[Bluez-devel] SBC codec, joint stereo mode

[Bluez-devel] SBC codec, joint stereo mode

[Siegbert Baude]

Hi,

I have a question with regard to joint stereo mode in SBC. According to
the A2DP spec. p. 53, sec. 12.4 "Syntax" in the frame_header there is
one bit RFA after the join[sb] bits. In the example implementations of
the SBC codec I got (one from TI, one from iAnywhere AV SDK, one from
the ALSA-Bluetooth project), this bit is however never accounted for.
Also in the sbc_test_14.sbc example you can download from bluetooth.org,
I find at offset 04FB the beginning of the frame as 9C7F797BBC87, where
the first bit of "87" should be this RFA bit (as this example uses 8
subbands). P. 11, sec. 1.3.2.1 "RFA" however says, RFA bits should be
set to zero, which is not the case for "87".

So is this RFA bit actually used or not?

Ciao
Siegbert

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Re: [Bluez-devel] SBC codec, joint stereo mode

[Frédéric DALLEAU]

Hi,

page 53 says join[sb] is nrof_subbands-1 bits wide =3D> wich is 7 bits. =

The 8th bit is the RFA which is 0.
0xBC =3D 10111100, The first 7 bits are the join table, the last bit is =

the rfa.
Then 87 is already the scale factors.

Frederic

Siegbert Baude a =E9crit :
> Hi,
>
> I have a question with regard to joint stereo mode in SBC. According to
> the A2DP spec. p. 53, sec. 12.4 "Syntax" in the frame_header there is
> one bit RFA after the join[sb] bits. In the example implementations of
> the SBC codec I got (one from TI, one from iAnywhere AV SDK, one from
> the ALSA-Bluetooth project), this bit is however never accounted for.
> Also in the sbc_test_14.sbc example you can download from bluetooth.org,
> I find at offset 04FB the beginning of the frame as 9C7F797BBC87, where
> the first bit of "87" should be this RFA bit (as this example uses 8
> subbands). P. 11, sec. 1.3.2.1 "RFA" however says, RFA bits should be
> set to zero, which is not the case for "87".
>
> So is this RFA bit actually used or not?
>
> Ciao
> Siegbert
>
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-- =

Frederic

Without the wind, the grass does not move.


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Re: [Bluez-devel] SBC codec, joint stereo mode

[Siegbert Baude]

Hi Fr=E9d=E9ric,

Fr=E9d=E9ric DALLEAU schrieb:
> page 53 says join[sb] is nrof_subbands-1 bits wide =3D> wich is 7 bits.

Oh, my blindness. Thanks a *lot* for the hint, maybe I didn't read it =

carefully enough, because the decoding code sample I looked first at, =

was evaluating nrof_subbands instead of nrof_subbands-1.

> The 8th bit is the RFA which is 0.
> 0xBC =3D 10111100, The first 7 bits are the join table, the last bit is =

> the rfa.
> Then 87 is already the scale factors.

Yep, now it makes sense.
BTW, is this the right list to ask SBC-specific questions? I have some =

more general questions, e.g. I don't quite understand the rationale =

behind the codec definition: If you choose joint mode for encoding, it =

happens that the encoder does not use joint mode for all subbands. But =

why? Saving bits? If anybody has a pointer to some documentation for =

SBC, which explains the design decisions I would be really happy.


Ciao
Siegbert

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Re: [Bluez-devel] SBC codec, joint stereo mode

[Brad Midgley]

Siegbert

> Yep, now it makes sense.
> BTW, is this the right list to ask SBC-specific questions? I have some 
> more general questions, e.g. I don't quite understand the rationale 
> behind the codec definition: If you choose joint mode for encoding, it 
> happens that the encoder does not use joint mode for all subbands. But 
> why? Saving bits? If anybody has a pointer to some documentation for 
> SBC, which explains the design decisions I would be really happy.

yes this is the right place.

I haven't worked on it in a while so your question doesn't ring a
bell... I do need to get back into it to re-roll the encoder and
eliminate the inner loop memory move.

Brad

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