After many years of being satisfied with my current rig I decided it's time to get somerhing new for mobile use.
This time wireless.
I had a very limited experience with BT prior to that, so, believing the advertisment, i thought BT with just SBC would sound just 'ok' and I'd need aptX for sure.
Now, with my new Momentum Wireless, the experience is very different from my expectations, and surprized by what I'm hearing I digged into the technical implementarion of SBC and aptX and came to a conclusion that aptX is only a marketing thingy. Ok, marketing and low latency.
I compared my Momentum with an iOS device (no aptX, Momentum does not support AAC) and a sony Z3(aptX) as sources. The difference in sound quality is about 0.
Now to the technical explanation, why the sound quality is the same:
SBC is a subband codec that works with 4 or 8 subbands.
aptX is a subband codec that works with 4 subbands, around 5kHz per subband. Means, SBC and aptX can be identical in this regards, if SBC is configured to divide the signal into 4 subbands.
aptX is using a fixed bit allocation for each band (in the BT implementation) following the pattern of 8:4:2:2 for each subband. It has a fixed compression ratio of 4:1
SBC is using a flexible bit allocation, that is, as the name suggests, flexible. It can be the same 8:4:2:2, or e.g. 8:3:3:2, depending on a signal it's fed. It can be also less bits per band, which means, for BT, SBC can be identical to aptX and have a compression rate of 4:1, or have less bits per band and a higher compression.
Both SBC and aptX use kind of ADPCM for encoding of the subbands and no psychoaccustics, SBC usually cuts everything above 20kHz with a hard band-pass filter.
Conclusion: aptX in the BT implementatin has fixed parameters and always delivers the same quality. SBC however has some flexibility and hence can sound exactly the same as aptX, but can also be used for higher compression with quality loss. Theoretically, SBC can even deliver better quality then aptX, because it has adaptive bit allocation per subband, and hence, if the source signal calls for it, can vary amount of bits used per frequency subband, while aptX in the BT implementation will alway allocate e.g. 8 bits for the lowest 5kHz and only 2 bits for the highest 5kHz (15-20kHz band).
This means, aptX is less a 'better' technology, but a bit more like a 'THX' logo for BT - you always know, what you are getting, it's a 4:1 compression, period.
SBC can have identical quality, but it depends on an actual implementation and bit rate, with lower bit rate and higher compression the quality will suffer.
I guss that most reputable manufacturers nowadays would use the full bandwidth available for SBC, that means, SBC and aptX will produce the same sound quality, since the technical implementation of both compression standards is almost identical.