Re:A2DP AAC codec bandwidth
Re:A2DP AAC codec bandwidth
- Subject: Re:A2DP AAC codec bandwidth
- From: Feng Yu Hong <email@hidden>
- Date: Tue, 12 Apr 2016 10:26:12 +0000
- Thread-topic: A2DP AAC codec bandwidth
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.
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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