Aarti,
Thanks for the quick reply. I can provide
the file for testing, I have the 15 second problem area isolated in its own
track as a 128kbps, 256kpbs and .wav file. Would you like those? If so, let me
know the best way to get those to you.
Where do I file a bug report? I don’t
have an air trace, but do have some data that shows the time between packet receives.
This data shows that once we hit the problem area of the song, the average data
period bumps up above 23 msecs. I can provide a few plots in a pdf format
if you’d like.
thanks
-----Original Message-----
From: Aarti Kumar
[mailto:email@hidden]
Sent: Thursday, January 31, 2013
2:31 PM
To: Shawn Steenhagen
Cc: email@hidden
Subject: Re: Issue with playback
of 256kbps AAC file via A2DP on iPOD
Shawn,
Thanks for bringing this up. To avoid
fragmenting AAC packets across two L2CAP packets, iPod (or any iOS
device) will limit the maximum AAC packet size to MTU. This will result
increasing the compression of audio data (if needed) to fit into maximum AAC
packet size.
We have not seen any issue regarding this so far and
will look into it with song you mentioned below.
Not sure if you have already done so, but will be good
to file a bug report for this and provide air trace if you have them.
On Jan 31, 2013, at 12:15 PM, Shawn Steenhagen <email@hidden>
wrote:
I have expanded the A2DP
capabilities of a Bluetooth SNK to support AAC.
I am using an iPod Gen4 with iOS 6.
Things are working well except for
one issue. A particular song (“Time” from Pink Floyd’s
Dark Side of the moon) when encoded at 256kbps. I am seeing the AAC packet
rate slow down and I think it is actually dropping packets. Because of
this packet rate slow down, unless I buffer about 25 AAC packets
before playback starts, I starve the buffer feeding my D/A converter.
This same song works fine if I put a
version on the iPod that has been encoded using 128kbps.
I am thinking the issue is related
to the MTU value being used in the channel. The Apple BT Developers Guide
recommends using a MTU value of 885 bytes. The max packet payload size I see
coming across is 880 bytes. HOWEVER, I know that there are about 20 packets in
this song, during the problem area (starting at the 20 second mark) which are
> 885 bytes. What will the iPod do if it encounters a packet that is
larger than the Channel’s MTU value? Does it split it up or just not send
it?
Anyone else see this issue?
Applied Signal Processing, Inc.
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