Re: Switching between SBC and MPEG audio on headsets

[Brian Gix <bgix@codeaurora.org>]

Hi Pedro,

On 3/17/2011 2:35 AM, Peter Dons Tychsen wrote:
> Hi Brian,
>
> On Wed, 2011-03-16 at 16:09 -0700, Brian Gix wrote:
>> This is not actually true. You can have multiple media channels open
>> simultaneously (for instance one for Video and one for Audio) which
>> are
>> opened with careful handshaking with the AVDTP_OPEN signaling
>> command.
>> In theory, this could be two audio channels as well.  If someone wants
>> a
>> Journaling channel, it would be opened without the AVDTP_OPEN
>> signaling.
>
> Yes, except protocol-wise the new channel on PSM=0x19 could also be the
> start of a new A2DP signaling connection (however very unlikely). As you
> mentioned, the problem with the Journaling channel can be filtered out
> with correct signaling.

AVDTP only allows a single signaling channel to exist between any two 
peers. You are allowed to have separate signaling channels to separate 
remote devices, but even if you are supporting multiple media streams 
(video/audio, or audio/audio) to the same device, all AVDTP signaling 
must be done on a single channel.

>
> I totally agree with you that your solution is the most elegant. However
> back when A2DP was introduced it did not seem clear to everyone how
> multiple streams would work. Some thought it would work with multiple
> media channels (like you suggest), and some thought it would work with
> multiple signaling channels (with max 1 media channel).
>
> Most of the implementations i have seen (for headsets), unfortunately
> ended up with the last solution. This means that they will not accept a
> secondary media channel. This is unfortunately basically true for all of
> the most popular chipsets and SDKs.

Actually, I thought I was agreeing with you about *not* having two media 
channels open simultaneously. There are too many legacy devices in the 
market that do not support a second media channel for audio. I was just 
pointing out that the limitations are in the implementations, not the 
specification. And I am a big believer in maintaining compatibility with 
legacy devices. It is possible to have two spec compliant devices that 
do not interoperate, which of course is why we have UPFs.

> Your suggestion is clever and follows the standard nicely (and would get
> my vote for a white-paper), but i doubt it will work with many headsets
> out there.
>
> Even worse, many headsets are hardcoded (not nice) to believe that the
> 3rd channel is always the Journaling channel. That is of course, just
> plain wrong due to the reasons you have stated.

I haven't actually even encountered any headsets in the wild that 
support Journaling.

>
> Thanks,
>
> /pedro
>


-- 
Brian Gix
bgix@codeaurora.org
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum