Apple

Matthias,

Thanks for the response. I forgot to mention that I am using an MTU size of 1000. It seems the apple device never takes advantage of this extra size, given that the max packet payload I measure is 881 bytes. Seems to be limiting itself to the 885 value mentioned in the developers guide. By the way, here is the guide:

https://developer.apple.com/hardwaredrivers/BluetoothDesignGuidelines.pdf

thanks,

-Shawn

-----Original Message-----
From: Matthias Ringwald [mailto:email@hidden]
Sent: Thursday, January 31, 2013 2:40 PM
To: Shawn Steenhagen
Cc: email@hidden
Subject: Re: Issue with playback of 256kbps AAC file via A2DP on iPOD

Hi Shawn

I have neither experience with AAC nor Apple's BT Dev Guideline, but can give some general Bluetooth feedback. The largest ACL packets (DH5 ) are about 1021 bytes (cannot remember the correct value) and allow for an L2CAP MTU of something around 1020 bytes. Could you try with an MTU of 1000 bytes? iOS tethering uses packets of size 15xx, which reminds me on some Ethernet constants. Anything larger than 1021 won't improve speed, as there is no baseband ACL packet that can hold it.

The L2CAP MTU limit is hard. If you try to send a packet larger than that, L2CAP is supposed to drop it or give you back an error. That's the idea of an MTU. So, it depends on the A2DP protocol/implementation what's going on.

Cheers

Matthis

On 31.01.2013, at 21:15, 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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