Filtering and recombination is the hard part. The most extreme case would be simply taking alternate 88.2/24 samples, truncating to make the lpcm compatible part of 44.1/24 and encoding the rest. This would be easy to implement, but suffer from aliasing.
As time and time again reviewers like aliasing dacs, this is a possible explanation for preference of MQA
Not being able to make a MQA file with test tones stops us ruling this out
Yes. One of my concerns wrt this 'split and then recombine' idea is that it can lead t all kinds of problems - particularly when people cluelessly 'mastering' get their hands on it.
Real-world filters have to trade off various competing requirements. So in this case there would be some tendency to add variations in the response around the 'dividing line'. Plus aliasing sprayed into the area of the spectrum. Trying to 'brickwall filter' the division reduces that, but then means a tendency for long filter profiles that will tend to ring.
And what might be optimum for split and combine may well *not* be optimum for those who are left with lpcm and then listen to the LF part alone as if it were LPCM. It is quite likely that what filter is best for one use is *not* best for the other
All this may be fine if you can cover the details in secret sauce. But may mean when people do comparisions and say "Having compared by ear I prefer MQA to the LF-only as LPCM" what they *might* really mean is that "The LF part has been degraded by the process altering it to get an LF part that recombines best with the HF hints."
It's the HDCD problem again. What you get as 'LPCM' has been altered to suit best some *other* form of replay. You then can't compare the fancy version with what would have been a well made plain version because that's *not* what you've been sold.
Given the demands on the split and combine imposed by the result having to be 'high quality' I can easily see the LF part alone ending up being degraded. Indeed, that happening might be seen by some as a useful way to nudge people towards paying for MQA decoding. All covered in secret sauce...
Given all this, keeping the sample rate and not splitting seems far more sensible to me. The tests we've done show you can reduce the filesizes that way in a way the matches MQA, and it avoids the almost-impossible - and conflciting! - demands of the magical split and recombine.