Apple

All,

The issue has resolved itself with iOS version 6.1, which I happened to update to at the end of the day yesterday. I’ve taken new data to validate. (Will try and attached a pdf file in a separate posting).

On the topic of the MTU size, I was using a size of 1000 during this testing. Will iOS attempt to take advantage of the larger MTU value or will it only use a max MTU of 885 bytes? It seems we could get closer to true 256kbps audio if we use the larger MTU size. Only a small percentage of the AAC packets will be > 885 bytes (about 2% for the 15 seconds of the problem area of the song in question) so it seems we would still be able to keep up the overall packet efficiency/throughput. Just a thought.

Thanks again for everyone’s help.

-Shawn

-----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.

Thanks.

Aarti.

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?

thanks

Shawn K. Steenhagen

Software Specialist

Applied Signal Processing, Inc.

www.appliedsignalprocessing.com

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