Support for the Bluetooth subband codec (SBC)

Support for the Bluetooth subband codec (SBC)

[Marcel Holtmann]

Hi guys,

besides the Bluetooth headset/handsfree support there is another type of
audio support. It is the high quality audio (A2DP) for headphones and so
on. To include a native support for these devices with every Bluetooth
adapter (for example the ones built into your notebook) it is necessary
to support the subband codec (SBC) from Philips. The codec description
is in the Advanced Audio Distribution Profile (A2DP).

I am not an audio expert and far away from doing any codec specific
things, but from my understanding the ALSA kernel part should be
extended with the SBC type for PCM devices and the library should know
how to convert any PCM data into the SBC format so we can use it inside
the kernel in a A2DP driver.

Any comments?

Regards

Marcel




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Re: Support for the Bluetooth subband codec (SBC)

[Jaroslav Kysela]

On Mon, 8 Nov 2004, Marcel Holtmann wrote:

> Hi guys,
> 
> besides the Bluetooth headset/handsfree support there is another type of
> audio support. It is the high quality audio (A2DP) for headphones and so
> on. To include a native support for these devices with every Bluetooth
> adapter (for example the ones built into your notebook) it is necessary
> to support the subband codec (SBC) from Philips. The codec description
> is in the Advanced Audio Distribution Profile (A2DP).
> 
> I am not an audio expert and far away from doing any codec specific
> things, but from my understanding the ALSA kernel part should be
> extended with the SBC type for PCM devices and the library should know
> how to convert any PCM data into the SBC format so we can use it inside
> the kernel in a A2DP driver.

Any datasheet for this format? Anyway, if SBC format only encapsulates 
linear PCM samples to packets, then it should be handled in the driver 
like the USB driver does.

						Jaroslav

-----
Jaroslav Kysela <perex@suse.cz>
Linux Kernel Sound Maintainer
ALSA Project, SUSE Labs


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Re: Support for the Bluetooth subband codec (SBC)

[Marcel Holtmann]

Hi Jaroslav,

> > besides the Bluetooth headset/handsfree support there is another type of
> > audio support. It is the high quality audio (A2DP) for headphones and so
> > on. To include a native support for these devices with every Bluetooth
> > adapter (for example the ones built into your notebook) it is necessary
> > to support the subband codec (SBC) from Philips. The codec description
> > is in the Advanced Audio Distribution Profile (A2DP).
> > 
> > I am not an audio expert and far away from doing any codec specific
> > things, but from my understanding the ALSA kernel part should be
> > extended with the SBC type for PCM devices and the library should know
> > how to convert any PCM data into the SBC format so we can use it inside
> > the kernel in a A2DP driver.
> 
> Any datasheet for this format? Anyway, if SBC format only encapsulates 
> linear PCM samples to packets, then it should be handled in the driver 
> like the USB driver does.

the datasheet for this codec can be found on the Bluetooth specification
webpage (www.bluetooth.org/spec/). It is part of the A2DP document. I
think you need to be logged in to get it and so I will send it to you in
a private mail later.

I don't think that this is linear PCM, but actually I am no expert when
it comes to codec specific stuff. I would be great if you can take a
look at it and tell me what you think how we should support it.

Regards

Marcel




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Re: Support for the Bluetooth subband codec (SBC)

[Jaroslav Kysela]

On Mon, 8 Nov 2004, Marcel Holtmann wrote:

> I don't think that this is linear PCM, but actually I am no expert when
> it comes to codec specific stuff. I would be great if you can take a
> look at it and tell me what you think how we should support it.

I see - SBC is an audio compression. It should be implemented in alsa-lib 
as you suggested.

						Jaroslav

-----
Jaroslav Kysela <perex@suse.cz>
Linux Kernel Sound Maintainer
ALSA Project, SUSE Labs


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Re: Support for the Bluetooth subband codec (SBC)

[Marcel Holtmann]

Hi Jaroslav,

> > I don't think that this is linear PCM, but actually I am no expert when
> > it comes to codec specific stuff. I would be great if you can take a
> > look at it and tell me what you think how we should support it.
> 
> I see - SBC is an audio compression. It should be implemented in alsa-lib 
> as you suggested.

my idea is to add a snd_pcm_t for every stream endpoint and then set the
type of the snd_pcm_hardware_t to SBC at runtime. The ALSA library then
must do the audio compression, rate conversion etc.

However my current problem is that SBC type has some codec specific
information elements (A2DP, page 20) and I don't know for some of them
how we should support them. For all that don't have the specification
these information are:

	- Sampling Frequency
		- 16000, 32000, 441000 and 48000 Hz
	- Channel Mode
		- Mono, Dual channel, Stereo, Joint stereo
	- Block Length
		- 4, 8, 12 and 16
	- Subbands
		- 4 and 8
	- Allocation Method
		- SNR and Loudness
	- Minimum Bitpool Value
	- Maximum Bitpool Value
		- Both are ranging from 2 to 250

Some of these settings are mandatory to support and some of them are
optional.

The only clear value for me is the sampling frequency. Mapping the
channel mode to the channels_{min|max} is not clear and for the rest I
have no idea.

Any ideas on how we should support this?

Regards

Marcel




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