I actually think this enormous thread, especially the last several pages, has produced some very helpful clarity despite the periodic acrimony.
That clarity comes in a very particular area: the entire debate about whether "real music" contains high levels of ultrasonic content is a debate that has arisen because of the fundamentally questionable claim at the heart of MQA's design. That claim is the claim that true high fidelity music reproduction requires a sample rate greater than 48kHz, because according to Stuart (in his AES paper), digital hi-fi music reproduction requires bandwidth up to 26kHz.
So is
@Raindog123 correct that "real music" includes electronic music that can have
increasing amplitude as frequency increases up to and beyond 20kHz? Yes, that appears to be correct.
Is
@DimitryZ correct that generally speaking music produces ultrasonic tones at lower amplitude than tones in the audible band? Yes, that appears to be correct as a general proposition even though there are exceptions per Raindog's comment.
Is
@amirm correct that if you are going to create, market, and distribute high-res music, then you have to deliver it in 24-bit form since 16-bit is the next practical step down from that, and that you more or less have to deliver it in at least 88.2k/96k form since 44.1k is not high-res and 48k is not consistently considered high-res by audiophile consumers? Yes, he is correct about that. We here can certainly critique that audiophile conventional wisdom, but Amir is basically correct about those market realities in terms of the available tiers of bit depth and sample rate.
Are Amir,
@John Atkinson , and Bob Stuart correct that a fully "empty"/"100% clean" 24 bits of depth is not necessary to produce a sufficiently low noise floor that meets or exceeds the limits of human hearing? Yes, they are correct. While Amir's, Atkinson's, and Stuart's particular arguments about this have some slight variations from each other, we can all acknowledge that 6.02 x 20 = 120.4, which means that 20 bits' worth of depth provides the necessary -120dB noise floor and therefore it is not necessarily a problem to store data in the lowest 4 bits, if there is some value or good reason to do so.
So none of these arguments is, in my view, wrong - they are all substantially correct as far as they go.
The underlying issue, of course, is that there is no evidence that ultrasonic frequencies are necessary at all - at whatever amplitude - for hi-fi music reproduction. And since there is no need to preserve ultrasonic frequencies, there is by definition no sonic value in doing anything - no matter how clever or allegedly harmless - to store them inside the digital data that encodes/stores the sound in the audible band. In other words, the way to reduce the file/bandwidth size of high-sample-rate digital content without compromising fidelity is simply to downsample it to the minimum feasible sample rate whose Nyquist frequency remains above 20kHz (plus some buffer for filtering).
So a lot of the stuff we've been arguing about doesn't actually need to be argued. The only real question from a sonic point of view is:
Is there potential harm caused by the way MQA stores ultrasonics
and other encoding info/data/flags in digital files? I would say Yes, for two main reasons:
- It stores some of that data within the first 16 bits. That means decoded, 24-bit MQA may have compromised bit depth because its effective bit depth is not contiguous in the most significant bits. It also means that undecoded MQA has a noise floor inferior to 16-bit redbook, which is a problem not only for the millions of playback systems that lack MQA capability, but also for the fact that undecoded MQA content has begun to pollute the pool of redbook digital music, and if/when undecoded MQA files begin to circulate outside Tidal (very likely/possibly already happening, because we know how lazy/indifferent the labels are about this stuff), they will not be marked as such and we'll have no way of knowing where they are or how numerous.
- It creates reconstruction-filter challenges for playback equipment. Hopefully this will be solved more fully as time goes on, but as of now there are lots of DACs out there that either emit audible clicks when switching between MQA and non-MQA filters on the fly, or else just keep MQA fllters on permanently. As MQA filters are slow/leaky, they produce undesirable aliasing. If you can't turn them off for non-MQA content, or you can turn them off only at the cost of audible clicks between MQA and non-MQA tracks, that's a problem.
IMHO the market case for MQA is very limited and rather weak. There is no significant streaming (or IMHO digital-file-for-purchase) demand specifically for files with bit rates above 88.2/96k. And MQA provides no significant size savings compared to 24/88.2 or 24/96 FLAC. Moreover, the ability to have one MQA file that can be served in multiple resolutions is also a highly overrated feature, since in practice that "multiple" is only two: redbook and high-res. There is no market for - and never will be a market for - a more split-up set of tiers like redbook vs 24/48 vs 24/96 vs 24/192, etc. Never going to happen.
In this vein, one can make a strong argument that the main - and only significant - force working to create a market for super-high sample rates like 352.8k these days is MQA itself, which is ironic since MQA does not deliver that sample rate and is totally incapable of ever doing so.