Home / Opinions / Sounds Good To Me / File Formats & Compression

File Formats & Compression

Before we get into our deeply dodgy, pseudo-scientific investigation, let's establish exactly what we're talking about. MP3, AAC and the other main audio codecs were designed to reduce the amount of data required for a digital audio file to provide an accurate representation of an original recording, enabling these files to be sent over the low-bandwidth 1990s Internet or packed en-masse onto the earliest, low-capacity MP3 players. All use some form of lossy compression through perceptual coding - essentially, analysing the music streaming from CD and using a psycho-acoustic model to work out which bits will be marginal to or beyond the aural capabilities of most human ears. This information can then be reduced or even discarded, resulting in a smaller file.

The Linn Records online store is one of a few to sell FLAC files for download in a higher than CD quality 'studio master' format.

Up to a point, it's a good approach, but it has limitations. First, much depends on the encoder being smart enough to work out which parts of the audio stream are crucial and which parts aren't, which is why you might hear a surprising difference in audio quality between MP3 tracks encoded at the same bit rate but using different encoders.

Secondly, there's a reasonable variation in the sensitivity of human ears. Those people who have naturally sensitive ears and/or who have trained their brains to recognise slight differences in audio quality (e.g. audio technicians, classical musicians) may spot flaws in MP3 tracks that most people wouldn't notice in a million years. Audiophiles take these things seriously, and when you've spent hundreds or thousands of pounds on Hi-Fi equipment, you aren't keen to make any compromises on their source material.

Finally, lossy encoding methods inevitably create certain audio artefacts. Instruments or sounds that have a short, fierce initial attack, like percussion, can be smeared or affected by 'pre-echo' (where the actual sound is preceded by its echo, with any recorded echo diminished). Some audiophiles claim that the simple fact that if any data is discarded, the result will always be a lower quality listening experience, particularly when high-end equipment is used.

Deutsche Grammophon sells FLAC downloads for much of its catalogue, but only in a CD quality 44.1kHz, 16-bit format

This is where lossless formats come in. Lossless formats, such as Monkey's Audio, FLAC, Apple Lossless and WMA Lossless, are far less efficient when it comes to squeezing lots of audio data into a smaller package, but by retaining all the information they ensure that the compressed version is the equal of the original. Now, this needs to be kept in perspective. To hear some people talk about FLAC or Apple Lossless, you might think that we're talking about the ultimate in audio quality. However, for obvious reasons, a FLAC encode ripped from CD can only ever offer the same quality as the 44.1kHz, 16-bit Linear PCM original. While it's possible to purchase tracks in a 96kHz, 24-bit FLAC format from specialist stores, most FLAC files you'll produce or come across won't match, say, Super Audio CD (2822.4kHz, 1-bit DSD) or DVD Audio (48kHz, 24-bit) material for quality. Given that these failed physical formats were designed to overcome the perceived limitations of CD, it's worth giving this some thought.

Ada Mari Frost

June 14, 2013, 6:07 am

High quality FLAC sounds warmer than MP3. Most people equate the warm sound to not being crisp, that's how you get a lot of people thinking the MP3s are better.

brownox

January 4, 2014, 1:41 pm

Those are the worst graphs ever. The axis are so poorly labled that I can't understand what they are supposed to show.

Me

February 16, 2014, 2:40 pm

This article is a few years old now and out if date, but even taking that into consideration......

Unless I missed it, no mention was made of the MP3 decoder being used. They're not all created equally in respect to accurate decoding. WMP had a poor decoder prior to version 7. The Winamp MP3 decoder prior to version 2.666 was known to be substandard.

Constant bitrate is never going to be the best method for audio encoding any more than it'd be the best method for encoding video.

Bitrate is not the only variable which effects quality. The LAME MP3 encoder has settings for CBR encoding which use different algorithms and result in different encoding speeds at exactly the same bitrate. The encoding time for 5 minutes of stereo audio using my (aging) PC is around 7 seconds for the LAME default of q3, and 38 seconds for q0 (CBR 192k, LAME 3.99). The article makes no mention of the quality setting used.

From version 3.99, LAME's CBR encoding uses the PSY model from the VBR code. Even today, at the same average (lower) bitrate, CBR encoding will never match the quality of VBR encoding, bitrate. Back in 2009 it was possible it wouldn't even at very high bitrates.

The golden eared folks over at hydrogenaudio have a page dedicated to LAME which states any of the LAME VBR presets from V3 to V0 should normally be "transparent". ie it's not possible to distinguish the MP3 from the original. The average bitrate for V3 is around 175 kbps. For V0 it's 245 kbps.

The article refers to lossless audio and bitdepths/sample rates higher than those used for CD sounding better. A claim which seems to have been debunked.
http://mixonline.com/recording...

Hannes Minkema

June 8, 2014, 2:00 pm

"Only one person could accurately pinpoint which tracks were MP3 and which tracks were FLACs in every case."

I appreciate the effort, and I believe the general conclusion is about right. Yet it is unwarranted to state that only one person *can* (or *could*, which is the past tense of *can*) accurately distinguish MP3 from FLAC. The statement should have been that only one person *did* this. But this, by itself, says *nothing* about his general capacity to do so.

This is not nitpicking. I am sure that many readers misinterpret this statement, and believe that this one guy actually has better ears than all the rest. Heck, you might even believe this yourself. But there is no proof of that.

Take one hundred people. Ask them to draw the Queen of Hearts from a stack of playing cards. Two, maybe three of them will do so at the first draw. How is that possible? Should we conclude that they are psychic? Of course not. They are just lucky.

Take one hundred people. Ask them to tell which of two playing cards is red, and which is black. Fifty of them will be totally right at the first guess. Ask these fifty people to do it again. Twenty-five will succeed. Ask them again: twelve of them will be so lucky as to perform the trick thrice. Six will do the trick four times in a row, and three are so 'psychic' that they manage to guess right five times in a row. Incredible! Not.

They have no special gifts, of course. They are just assisted by blind chance. The next time they are put to the same test, they most probably won't be so lucky. So far for their 'general capacity'.

During the MP3-to-FLAC trials, the one guy who was able to guess right four times out of four trials was assisted by blind chance either. You can't erase blind chance. That's why scientific inductive statistics take 'blind chance' into account. And that's why the statement 'one guy *could* do it' is misleading. He couldn't. He just did. The combination of his ability AND blind chance did the trick. But that's not the same.

The question was not if one person *could* distinguish MP3 and FLAC, but whether a more general null hypothesis was rejected or not. That null hypothesis would be that out of seven people trying four times to distinguish MP3 from FLAC, a greater number of guesses turned out to be correct than can be accounted for by blind chance, with an uncertainty of less than 5%.

Anyone with a scientific training could have told you this. It is Statistics 101.

comments powered by Disqus