[pulseaudio-discuss] Bluetooth A2DP aptX codec quality

Pali Rohár pali.rohar at gmail.com
Wed Sep 12 10:42:56 UTC 2018


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":



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

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.

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?

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?

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.

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.

Pali Rohár
pali.rohar at gmail.com

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