Apple

Thank you, Craig.

We modified MTU size, and the measured bandwidth reach 20kHz.

Yuhong

发件人: email@hidden [mailto:email@hidden]
发送时间: 2016年1月22日 3:07
收件人: Feng Yu Hong
抄送: email@hidden
主题: Re: A2DP AAC codec bandwidth

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

On Jan 20, 2016, at 7:13 PM, Feng Yu Hong <email@hidden> wrote:

IOS devices support SBC and AAC codec in A2DP profile. We measured the bandwidth of AAC codec by playing 20~20kHz log sweep sine wave signal. We tested three iPhones ( iphone4S: ios 6.1.3; iphone5: ios8.1.3 iphone6 ios9.2). We compared two Bluetooth headphone (one uses CSR8635 chip, another uses our own chip). CSR’s bandwidth is 20kHz, but our chip’s bandwidth is 17.5kHz. We captured Bluetooth communication data through Ellisys Bluetooth analyzer. We have excluded AAC decoder of Bluetooth headphone. AAC encoder of Iphone sends the different bitstream when it connects the two headphones. But AAC codec capabilities of the two headphones are the same, and the negotiated AAC capabilities through AVDTP are the same. The negotiated AAC format is listed.

Media Codec Type          MPEG-2,4 AAC

Object Type                     MPEG2 LC

Sampling Frequency      44.1kHz

Channels                          2

Variable Bit Rate             Yes

Bit Rate                            262 kb/s

The issue seems to involve a configure parameter of aac bandwidth (or cutoff frequency). But A2DP doesn’t define the configure parameter of bandwidth, and only define the configure parameters of object type, sampling frequency, channels, VBR and bit rate.

Does IOS use private parameter that isn’t defined by A2DP? CSR chip seems to know this private parameter.

Yuhong



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References:
	>A2DP AAC codec bandwidth (From: Feng Yu Hong <email@hidden>)
	>Re: A2DP AAC codec bandwidth (From: Craig Dooley <email@hidden>)