[pulseaudio-discuss] Timing interval for Bluetooth Wideband speech

[Luiz Augusto von Dentz]

Hi Sathish,

On Tue, Mar 27, 2018 at 9:39 AM, Sathish Narasimman
<nsathish41 at gmail.com> wrote:
> Hi Tanu,
>
>
> On Sat, Mar 17, 2018 at 1:26 AM, Tanu Kaskinen <tanuk at iki.fi> wrote:
>>
>> On Thu, 2018-03-15 at 14:49 +0530, Sathish Narasimman wrote:
>> > Hi,
>> >
>> > I would like to know the PulseAudio module that is responsible for
>> > pushing
>> > the audio for every 1 millisecond.
>> >
>> > For NBS the packets are pushed with 48bytes and 1milli sec interval.
>>
>> Where does this 1 ms interval come from? The audio that pulseaudio
>> writes with SCO is mono, 16-bit, 8000 Hz. With that specification 48
>> bytes means 3 ms packets, not 1 ms.
>>
>> > But for WBS the timing between 2 SCO packets needs to be *7.5 Milli
>> > Sec.*
>> >
>> > *May I know how to change the packet interval between the 2 SCO
>> > packets.*
>>
>> The write_block_size variable in src/modules/bluetooth/module-bluez5-
>> device.c controls how big packets pulseaudio writes. That in turn is
>> calculated based on write_link_mtu, and that comes from the transport
>> acquire() callback. The ofono backend always returns 48 bytes. The
>> native backend returns 48 bytes by default, but it can also be
>> configured to ask the kernel what the real MTU is.
>>
>
> I am able to set the MTU to 60 bytes in the and able to make the SCO
> connection.
> But still, the data rate is maintained as 1 msec.
>
> Do we need to introduce any latency in the PA?
> As per core spec 5 , Vol 4 Part B of USB transaction the HCI packet interval
> need to be 7.5msec.
> Do we need to maintain the rate in PA?

I suspect something in btusb.c might be the problem:

https://git.kernel.org/pub/scm/linux/kernel/git/bluetooth/bluetooth-next.git/tree/drivers/bluetooth/btusb.c#n1272

Either the interval of endpoint is wrong or we shouldn't set the ASAP
flag since that might ignore the interval completely.

>>
>> --
>> Tanu
>>
>> https://liberapay.com/tanuk
>> https://www.patreon.com/tanuk
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>
>
>
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-- 
Luiz Augusto von Dentz