- Aug 14, 2018
Isn't this a good example of exact issue we are having in using the word "lossy:? Downsampling is mathematically lossy with reference the available information. What you wrote implies that downsampling, despite discarding information, is not lossy because it satisfies a perceptual criterion.
It's mixing domains, goals, purposes. MQA did the same when they called their ability to deliver a digital file with known provenance to a streaming service or download site, and then through known hardware, "lossless".
Put it this way: In another, past thread here on another topic, I stated that the resampling process of taking a 96k source down to 44.1k changed the audio data, and therefore a 96k PCM file and a downsampled 44.1k PCM file could sound different because of the non-integer sample-rate conversion, and that the sonic difference could be seen in a difference file from trying to null-compare the two.
In response, our knowledgeable and currently thread-banned friend @mansr [edit: it was @danadam actually] told me I was mistaken because my method of trying to null-compare the 96k original with the 44.1k downsampled version couldn't work. Instead, he explained, I should downsample the 96k to 44.1k, then resample the 44.1k back to 96k and compare the two 96k files. When I did so, they nulled out 100% for all frequencies up to 22.05kHz, indicating that the audible-range information from the original 96k file could be perfectly reconstructed. So I had to admit I was mistaken in my initial claim, which I was happy to do since I had learned something - I didn't realize that non-integer resampling was still lossless; in other words the different Nyquist limits of course made a difference in the ultrasonics, but in the audible range the non-integer resampling was a non-issue in terms of the ability to perfectly reconstruct the original content that was originally in a higher sample rate.
My point is that I think it obscures more than it reveals to call the well-documented limits of human hearing "perceptual" in the same way that lossy codecs' compression algorithms are perceptual - and therefore it also obscures more than it reveals to call downsampling that perfectly preserves, bit for bit, the audible-range musical data, "lossy." By that logic, a 176.4k file created from a 352.8k original is lossy. Sure, there is a clear logic by which that claim can be made - but in order to employ that logic you have to stretch the term "lossy," in the context of sound reproduction for humans, to the point where it becomes meaningless (which is not what Amir is trying to do, but which is most certainly what many promoters of MQA have attempted and are continuing to attempt to do).
We can certainly debate the relative merits of various encoding and compression algorithms independently of questions of mathematical lossiness/losslessness, and I have absolutely no problem with doing so.
But to lump something as fundamental as frequency and sample rate into the same lossy bucket as perceptual encoding - to me that is a mixing of domains, and when it comes to the discussion of MQA, a mixing of goals and purposes as well. Amir says he can pass a blind test distinguishing 320k mp3 from lossless. He would never make any parallel claim that he could do so with two files that were bit-identical except for frequencies above 22kHz - nor, I think, would he or most others here be inclined to believe such a claim made by anyone else. Returning to my prior example, the difference file between the musical data in a PCM file and an mp3 file made from that PCM file will be audible. By contrast, the difference file between the data in a 96k PCM file and a 44.1k PCM file made from that 96k original will not be audible. That's a meaningful difference.
I think at some point this becomes a philosophical, perhaps even semantic, debate. But I think it is both practically and epistemologically improper to equate (implicitly or explicitly) downsampling and perceptual encoding under a simple heading of "lossy."