AAC requires 1024 frame packets, and at 44.1kHz, that gives ~43 packets per second. If you lose ~10 bytes for L2CAP and AVDTP headers, that leaves 43 * 662, or ~227kb/s. If your device only supports 2Mbit EDR, 671 is a good compromise since any packet sent will fit in a 2DH5 packet, instead of needing to be split into a 2DH5 + a 2DH3. - Craig
On Jan 22, 2016, at 12:53 PM, Gordon Rankin <gordon@wavelengthaudio.com> wrote:
Craig,
We are releasing an A2DP AAC solution this month. I looked at our code after reading your response on the MTU subject and found we are negotiating at 672 bytes. Is there some sort of calculation with the MTU number and the bandwith? I searched older posts which indicated a maximum of 885 bytes. Any information would be appreciated.
Thanks, Gordon
Date: Thu, 21 Jan 2016 11:07:01 -0800 From: Craig Dooley<cdooley@apple.com> To: Feng Yu Hong<fengyuhong@vimicro.com> Cc:"bluetooth-dev@lists.apple.com" <bluetooth-dev@lists.apple.com> Subject: Re: A2DP AAC codec bandwidth Message-ID:<B6156406-1236-4065-B7AF-C5C054D7581C@apple.com> Content-Type: text/plain; charset="utf-8"
The encoder takes into account the MTU negotiated when determining the maximum frame size available. For example, if the device has an MTU of 565 for the AVDTP channel, that only allows for ~192kb/s, even if the endpoint configuration says the device can handle 262 kb/s.
- Craig
-- J. Gordon Rankin Owner and Chief Scientist ====== Wavelength Audio, ltd ======
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