Are there any audible differences between 192 and 320 kbit/s .mp3 files?

Question

This is a question that's currently being discussed on skeptics.stackexchange. It was thought reasonable to get some input from the audio experts. So with the author's permission I'm posting it here.

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I have friends that are very picky when it comes to mp3 bit-rate, and will always look for the 320 kbit/s version of a file. However, I have never noticed any differences, they sound the same to me. I remember reading somewhere, can't remember where, that the human ear is simply incapable of sensing the difference, even if present.

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Are there any good quality tests that have been done in this area? What types of factors would need to come into play to tease out the differences (if any) in audio quality between a 192 vs 320 kbit/s .mp3?

Answer 1

If you're after audible differences the only test you can use is a double blind test. Empirical tests such as waveform analysis will most definitely show differences in the output, but that doesn't mean the differences are audible.

In this case you could have some one listen to samples of music and have them guess the bitrate of the samples. The ABX type test seems to be popular for bitrate comparisons: you listen to sample A and then B and then X and decide which of A or B sample X is from. So A might be 192 kbps and B might be 320 kbps and the listener has to decide whether X is from the 192 or 320 kbps encoded track. This kind of test is a little better than an "assign the bitrate to a sample" type test because the listener can't guess their way through it with any accuracy and skew the results.

There are other double blind approaches, see this article for details.

There's been a few magazine attempts at uncovering codec and sample rate differences. This Maximum PC article gets a lot of attention on the net, though they're doing 160 vs 192 and 320 vs uncompressed.

I couldn't find any academic papers on the matter (but I didn't look that hard), which surprised me a bit. I would have thought there'd been a bunch of grad student research in psycho-acoustics available.

Answer 2

I find it comes down entirely to the type of music and the speakers you listen to it through:

I can happily listen to pop music on an ipod through earphones at low bitrate (well, I say happily - not really a fan of pop music, but what I mean is it sounds as it should)

If you play the same mp3 through a decent or high end audio system it will sound worse, but again, with pop music it is still acceptable.

The real problem I have is in trying to listen to either heavy metal or classical music at low bitrate through a high end system. The jangles, splashes and other digital artifacting ruins the pleasure I get out of a good piece of music. This happens mostly at the top end (cymbals, distorted guitars etc) and especially when there is significant bottom end at the same time (1)

That said, I never bother with 320 constant bit rate because it just leads to large files. You should be better off with variable bit rate as it does intelligently reduce the file size where it isn't needed.

1- recording one of our new tracks found a major problem when generating mp3s - there are sections where not only are there a lot of high pitched distorted guitars, but some major subsonics that dive from around 80 to 15 Hz. That required top quality VBR to cope:-)

Answer 3

To an untrained ear it would be borderline impossible to tell, and even a trained ear needs to pay attention. As such, it is hard to quantify perceptible differences as everyone will perceive it differently, if at all.

My approach to audio engineering, which I picked up from my time at SAE, is to go by feel. You can monitor everything, tweak to the nth degree, but as audio is inherently analogue, that is all you should really measure. As Ian C says, an ABX test is a great way to see for your self.

To answer your question from my experience the answer would be yes absolutely.

The degree to which you can perceive the differences will largely depend on the content of the song. Orchestral music is in my experience the perfect subject matter to test with. It is both incredibly complex in it's sound makeup (usually covering the entire frequency of human hearing) and has a very large dynamic range.

Bit depth also has an effect, my experience is that an larger bit depth 24bit as opposed to the cd standard of 16bit, also makes a great deal of difference. The next natural step up is vinyl.

As noted by some other people, the gear you use will also influence the perception. On my little white earbuds the difference is not nearly as instantly noticeable as on my large studio monitors. So if you were to test, you should use the highest quality gear possible to take harmonic distortion from the gear completely out of the equation. You could use a boom box from 1980 and not notice a difference between A or B.

Answer 4

There are some neat studies in the field of psychoacoustics that claim that lossy audio compression tasks our brains more than less lossy formats. I cannot for the love of me remember where I got this but I believe it was from TED.com I did do some background checking on those claims after the fact and they seemed to be fairly accurate.

Most of us can certainly tell the difference between 128 and 192 bit rates but the difference is less audible between 192 and 320. However, for the trained ear I purpose that there's a difference. I certainly feel that listening to some 320 bit rates compared to their 192 counter parts is less tasking. But it's really hard to apply this in the general sense. A lousy MP3 can definitely mess with my hearing even at higher bit rates but I cannot apply such a theory to the general population.

Listener experience (and training) definitely factors in. Unless your are running a streaming service and need to be conservative with the bandwidth, I say go with the higher, 320 bit rates.

Answer 5

I work with sound for a living.

I have placed the same cut from 128, 192, 256 and 320 into a human ear sound analysis test range with a $100,000 sound sampling oscilloscope and the full wave form across the entire spectrum of the human ear is filled.

Further analysis shows that the main problem with mp3 sound samples in not in the bit but the distorted recording setting at any bit. Other words, people have a tendency to rip music at a height of input much higher then is needed causing envelop folding of all wave forms adding distortion in the applied recording.

Higher then 95% input is damaging to the recording as it leaves little overhead. My rule of thumb is to not record any audio over 89.2% as this will give your recording a level for overhead. All sounds have peeks and valleys which has to be given space for or its distorted by envelop folding for highs and crashes (base muffling) for lows.

To acquire a good recording you must stay within a channel of tones and then it is only replicated by the quality of the listening equipment and the receivers ear. Both of which by the way are never the same or ever perfect.

What I am saying is there is a heck of a lot more to the quality of sound then just the bit it is replicated at. Everyone is tone deaf at some level. You can play one sound clip to 100 different people and everyone of them will hear the same clip differently at some point. This has been proven in sound lab testes over and over. Then you put in the factor of human acceptance (like this sound or not) the best you can come up with from any recording is 88% acceptance.

So, you see, your fighting a loosing battle to try to explain the best quality. Just don't over peak your recording and let the individual receiver adjust the listening level for their own ear and equipment. Do your part by just sampling at a modest level. That's all that you or anyone else can or ever do to appease anyone.

Quality of input recording is the best answer as bit replication depth has little effect as long as it is kept within acceptable reason. Just my opinion after 30+ years experience.

Answer 6

Another thing to consider is who/how is/are they encoding it. I find a lot of the time that people are encoding and playing with settings they may not understand. This question is so finite though that it almost parallels the "analogue vs digital audio" debate.

Can it be perceived? Yes.

Will it be bothersome to listen to a properly encoded 192kb/sec mp3? Not likely...

Will it be bothersome to listen to an improperly encoded 320kb/sec mp3? Yes.

Is it personal preference? Mostly.

I can't listen to 128kb/sec mp3's... can't do it. Yuck! 192 is fine for me. Saying that, are there folks out there who have stereo systems that replicate the audio well enough that developing a preference for 320 over 192 is believable? Definitely... I don't have that stereo system though ={

Answer 7

I am a very picky listener. I notice that with acoustic instruments, you will hear the difference on a good audio set compared with mp3 players that are not high end audio.

I like dance organs from Decap Antwerpen (Belgium) and they are equipped with jazzflute ,vibraton, and flute. They have 8 registers that are natural pipes and the rest is electronics. It uses an inline synthesizer for sax, baritone, and trumpet.

I have all these music cd's stored as .wav and that sounds perfect. I tried .mp3 at different bitrates and is sounds less good with less ambiance... There is something missing in the warmth of the music.