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Topic: FLAC comparison - perceptual shelving? (Read 7347 times) previous topic - next topic
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FLAC comparison - perceptual shelving?

I thought turning to direct CD FLAC conversion rather than 320 would get rid of the annoyance of cross-checking for transcoding and proving spectral transparency but I find it is just as hard unless I do it myself. My question is, I've recently found a lot of FLACs appear shelved below 22.1khz and whether this was a cause for concern as I am relatively unlearned in comparative FLAC analysis. I understand that a pure accurate rip of an older recording or a less active recording will not necessarily stretch continuously to 22.1khz and might even peak at around 16khz, whilst remaining (sounding) entirely 'as intended' but the two songs were processed exactly the same. I have two songs in FLAC. The one on the right is continuous to 22.1khz in Adobe Audition and was conditionally verified in auCDtect and Spectro etc. The one on the left appears degraded by comparison even though it was processed similarly and also has been identified as having a 22.1khz cut-off.

FLAC comparison

My query is: can this be normal? There appears to be material above the shelf but the shelf itself seems more forced than a natural declination in activity. Can any spectres give me some explains or direct me to some hydro links as my searches have come to no avail. Thanks.

FLAC comparison - perceptual shelving?

Reply #1 –
Quote from: Blaice on
My question is, I've recently found a lot of FLACs appear shelved below 22.1khz and whether this was a cause for concern as I am relatively unlearned in comparative FLAC analysis.


Generally CD audio is low pass filtered at 18-20kHz or so, although not always. 

FLAC comparison - perceptual shelving?

Reply #2 –
Quote from: Blaice on
I thought turning to direct CD FLAC conversion rather than 320 would get rid of the annoyance of cross-checking for transcoding and proving spectral transparency

Aiming for spectral transparency does not mean retaining all the frequency range of the original. Lossy encoders take out parts of the spectrum it assumes you won't hear, by understanding how human hearing works, masking and other things. Though it depends on many factors (bitrate/quality setting, each person's hearing, etc.), they are quite successful at that, and for content above 16khz in real music most of us mere mortals seldom have problems.

FLAC of course, being lossless, retains all of that. Prove me wrong, but I don't really think there's a problem in those files: some songs go up to 22khz, some don't, even if they come from the same album. We only hear up to 20khz anyway, even less than that being actually meaningful to music. Deal with it.

FLAC comparison - perceptual shelving?

Reply #4 –
To get the basics out of the way - FLAC is, with mathematical certainty, lossless. This has never been in question, aside from people who might argue that 2+2=7. The output of a FLAC decoder will be identical to its input with no variation. You can pretend you're working with WAV files. Or, you can in fact work with WAV files, and you'd get identical audio and spectrograms - just using a lot more disk space. The bottom line is that you can ignore that these files are FLAC since that's irrelevant to any questions regarding the audio itself.

So now we get to the audio itself. The big mistake with your method there is that you are comparing two different tracks. These tracks might have been produced and mastered differently - not to mention they are different pieces of music - so the fact that their spectrograms differ is to be expected, and it indicates nothing.

Many CDs are mastered so that they have sound content all the way up to the Nyquist limit at 22.05 kHz. Many other CDs are mastered with a lowpass filter a few kHz under the limit - this is mostly done to avoid aliasing problems when digitizing the signal.

You are correct in that older music won't have much if any signal above 15 - 18 kHz or so. In those cases you'll see a gradual roll-off on the spectrogram. In newer music, where you have signal up through 22 kHz, but the audio has been lowpassed at 20 kHz for CD, you'll see the "shelf" you described.

Lastly I'd like to say that while looking at spectrograms in this manner can sometimes be a useful tool in determining how a piece of audio was processed, it's really just an academic exercise. The frequency ranges we're looking at have little-to-no bearing on how it sounds to your ears. Take one of the files with sound at 22 kHz, lowpass it at 19 kHz, and you will not hear a difference.

FLAC comparison - perceptual shelving?

Reply #5 –
@saratoga: slks brought this up too; why are low-pass filters then even used if they support acoustic degradation? I'm not sure I understand.

@TomasPin: was unaware of 'lossy encoders' and the phenomenon of auditory masking. Thanks. I just find it hard to come to terms with two separate albums that, for whatever suite of reasons and variables, appear dissimilar spectrally. As you have pointed out, the difference is not meaningful and you're right.

@greynol: lol. I was just looking for a pragmatic and very much conceited (albeit arbitrary) explanation that would set things either black or white and for all time. But, as per your signature, I am quickly seeing just how grey things are in the world of compression, conversion, and comparative analysis. Thanks.

@slks: I appreciate your taking the time with 'the basics'; essentially this is what I wanted help with but could see no non-cavalier way of bringing it up on a forum. Thanks.

ALSO: how do you set up your (casual) FLAC libraries? Are there any guides for beginners as to best practice etc.?

FLAC comparison - perceptual shelving?

Reply #6 –
Quote from: Blaice on
@saratoga: slks brought this up too; why are low-pass filters then even used if they support acoustic degradation? I'm not sure I understand.


I'm not sure I understand your question either, but low pass filters are used to remove ultrasonic content that could introduce distortion in subsequent processing.  On modern equipment its not really  necessary since the DAC will remove it anyway, but its good practice just to be safe.
 

FLAC comparison - perceptual shelving?

Reply #7 –
Quote from: Blaice on
ALSO: how do you set up your (casual) FLAC libraries? Are there any guides for beginners as to best practice etc.?

I just use folders by artist name, then album, and then either one file per track or the whole album as one file and a CUE for track indexes. Try to have everything properly tagged and sorted, so you can then transcode to whatever you want or need and have all in order. (I did not do so from the start, and it can be quite a pain to start tidying up later...)