[pulseaudio-discuss] Bluetooth A2DP aptX codec quality

[Pali Rohár]

On Wednesday 12 September 2018 19:03:41 Luiz Augusto von Dentz wrote:
> Hi Pali,
> 
> On Wed, Sep 12, 2018 at 1:42 PM, Pali Rohár <pali.rohar at gmail.com> 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.
> >
> > 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?
> 
> I don't think, not the level of latency necessary for speech and to
> avoid lip sync issues, so that would leave aptX at the same category
> as SBC.

That is what I thought. So seems that aptX (non-HD) has no benefit over
SBC in pulseaudio.

> > 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?
> 
> I recall setting it to 64 before, but perhaps we are using 53 given
> that most headset set that as maximum influenced by the spec suggested
> values, I wouldn't go above 512kbit/s since then leave very little
> room for any other traffic.

So... seems that maximum value is 53:
https://cgit.freedesktop.org/pulseaudio/pulseaudio/tree/src/modules/bluetooth/bluez5-util.c#n1599
https://cgit.freedesktop.org/pulseaudio/pulseaudio/tree/src/modules/bluetooth/bluez5-util.c#n1260

Even when headset advertise higher value. Should not we increase when
headset support higher value?

Also question about allocation method SNR vs loudness. Default is
loudness.

> > 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.
> 
> That is probably useful as something that provides a quality
> improvement compared to SBC.

Ok, I would let this part to other people as currently I do not have
aptX HD native hardware.

> > 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.
> 
> I had some suspicion before given that no manufacturer provided any
> evidence aptX would beat SBC at the same bitrate, it is probably
> better just because we are stuck at 53 bitpool with SBC while aptX can
> probably have much higher bitrate. Anyway thanks to the researcher for
> putting the time to evaluate the codecs we now have a good reference
> for the quality each codec provides.

-- 
Pali Rohár
pali.rohar at gmail.com