic into a2play. This is somekind of working, but the
quality is not that good with the HP headphone.

I also figured out that the HP headphone has somekind of buffer and you
can send more than one AVDTP frame (equals 8 SBC frames). But it seems
that this is somekind of ringbuffer. Too many frames will overwrite
previous frames and make the sound a mess.

So the best thing is too encode two or three full AVDTP frames in
advance and have them ready to send. The sending can't be controlled be
usleep(). We need a time that kicks in every whatever msec and makes us
to transmit the next frame.

Regards

Marcel


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