An observation about some purchased FLAC Files.

One of the ideas which I’ve blogged about often – a pet peeve of mine – is how lossy compression is not inaudible, although some people have claimed it is, and how its use degrades the final quality of modern, streamed or downloaded music.

And so if this is taken to be real for the moment, a question can rise as to what the modern methods are, to purchase High-Fidelity, Classical Music after all. One method could be, only to purchase Audio CDs that were mastered in the 1990s. But then, the eventual problem becomes, that even the best producers may not be mastering new recordings in that format anymore, in the year 2019. We may be able to purchase famous recordings made in the 1990s, but none from later, depending on what, exactly, our needs are. But, an alternative method exists to acquire such music today, especially to acquire the highest quality of Classical music recorded recently.

What people can do is to purchase and download the music in 16-bit, FLAC-compressed format. Ideally, this form of compression should not insert any flaws into the sound on its own. The sound could still be lacking in certain ways, but if it is, then this will be because the raw audio was flawed, before it was even compressed. By definition, lossless compression decompresses exactly to what was present, before the sound was compressed.

I have just taken part in such a transaction, and downloaded Gershwin’s Rhapsody In Blue, in 16-bit FLAC Format. But I made an interesting observation. The raw 16-bit audio at a sample-rate of 44.1kHz, would take up just over 1.4mbps. When I’ve undertaken to Flac-compress such recordings myself, I’ve never been able to achieve a ratio much better than 2:1. Hence, I should not be able to achieve bit-rates much lower than 700kbps. But the recording of Gershwin which I just downloaded, achieves 561kbps. This is a piece in which a piano and a clarinet feature most prominently, and, in this version, also some muted horns. And yet, the overall sound quality of the recording seems good. So what magic might be employed by the producers, to result in smaller FLAC Files?

(Updated 8/27/2019, 14h45 … )

(As of 8/18/2019 : )

Well there’s one trick which I can think of, which I could not insert when compressing to FLAC format myself. Specific instruments could be put into two categories:

  • Instruments which must contain a significant high-frequency component, such as clarinets, violins, other strings, French Horns, cymbals etc.,
  • Instruments that can be considered complete in their sound, even if they do not contain much ‘energy’ or ‘variance’ at the highest audible frequencies. These instruments could include the piano (for certain performances), organs, certain bass instruments that are neither strings nor acoustic drums, etc. And arguably, those might include muted brass instruments.

What some producers might be doing, is to put the second category of instruments through a low-pass filter (e.g., a second-order filter with a corner-frequency of 5kHz), but not the first category. It’s really in the ‘air’, the ‘hiss’ that’s present at the highest audible frequencies, that the FLAC format becomes inefficient, because this variance will not follow predictably from the predictive coefficients used in the Linear Predictive Coding. Thus, hiss and air -like sounds will give higher residuals, which must in turn be encoded exactly, with some variable-length encoding, and which tend to make even the best FLAC-compressed recordings longer in bits and bytes.

Yet, as long as that clarinet is only playing for part of the duration of the piece, while the piano is playing for the entire duration, and if the muted horns have indeed been put in the second category, the result will still be, shorter FLAC Files, that are highly listenable, and which I cannot match because I do not have access to all the contributing instruments’ individual tracks.


 

(Update 8/19/2019, 13h40 : )

I’ve just re-examined the collection of FLAC Files, which I encoded myself, from Audio CDs, that were mastered in the 1990s. Some of those recordings included Beethoven’s 7th Symphony. When looking at the bit-rates of my own FLAC Files, I find that, as expected, most of those are in the vicinity of 700kbps. However, my own FLAC encoding of the 2nd Movement, of this composer’s 7th Symphony, also came down to a bit-rate of 534kbps.

What this could mean is that, just by accident, certain compositions have simply been orchestrated in a way that gives them a ‘warm sound’. Each part that plays for most of the recording could have been handed to an instrument, that just doesn’t have very high frequency-components, and the parts which are being played with such high frequency-components, could just happen to play only for a short duration. And then in such a case, a FLAC File with a lower bit-rate can result, even though the producer had no way to know that FLAC Files would one day exist.

But what I also observe about the 2nd Movement of Beethoven’s 7th, is that this Movement has a lot of very quiet passages, and FLAC format just happens to compress those well.


 

(Update 8/19/2019, 17h00 : )

There remained a possibility which I needed to rule out, which was, that the professionals who sold be the FLAC File in question, may have had better methods or parameters, to FLAC-compress their audio, than I do, even though I’m able to play back and therefore decompress the resulting FLAC File. To test for this possibility, what I did was to import the FLAC File in question into ‘Audacity’, at which point it was also decompressed, and then to export it again, as a FLAC File, compressed the way I would normally compress them, with parameters simplified by the command-line option ‘-8′.

The result was a FLAC File with an average bit-rate of 567kbps, instead of 561kbps. In other words, even though there was a slight difference in the encoding itself, the resulting difference in the average bit-rate was not dramatic. And this is also approximately what I’d expect.


 

(Update 8/27/2019, 14h45 : )

I can think of a different reason, why my purchased FLAC File may be achieving a lower bit-rate, than the ones do, which I compressed myself, starting from Classical Music originally recorded on Audio CDs by famous orchestras.

It could very well be that in today’s age, Professional Musicians record their music, be it Rock and Roll or Classical, on equipment that also stores this music on internal storage devices, and that the Sound Producers rely on this feature of their Audio Equipment.

What could be, is that this sort of equipment may already store the Audio in a compressed, lossy format. For example, their equipment may record the Audio at a 24-bit sample-resolution, but may simply use DPCM to compress it, maybe not even using ADPCM, so that for each recorded sample, only ?12 bits? need to be stored. This would be a form of lossy compression much harder for me to recognize, than MP3-compression would be. But the effect would still be similar in some ways to using a low-pass filter. Only, what DPCM does to sound is more subtle, especially if the original instrument was mainly generating mid-range frequencies. And the net effect would be, that once the Sound Producers mix this format down to a 16-bit FLAC File, the result is a FLAC File with a noticeably lower bit-rate.

Dirk

 

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