On 16.08.2018 23:23, ValdikSS wrote: > On 16.08.2018 22:53, Luiz Augusto von Dentz wrote: >> The reason of 53 bitpool is probably due to the spec. suggesting it as >> high quality, which as you said is not quite right, so BlueZ never >> really imposed any limitation on what you can configure, even going >> above 512 kbit/s. As for not using dual channel that is known to >> consume more bandwidth which may have an impact on airtime and >> coexisting with wifi which is why joint stereo is preferred, the real >> problem I would say is limiting to just 53 especially because the >> bitpool can be adjusted on the fly so this is on the headset >> manufacturers, perhaps because they actually want SBC to be perceived >> as a bad codec so user would favor their proprietary codec? Or perhaps >> because SBC has a quite bad compression rate and latency compared to >> other codecs, still there is no reason to limit the bitpool. >> > I see the problem in the specification. Imagine you're a headphones manufacturer and your chip limitation is 512 kbit/s. There's no way to properly indicate this constraint. If you return bitpool max 53 in GetConfiguration/GetAllConfiguration GetCapabilities/GetAllCapabilities* > for the connecting device, everything would be fine with joint stereo/stereo/mono, 8 subbands and 16 blocks, but the connecting device can use DUAL_CHANNEL with bitpool 53, which will produce 617 kbit/s SBC audio for 44.1 kHz, or even just use 4 subbands with joint stereo, which will produce 633 kbit/s and is higher than a bitrate constraint. > > The decoder should support all channel types, all block sizes and all subbands. The headphone manufacturer can't restrict connection only to joint stereo/stereo as it wouldn't be compatible with the specification. > > As far as I know, only source can decrease the bitpool, the headphones have no way to notify the source that high bitrate audio can't be decoded and played properly. > > To circumvent max bitpool 53 found on most headphones, bluetooth stacks could use either DUAL_CHANNEL, or joint stereo with 4 subbands. I prefer the former. >
