Bluetooth Headset - Voice Commands

Nils Faerber nils.faerber at
Mon Mar 5 12:27:31 CET 2007

Sorry for beeing late here ;)

Mike Hodson schrieb:
> On 2/28/07, Jonathon Suggs <jsuggs at> wrote:
> By being connected to the USB bus, this works exactly like every
> current Linux computer with bluetooth: as of now, the BlueZ stack can
> do SCO / headset, and they are working daily on properly working A2DP
> (advanced audio) stereo codec support both as alsa modules.  It would
> then be my guess, that all the OpenMoko software would have to do, is
> change the alsa input/output by responding handsfree button or avrcp
> commands (for stereo headsets).

SCO uses another transfer interface than A2DP - A2DP runs through the
normal data channel und SCO audio is routed through a synchronous
channel though also ending up somewhere else, namely the PCM interface
of the BT chipset.
The PCM interface can be routed either to HCI (the host controller
interface) or a dedicated PCM output of the chipset. This can be used on
dedicated hardware to channel the PCM data directly to some audio codec
and amplifier not wasting HCI bandwidth and making building headsets
easier ;)
Some chipsets can have this routing hardwired to PCM so that you will
never be able to get the SCO audio data into your software.

So buttomline here is that it largely depends on the BT chipset and its
firmware if or if not the SCO data can be routed to HCI.

Since AFAIK the NEO will use a CSR chipset the probability is high that
it can be rerouted if HCI is not yet default.

For A2DP the whole thing is easier since A2DP is always a normal data
connection sending out audio data packets through userspace.

> Furthermore, it is definitely plausible that the bluetooth controller
> in your pocketpc is somehow intertwined with the GSM chipset.  If this
> chip has no provision of routing audio into the software, and only
> considers bluetooth a voice service, then it would talk directly to
> the wireless interface and its GSM chip. The windows mobile/ppc
> software can't grab it.

In theory this could be possible, if the GSM chipset has a matching PCM


> Mike
  nils faerber

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