question about 'read' system call of sco socket

question about 'read' system call of sco socket

[liu yang]

Hi experts,
I have one question about 'read' system call of sco socket. I'm using
an open source  library which implements bluetooth handfree profile on
linux platform. About our use case, cell phone works as audio gateway
that all audio received on cell phone will be streamed to linux PC,
and vice versa. Everything works just fine of audio streaming above
bluetooth channel. But our special application is very sensitive on
timing that we need send/receive bluetooth audio frame every 20ms.
Regarding sending direction, I can tweak MTU of bluetooth donglel
installed on my linux PC. Let's say, MTU is set to 320 bytes, so every
20ms 'send' system call only needs be called one time. ( the audio
frame conveyed aboved bluetooth is of signed linear 16bit per sample,
8K sampling-rate )  But this does not affect receiving direction that
every 'read' system call on sco socket returns exact 48 bytes. Also I
find such 'read' event on linux 'poll' is triggered every 10ms. Every
10ms, such 'read' action is invoked, but it takes 3 or 4 times
invokation ( sometimes 3 , sometimes 4) of  'read' to consume this
'read' event, which results in 'EAGAIN'. So every 20ms, I get 288
(48*6) or 336 (48*7)  bytes, not exact 320 I expect.

Shortly, my question is how to receive ( 'read' system call ) exact
320 bytes every 20ms.

Thanks

Kandy

Re: question about 'read' system call of sco socket

[Zhu Yanhai]

On 03/02/2010 10:28 PM, liu yang wrote:
> Hi experts,
> I have one question about 'read' system call of sco socket. I'm using
> an open source  library which implements bluetooth handfree profile on
> linux platform. About our use case, cell phone works as audio gateway
> that all audio received on cell phone will be streamed to linux PC,
> and vice versa. Everything works just fine of audio streaming above
> bluetooth channel. But our special application is very sensitive on
> timing that we need send/receive bluetooth audio frame every 20ms.
> Regarding sending direction, I can tweak MTU of bluetooth donglel
> installed on my linux PC. Let's say, MTU is set to 320 bytes, so every
> 20ms 'send' system call only needs be called one time. ( the audio
> frame conveyed aboved bluetooth is of signed linear 16bit per sample,
> 8K sampling-rate )  But this does not affect receiving direction that
> every 'read' system call on sco socket returns exact 48 bytes. Also I
> find such 'read' event on linux 'poll' is triggered every 10ms. Every
> 10ms, such 'read' action is invoked, but it takes 3 or 4 times
> invokation ( sometimes 3 , sometimes 4) of  'read' to consume this
> 'read' event, which results in 'EAGAIN'. So every 20ms, I get 288
> (48*6) or 336 (48*7)  bytes, not exact 320 I expect.
>
> Shortly, my question is how to receive ( 'read' system call ) exact
> 320 bytes every 20ms.
>
> Thanks
>
> Kandy
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Hi Yang,
I'm not a expert at this. IIRC SCO connection is designed to carry fixed 
rated data, which is 64kbps[1]. In your case, you are expecting about 
16bit*8K = 128kbps, which is beyond of SCO's capability. Maybe you can 
consider creating two SCO links (the upper limit is 3 between two 
Bluetooth devices) to carry your data or maybe you could consider using 
a eSCO link?
Again, I'm not good at this, just saying...

Regards,
Zhu Yanhai

[1] Bluetooth specification Version 2.1, Vol 1, Page 44,
     3.5.5 Synchronous connection-oriented (SCO)