Apple

Hi Craig

Thank for your reply.

I have read the technical note tn2271. but I don’t know the difference between ios 6.1.3 and ios 8.3 (ios 9.2). MTU size that we used is 895 byte. The quality of aac bitstream in ios 8.3 and ios 9.2 is not as good as the quality in ios 6.1.3.

Yuhong

发件人: email@hidden [mailto:email@hidden]
发送时间: 2016年4月28日 12:39
收件人: Feng Yu Hong
抄送: email@hidden
主题: Re: A2DP AAC codec bandwidth

Hi Feng,

The AAC codec is always configured as 44100Hz, but the MTU determines the max packet size that will be encoded. You can read about the encoder here. The encoder is configured as kAudioCodecBitRateControlMode_Variable.

Technical Note TN2271: Bit Rate Control Modes for AAC Encoding

- Craig

On Apr 12, 2016, at 3:26 AM, Feng Yu Hong <email@hidden> wrote:

Dear Craig

We have modified MTU size from 672 to 895.   Iphone 4s (ios 6.1.3 ) can be up to 20kHz bandwidth. But Iphone4s( ios 8.3) and iphone6( ios 9.2) only be up to 18.9kHz bandwidth. CSR8635 has the same issue.

Is it possible that AAC bandwidth can be up to 20kHz, rather than 19kHz in a high version of the ios system?

Thanks!

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