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MQA ... why bother? ... why not just use least significant bits truncation?

Posted by bjh (A) on May 22, 2017 at 12:34:09

In a recent post here [on Digital Drive] by Charles Hansen he says:

"MQA apparently asserts that the at least 6.8 (and possibly as much as 11.8) of the lower-order bits are inaudible."
https://www.audioasylum.com/audio/digital/messages/18/184097.htm

This is in reference to the lower order bits being re-purposed by the proprietary MQA encoding scheme ... per Hansen:

"However the seven or eight Least Significant Bits (LSBs) of the container are used to store the "folded" dual-rate audio data (encoded losslessly) and the quad-rate audio data (encoded with lossy compression). They *replace* the low-level bits in the original 24-bit file. . reducing the resolution of MQA below 24 bits."
https://www.audioasylum.com/audio/digital/messages/18/184054.html

I will not concentrate on the comment "... reducing the resolution below 24 bits", or even that MQA resolution is "... limited to a maximum of 17.2 bits" (same post). Those who would like to contend those assertions should have at it directly with Hansen ... and in advance, good luck to you, you'll need it!

Rather Hansen's comment triggered another thought ... *if* the lower order bits are relatively insignificantly (1) why not just truncate [the lower order bits] and stream the remainder? ... with the missing bits filled in on the receiving side (e.g. some dithering scheme or whatever).

So we still lose the low order bits ... no change fundamentally since that are effectively lost already with MQA ... but forego the need for the fancy "origami" folding, including the lossy compression of 4FS audio data.


(1) To be clear Hansen make no claim the lower order bits are insignificantly, in fact he casts doubt on just that. However if the idea of tossing the LSBs is a non-starter then MQA is a non-starter ...for very same reason!

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