[pulseaudio-discuss] Bluetooth A2DP aptX codec quality

Arun Raghavan arun at arunraghavan.net
Tue Sep 18 18:02:30 UTC 2018

On Wed, 12 Sep 2018, at 4:12 PM, Pali Rohár wrote:
> Hello!
> I would like to let you know that Serge from soundexpert.org did in last
> month some research on aptX and its quality. Detailed article is on the
> following website, specially see parts added around "August 2018":
> http://soundexpert.org/news/-/blogs/audio-quality-of-bluetooth-aptx
> ============
> Conclusions:
> aptX codec used in BT applications is no better than SBC at 328. Despite
> slightly lower algorithmic delay of aptX both SBC and aptX codecs
> provide the same 100-150ms latency in real-life BT applications.
> If you hear the difference between SBC and aptX in some BT product,
> there can be only two explanations - placebo effect or using SBC in
> Middle or Low Quality modes.
> AptX is just a copper-less overpriced audio cable.
> aptX HD is high-bitrate version of aptX. It has clearly noticeable
> increase in sound quality (not dramatic though taking into account the
> increase in bitrate)
> ============
> And it just confirms my own testing. Whatever I did I was not able to
> either hear or see difference between aptX and SBC encoded-->decoded
> audio.
> I had discussion with Serge and there are some ideas which Linux
> Bluetooth A2DP software could supports:
> 1) Allow user to specify SBC codec quality. In most cases, including
> pulseaudio, SBC quality is chosen either to middle or low, not to
> maximum bitpool. In some cases SBC at high quality can provide better
> quality as aptX and more important -- SBC is supported by all headsets.
> 2) Show user current SBC codec quality. So user would know what was
> chosen and what should expect. I was told that Windows's Toshiba
> bluetooth stack has support for this indication.
> 3) In some cases SBC SNR bit allocation method can provide better
> quality as SBC loudness method.

Thanks for sharing, this is very interesting.

> So then I could ask question:
> 1) What to do with aptX? It is really useful for users to have it in
> Linux & pulseaudio? Because it looks like that the only thing which it
> has better is lower latency. But can pulseaudio on Linux system really
> achieve it?

What would prevent us from doing so?

> 2) Should we rather look at increasing quality of SBC codec in
> pulseaudio? And if yes, how should be quality of SBC configured? Via
> profiles? Or to invent some new protocol options? Can we increase
> default SBC bitpool allocation?

My preference is to not expose things to the user but try to move towards 

> 3) If aptX is decided as useless, what about aptX HD codec? aptX HD
> codec is supported by less products (currently I do not own any), but
> this one may provide better quality as SBC according to that research.

Right, could still be worth it indeed.

> PS: That aptX research is the first and the only one about which I know.
> All other information about quality or other details which I found on
> internet are just marking informations.

In general, it seems the work to support other codecs could still be valuable for AAC and maybe in the future, LDAC? Is anyone aware of a similar comparison for the either of these codecs?

AAC is still interesting for passthrough media, of course (I hope to have more on the ability to support that in coming weeks/months). Any objective information on LDAC would be interesting too.


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