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Old 05-24-2009, 04:51 AM   #1 (permalink)
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Post Lossy Audio Formats (mp3, ogg, m4a, etc), Quality and Comparisons

Index
  • Introduction
  • Explained : Analog vs. Digital Recordings
  • Explained : Lossy Compression File Formats
  • Explained : Constant and variable bitrates
  • Question : Is one format better than others?
  • Question : What bitrate should I get for the best listening experience?
  • Question : Should I choose Variable Bitrate over Constant Bitrate?
  • Summary


Introduction

Most of us by far have music collections on our computer these days and most of it are in lossy formats like mp3, m4a, ogg, mpc and more. Aside from potential differences between formats, the different formats also typically come in different bitrates representing the sound quality of the file. With all this to keep in mind, some questions may arise :
  • Is one format better than the others?
  • How high bitrate do I need to get if I wanna have the best listening experience?
  • Is constant bitrate favourable over variable bitrate?

And so on. As I'm writing this intro, I don't actually know the answers to any of these questions yet, but I plan to look up sources and find out. I'll post my findings here and summarize them at the end. Before I do, let's just sort out some terms and expressions for those who don't know them.



Explained : Analog vs. Digital Recordings

An old-fashioned recording method like vinyl records sound as changes in a physical medium. Although it is an exaggeration, you can think of these formats as lossless. Conceptually, if you think of a needle scratching a vinyl surface, any sound waves will make that needle reverberate and those vibrations will be recorded in the vinyl medium. Although the representation is not exactly the same as the sound it records, it still allows for a very accurate representation of it. However, computers don't deal with physical mediums like vinyl, they deal with 1s and 0s. The "Yes or no" or "on and off" manner of which digital formats are recorded just doesn't allow for the same kind of representation that physical mediums do, therefore they are considered "lossy", meaning they lose some of the information you want to record.



Here you can see illustrated how the digital format is not able to accurately represent the soundwave that the analogue format represents. However, just like a digital picture format (like jpg) and how it's quality depends on the resolution of the picture, so you can raise the quality of a digitally recorded sound by raising it's resolution. When you look at the digital wave, everytime you see a 90 degrees bend or just a corner in the wave, that's a point where the sound has been recorded or rather - "sampled". If you drew a line between them, they would look like a wave. If you're familiar with maths, it's a bit like trying to calculate the area behind a line in a graph by the use of integration theory (squares basically), like in the graph below.



This is not a digital soundwave, however it illustrates the principle. If you want to accurately represent the area under the red wave, you can try and do so by filling the area with squares. In other words, the quality of representation depends on how many samples (squares) are recorded per time unit. This is known as the sample rate. The idea is that if you sample the sound often enough, you approximate the real "analog" sound. CD quality is 44100 samples per second (44.1KHz) and is regarded a good enough representation for practical use (although many argue still that vinyl is better, which they can be, although noise from dust and imperfect pins etc. may ruin the potential quality gain and who knows if the difference is even perceptible?).

There's also quantisation which basically means how detailed every sample is. In digital audio, quantisation involves representing the entire audible range of sound (20 Hz to 20 KHz) by a range of numbers. For example, audio CDs represent sound using a 16 bit signed number which denotes the numeric range -32768 to +32767. If we were to use a bigger range of numbers (which would require more data), then we would attain an even more precise representation of the original audio. (Thanks Seltzer!)



Explained : Lossy Compression File Formats

You can tell what the quality of your audio file is by looking at it's listed kbps, it's bitrate - or - how much information is processed when you play that file each second. A 128 kbps bitrate recording processes only half the data when played as that of a 256 kbps bitrate version of the same file. Also worth mentioning is that compressed file formats like MP3, OGG Vorbis, MPC and WMA are even more lossy than CD quality because they purposely discard all the information that we can't hear. In other words, a CD records "everything", also information you likely won't hear. Lossy file formats discard a lot of information (supposedly ~90%) to minimize file-size. How they figure out what to discard and how to do it (the "compression method") differs between file formats which means that there will be potentially audible differences between them.

Some lossy compression file formats : AAC, ADPCM, ATRAC, Dolby AC-3, MP2, MP3, Musepack, Ogg Vorbis (open source), WMA



Explained : Constant and variable bitrates

Lossy formats can either have constant bitrate, meaning they have the same quality from start to finish - or they can have variable bitrate which is an attempt to combine the best possible listening experience while preserving disk space. On a variable bitrate, the program that makes the file tries to figure out how much bitrate is needed to accurately represent the different parts of the songs it's recording. The higher frequency sounds there are and the more "complex" the sound is, the higher the bitrate needed to represent that sound becomes.



Okay, hopefully I was able to explain these things so we've got that sorted out. I'll now take a look at some sources and then post my findings below.
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Old 05-24-2009, 05:45 AM   #2 (permalink)
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Question : Is one format better than others?

I've read several tests and will try to summarize the results I've come across. These tests typically pitch different formats against eachother in blind tests, testing the same recordings in the same bitrates, different bitrates and so on. It seems the differences are most noticeable at low bitrates and in general, a music library should not go lower than 128 kbps in quality for any format. Because 128 kbps is quite popular, I've summed here the results from a test testing OGG, WMA and MP3 at that particular bitrate.
  1. OGG & WMA - 95 %
  2. MP3 - 80 %

>> source : CD BURNER.CA - Comparison of Digital Audio Formats - MP3, WMA, OGG, WAV & more!

The percentages say how well they approach CD quality for the track they tested. As you can see, both WMA and OGG do better than MP3 at 128 kbps, but at higher bitrates, this can change and differences are often regarded as too miniscule to really notice. Note also that MP3 encoding may have improved since then, making it unsure if this description is accurate today.

Quote:
I have received caustic comments by fans of various formats on whether such a comparison is necessary. PC Magazine, goes even further by proclaiming that their "Labs tests show that even to an audiophile, the differences are virtually imperceptible". Yes, testing high bitrates such as 128kbps shows little difference.
>> source : OGG vs. MP3 vs. WMA vs. RA

There are noticeable differences at lower bitrates, but these low bitrates are mostly important considerations for streaming audio and not so much for listening to your local music collection. At lower than 128 kbps, MP3s tend to do bad and produces "compression artifacts", sounds that are not part of the recording. As such, other lossy compressions available at those bitrates tend to do better.



Question : What bitrate should I get for the best listening experience?

Of course, the higher you go - the better the representation. But when storing music on the hard drive, of course it's a compromise between quality and preserving disk space. The following graph shows mp3s at different bitrates and how accurately they represent the audio stored on the CD they are ripped from. We see herz along the X-axis and the closer the lines are to the red, the more alike they are.



>> source : MP3 Producer Bitrate Comparisons - winamp bit rate mp3 mpeg3

As you can see, all bitrates fairly accurately represent the lower Khz audio-range and it's not until ~15Khz (which is within the audible range of most) that 112 bitrate file does a drop. At higher frequencies, it does not do a good job meaning information is lost. Because this is within audible range, 112 kbps (CBR) ripping should be frowned upon. 128 does quite a bit better and doesn't drop off until ~16Khz and I'm thinking that's probably right on the edge of what I can hear. The logical assumption here is that if you're like me and your hearing is not perfect, the difference between bitrates of 128 kbps and up compared to listening to the actual CD the files are ripped from is probably miniscule. However, as you can see on the chart - if your hearing (and equipment) is good enough (typical for you youngsters), you could potentially hear a difference between 192 and 256 kbps bitrate as there is quite a difference between those two bitrates at the ~17Khz .. provided you can still hear it. Also, what we see here and what I've gathered from other tests is that an increase of 64 to 128 will be a considerable change in quality whereas an increase from 192 and 256 likely won't because the difference then is only at the highest frequencies.

However, these are just charts and numbers. I wanted to see if I could find an actual listening test and see what that said. I've looked at tests where participants listen to files in uncompressed as well as compressed versions. These tests were all blind tests so the participants did not know the quality of the song they were listening to. The first blind-test had 4 people listening to 3 versions of 4 songs. One uncompressed, one mp3 encoded at 320 bitrate and one at 160. Participants are expected to guess right every now and then and the test seems to show that people (even audiophiles) are on average not able to distinguish between these bitrates.

>> source : Do Higher MP3 Bit Rates Pay Off? - Page 1 | Maximum PC

However, some other blind tests do report people claiming they were able to hear differences between 192 kbps and CD quality and 192 kbps and 256, but I've yet to find reports of someone hearing any kind of audible difference between 256 and 320. Looking at the charts, it makes sense that those with good hearing will be able to tell the difference between a 192 kbps and a 256 kbps bitrate.

>> source : MP3 vs. CD Audio Quality Tests & The Great MP3 Bitrate Test: My Ears Versus Yours - Gizmodo Australia

I did my own comparison just for fun where I listened to a 112 kbps mp3 to see if I could hear a difference from the actual sound on the CD. Remember now that I have no audiophile equipment and I have a tendency to say "huh?" when spoken to. I listened to Gartnerlosjen's song Kinosangen and slightly surprised, there was not much difference. The hats and cymbals that typically makes sounds in the high Khz ranges were crisper on the CD, but as far as the enjoyment of the listening experience went, it made basically no difference.

The answer to the question "what BR should I get?" becomes less straightforward, but here's an attempt to summarize :
  • Recommended BR varies with the song. Classical music with fine string sections or electronica with high pitched sounds should be encoded at a higher BR. Rock, alternative rock, folk music and so on may not need the same bitrate.
  • For young listeners with good hearing who want the best quality sound : 256 kbps
  • For the average aged listener : 160 / 192 kbps - depending on the song. 192 should be a safe choice for most.



Question : Should I choose Variable Bitrate over Constant Bitrate?

Although it was harder to find reliable tests on this, it seems that in most cases, VBR is the ultimate compromise between disk space and quality. A VBR file will have better quality than a CBR file at the same filesize because the simple parts of the audio can be compressed better than in CBR, freeing up more bits for the complex parts. For the average listener, a VBR setting like 112 or 128 bitrate at the lowest up to 256 at the highest should produce files that closely approximate the experience of listening to the CD while preserving the most amount of diskspace.

Beware though that there is a margin of error and there is a slight chance VBR could produce the odd "artifact" in the sound or encode a complex part at lower bitrate than it should. However, most encoders have supposedly become so good at this that these problems are now unlikely. Hardware compatibility problems are now mostly a thing of the past.

Of course, the highest quality MP3s are of course CBR files at 256 / 320 kbps. However, the average listener is not likely to hear a difference between this and a 128-256 VBR file.

>> source : Constant Bitrate - Hydrogenaudio Knowledgebase



Summary

After reading through a number of tests, I've concluded with the following stuffs :
  • At low bitrates up to 128 kbps, MP3 performs worse and may produce compression artifacts (sounds you don't want) more compared to other lossy formats like AAC, WMA and OGG.
  • At higher than 128 kbps, differences between formats become miniscule
  • VBR is a perfectly good way to preserve disk space while still enjoying high quality lossy files.
  • Most people can't reliably discern 160 kbps bitrate from 320 and even uncompressed.
  • 160 / 192 kbps (192 considered the "safest choice") should be good enough quality for most people.
  • 256 or higher is only recommended for those with very good hearing and good audio equipment.
  • Files with a VBR from 128 to 256 (though you could as well cap it at 320) should satisfy most quality-wise while being the most disk-space preserving option. It's likely a good safe bet for all.
  • From a practical point of view, 256 and 320 kbps lossy files sufficiently approach CD quality enough to make "lossless" files like flac files an unnecessary waste of disk space.
  • Also, lossy formats that can be encoded higher than 320 - no need.

Thanks again to Seltzer for sharing his wisdom and knowledge. That's it! Now it's time for comments, crass critique and - of course - any questions you might have.
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Old 05-24-2009, 05:10 PM   #3 (permalink)
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Placeholder yos!
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Old 05-24-2009, 05:25 PM   #4 (permalink)
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Very informative thread, I enjoyed reading it and looking at the charts and trying to piece two and two together. The science of sound is still something I have a hard time comprehending, it wasn't my niche for science (Earth science was more my thing), but I really enjoyed it. The chart of the bit rates was extremely accurate. Anything less than 128 kb/s and I discard it and look for something better.
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Old 05-24-2009, 05:40 PM   #5 (permalink)
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Originally Posted by Pobodys_Nerfect View Post
Very informative thread, I enjoyed reading it and looking at the charts and trying to piece two and two together. The science of sound is still something I have a hard time comprehending, it wasn't my niche for science (Earth science was more my thing), but I really enjoyed it. The chart of the bit rates was extremely accurate. Anything less than 128 kb/s and I discard it and look for something better.
Thanks for the feedback

After reading these tests and experimenting a bit myself, it's become increasingly appearant for me that I myself don't really need albums at 320 kbps. After playing too much drums without hearing protection and so on, I've become a sufferer of tinnitus and my hearing ability has taken a blow so .. Actually, 160 kbps is probably more than I need.

People with young ears like Anteater might wanna "safe it" with higher BR files though, if they wanna make sure they get the best. However, the listening experience will probably be as enjoyable with lower BR files.

192 kbps is the middle ground between the tired old ears and the audiophile's requirements.
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Old 05-24-2009, 05:56 PM   #6 (permalink)
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Nobody "needs" it at 320 but if that's what's available then I'll take it. I'm not enough of an audiophile to really care, but FLAC format is just wasted space, I mean does an entire album need to be over 200 mb? for 11 songs?
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Old 05-24-2009, 06:13 PM   #7 (permalink)
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lol, using calculus to explain digital recordings. but yes, it's a good analogy.

assuming i understood everything which was presented here, this is a brilliant fucking guide, especially for our audiophile nazis out there. personally, i can't even tell the difference as long as it's at least 128 kbps, no need for me to pursue lossless formats.
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Old 05-24-2009, 07:29 PM   #8 (permalink)
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I can tell the difference between 128 and 320, but I can't tell the difference between 256 and 320.

Anyways top notch guide!
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Old 05-24-2009, 09:19 PM   #9 (permalink)
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I always preffered v0 variable mp3 rips to 320, sound richer than the constant BR which can quite possibly be ripped poorly.
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Old 05-25-2009, 02:19 PM   #10 (permalink)
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Originally Posted by Pobodys_Nerfect View Post
Nobody "needs" it at 320 but if that's what's available then I'll take it. I'm not enough of an audiophile to really care, but FLAC format is just wasted space, I mean does an entire album need to be over 200 mb? for 11 songs?
Yes. If I can get FLAC I'll get FLAC because it sounds better to the ear. 320 sounds horribly compressed imo.
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