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MP3 and audio data compression

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  • #31
    Speaker distortion and MP3 encoding

    Originally posted by A.S. View Post

    Incidentally, when DCC appeared (and I still use it - 256kb, 4:1 encoding, completely transparent) I approached Philips and asked them if I could somehow incorperate their compression technology in our active speakers. My concept was that if we were erasing and not feeding the speaker with all the fine detail that was masked in the ear anyway, this would reduce the strain on the speaker, distortion would diminish and the result would be a cleaner sound entirely due to data reduction. I still believe that is a valid argument. The easier we make the loudspeaker's task, surely the less distortion it will produce. ...
    If this hypothesis is valid, wouldn't it be just as easily tested with a passive loudspeaker, by comparing distortion produced by a speaker fed a WAV file, and then comparing that to the distortion produced by the same speaker when fed a 256 kbps MP3 file that has been bit-reduced from the original WAV file? The latter would have the fine detail stripped out, so if the theory holds, the speaker should produce less distortion with the MP3 than with the WAV. Or am I missing something?

    Comment


    • #32
      Data reduction and speaker stress?

      Originally posted by EricW View Post
      If this hypothesis is valid, wouldn't it be just as easily tested with a passive loudspeaker......
      No, I don't think that's the same. Alan is postulating a new idea about making the loudspeakers to produce only the sound that would be heard by us. He is making the loudspeakers to ignore all other frequencies that would have been masked in the ears. Clever indeed. It doesn't matter if the source is a CD, SACD or other format the loudspeakers play the sound as we would hear them.

      My only concern is the loss of frequencies above 18 or 19kHz when the file is compressed, which defeats the purpose of SACD or other high resolution format, which supposedly captures frequencies up to 50kHz. But then the whole argument in favour of MP3 is that human do not need to hear frequencies above 16kHz for musical enjoyment.

      I wonder if the wireless home theater system for rear speakers already implemented similar concept.

      ST

      Comment


      • #33
        About time we had a paper from BBC R&D?

        This paper (link to pdf) actually addresses the issue of cascaded audio coding, ie what happens when several compression techniques are applied successively - as happens in broadcast situations.

        It's authors (Andrew Mason & David Marston) are I believe employed at Kingswood Warren and as such may either be known to A.S. and/or (it's just possible) call in here. That being the case, I ask that mods feel absolutely free to edit/comment/delete as they see fit.

        Introduction

        The production and broadcast of audio is a technically complex operation. The audio signal will typically pass through several distinct processes including recording, sending to the studio, postproduction and so on. Increasingly, people have been turning to bitrate reduction to reduce the cost, or to increase the speed, of these processes. In isolation, the impact on audio quality of a single application of bitrate reduction can appear negligible. However, the reality is that the cumulative effects of bitrate reduction throughout the broadcast chain is far from negligible. If each process removes all redundant audio information, or uses the signal to mask the noise being introduced, then the next process might have nothing left to remove, or will see previously introduced noise as signal to be used to mask more noise.
        I was interested in simply seeing how such a study is carried out, but if you scroll straight down to the conclusions you will find this:

        The results clearly show that the cumulative effect of cascaded audio coding can be highly detrimental to audio quality, even when each stage in the chain makes only a small reduction in quality.

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        • #34
          Originally posted by STHLS5 View Post
          No, I don't think that's the same. Alan is postulating a new idea about making the loudspeakers to produce only the sound that would be heard by us. He is making the loudspeakers to ignore all other frequencies that would have been masked in the ears. Clever indeed. ...

          ST
          Yes, but isn't this exactly the same as what happens when you amplify an MP3 file and send it on to the (passive) loudspeaker? Either way, the "fine detail" has been stripped out of the signal and in theory the loudspeaker should have less work to do. If it's different, what exactly is the difference?

          Comment


          • #35
            Originally posted by weaver View Post
            [URL="https://www.ebu.ch/en/technical/trev/trev_304-cascading.pdf"] .... are I believe employed at Kingswood Warren....
            As we reported in a Newsletter earlier this year, BBC Kingswood Warren no longer exists and has been demolished for housing development.

            As far as I know, those (very) few remaining stall in the BBC's Research Dept. are engaged in non-audio work. As I understand it, blue sky audio research has ceased at the BBC. Sadly, that sort of erudite paper hasn't found a practical application in a world where digitally compressed audio is commoditised. The BBC does not have the remit or the resources to try and persuade the global audio industry to address issues that they don't see as a problem - especially if the solutions require more training, more equipment or changes in working practice.
            Alan A. Shaw
            Designer, owner
            Harbeth Audio UK

            Comment


            • #36
              Pitch and tones

              Originally posted by A.S. View Post
              To quote from your linked paper ...

              Empirical results also show that the human auditory system has a limited, frequency dependent, resolution. This frequency dependency can be expressed in terms of critical band widths which are less than 100 Hz for the lowest audible frequencies and more than 4 kHz at the highest. The human auditory system blurs the various signal components within a critical band although this system's frequency selectivity is much finer than a critical band.
              Considering the second quote: we sense high frequencies in 'blocks' or bands that are up to 4000Hz wide. Yes, that's right: a whopping 4kHz wide. So, at the top end of the scale, 16-20kHz is sensed in one wide spectral block.
              Ok, I may need help with the maths again here:

              At the bottom end of the piano scale 100 Hz is the equivalent of roughly two octaves: eg. C at 33 Hz to C at 133 Hz.

              In the 16-20kHz range that you are talking about, 4000 Hz is actually around a quarter of an octave, or two whole notes.

              [This is way off the top of the keyboard, but starting with C at 4kHz (4186 Hz) then roughly the next C would be 8 kHz the next 16 and the next 32. So between 16 and 32 it's 16 kHz to an octave, or 4 kHz is roughly two whole tones. Very approximate, and as I said: I may need help with the maths.]

              I'm not sure whether this agrees with (the sense of) what you are saying or not: to differentiate between two (imaginary piano) tones above 16 kHz they need to be 2 kHz apart - and a semi-tone would be 1000 Hz. We can therefore say that the ear senses a wide frequency range as a block.

              But, in terms of sensitivity to pitch as opposed to frequency the picture is different isn't it? 4000 Hz at the top of the audible range is a much smaller range of tones than 100 Hz is at the bottom.

              I just want to be clear that in musical terms we are talking about pitch, about tones and semi-tones; whereas in audio terms we talk about frequencies. So what may be a wide range in audio terms is actually a relatively small range in human, musical terms.

              Comment


              • #37
                Frequencies, pitch etc.

                It's not your maths I have a problem with, Weaver, it's your reasoning.

                I think all we can say is the ear much much more discriminating around middle C (256Hz) and concert pitch A (440Hz) than it is of the top of the piano keyboard counting in kiloHertz.

                And I don't think you can decouple pitch and frequency as you attempt; even if we understand that they are not as closely locked as simple science would predict - the upper octaves of the piano are "stretched" otherwise the intervals sound flat!

                http://www.blackstonepiano.com/tutor...techniques.htm

                Furthermore, music is not usually played with pure notes (sine waves) - each basic note (fundamental) comes with a family of higher notes, the harmonics, which even for a fundamental note below or in the low hundreds of cycles per second (Hz) will run up into the 1000s (kHz)

                Being able to differentiate these high harmonics helps us tell the difference between (say) a clarinet and an oboe.

                Where all of this get us I don't know, but I do know I don't know enough!

                Nevertheless I am uneasy with decoupling pitch and frequency!

                Comment


                • #38
                  MP3 coding bands ....

                  Originally posted by Labarum View Post
                  Nevertheless I am uneasy with decoupling pitch and frequency!
                  My aim was not to de-couple them, but to re-state the point that the relationship between the two is not linear. Also, in audio discussions, I think it is all too easy to forget what the numbers actually mean - maybe that's just me though.

                  If we are talking about a block 4kHz wide we need to remember where that block is located - the entire (fundamental) range of the piano is only 4kHz after all. I do appreciate what you are saying with respect to harmonics though.

                  One of the issues with the mp3 codec is that the first thing it does is to

                  divide(s) the audio signal into 32 equal-width frequency subbands*
                  You might guess that this "equal width" approach is not without compromises.

                  *from the same paper referred to above.

                  ps I think I remember that you have a musical background, if that is the case then not only do you know far more about this than me, but what a given frequency actually means, ie what it sounds like will be built in to your thought process. Whenever Alan talks about a frequency range I have to look it up and put it in context first.

                  Comment


                  • #39
                    CD WAV v. MP3 - am I kidding myself? A test ....

                    Originally posted by A.S.
                    .....So now, perhaps we can return to - not abandon as you suggested - your technical deconstruction of dynamic range compression. Can you imagine a better way of visually presenting your (undoubtedly true) observation that sound just isn't what it used to be? Thoughts?
                    Not so sure I said that nor I am technically competent to deconstruct the workings of MP3. But this topic is important to me because I see a bigger role in server based media player and may have to convert my CD collections to other formats such as MP3, AAC and FLAC. Still not sure what to do with my SACD collection.

                    Previously, Alan has said that MP3 format at 256kb is as a good as CD quality. I agree that some MP3 do sound as good as CD but on closer listening it falls short of my expectation when I feel they tend to be monotonous in their dynamics, especially at higher volume. This is my opinion and may be wrong since I am prejudiced against MP3. Not only that, it would be embarrassing to me that I am lying all these years to myself to "claim" that SACD sounds better to my ears than CD because the imperceivable high frequencies made a different to my musical enjoyment.

                    During this weekend, I am downloading from Linnrecords one track in Studio Master FLAC, CD quality FLAC or MP3 files formats, to experiment myself if I can tell the difference. Anyone want to suggest which would be the best track for comparisons?

                    ST

                    Comment


                    • #40
                      Selecting music for testing

                      Originally posted by STHLS5 View Post
                      Anyone want to suggest which would be the best track for comparisons?

                      ST
                      If you like you can also try Bluecoast records, just register and recommend Alex De Grassi - Greensleeves. Its a fine tune and available in flac, mp3 and wav.

                      {Mod's comment: before you select music tracks maybe you should anticipate the results of your test (what you hope it will show) and work backwards to the music that will best highlight whatever you expect? What do you think will be the result?}

                      Comment


                      • #41
                        Test music

                        Thanks Kittykat. I just realized my CD player may not be able to play FLAC or AAC.

                        I just finished burning Bach: BMV. 565 Toccata (for the deep organ sound) and Romance de Amour (for guitar) using 256kb MP3 and copied the same in wav format. I used Window Media Player to do this. Without knowing how the sequence of the tracks format, I managed to pick wav files to be better. However, whatever difference I can tell is only when it is played side by side for comparison. If I were to walk into the room and asked to identify which format is playing, I would not be able to tell.

                        Interestingly, when I listened to the same tracks using my computer, they all sounded the same to me. Maybe, I should have burned them at 320kb. I was told to use VBR setting, but I can't find that option in WMP.

                        ST

                        Comment


                        • #42
                          Dynamic range compression and data compression

                          As some readers here will already know, the BBC is experimenting with a high quality internet stream for the remainder of the Proms concerts. This applies to the live broadcasts and the daily repeat programmes but not the on demand 'Listen Again' feature.

                          [It can be accessed here]

                          This stream is at 320 kbps as opposed to 192 kpbs for Radio 3 at other times.

                          There is also a blog attached to the project here.

                          Reading the comments that have been added it quickly becomes apparent that listeners are delighted with the improvement in quality but also that they believe the dynamic range of the broadcasts to be greatly improved.

                          It has been a criticism of Proms broadcasts, be that via TV, FM radio or internet, that the dynamic range of the concerts has been heavily compressed; this current stream appears to show a vast improvement in this area.

                          As I understand it, compressing the data prior to broadcast would not of itself lead to dynamic compression, but clearly the two are lumped together (in the listeners mind) in this instance.

                          I have no knowledge of the dynamic limitations of the various broadcast channels but my question would be: in the case of broadcast music (TV, FM radio, web) does dynamic compression necessarily go hand in hand with data compression?

                          Comment


                          • #43
                            The choice of music ....

                            Originally posted by STHLS5 View Post
                            ...I just finished burning Bach: BMV. 565 Toccata (for the deep organ sound) and Romance de Amour (for guitar) using 256kb MP3 and copied the same in wav format.
                            We're curious as to why you picked organ and guitar as a WAV v. MP3 comparison. As I've reported here before, the organ sounds good on every loudspeaker (it has a narrow frequency range and limited harmonics) and obviously the (acoustic) guitar has a wider frequency range but almost no dynamic range at all.

                            As Mod1 said, what are you actually trying to listen for? You know what the secret of a really good research test is? It's thinking through the likely outcomes, and constructing the test to maximise the comparative differences long before lifting a finger to actually perform the test. In this example, musical selection is of paramount importance.
                            Alan A. Shaw
                            Designer, owner
                            Harbeth Audio UK

                            Comment


                            • #44
                              Originally posted by A.S. View Post
                              As I've reported here before, the organ sounds good on every loudspeaker (it has a narrow frequency range and limited harmonics) and obviously the (acoustic) guitar has a wider frequency range but almost no dynamic range at all.
                              Alan

                              Do all organ stops have limited harmonics? I am guessing reed stops are rather rich in harmonics and can have a rather steep starting transient.

                              Doe the guitar realy have greater frequency range than an organ? Can it reach down to CCCC, or sound as high as a 2ft Mixture Stop?

                              Of course the organ will test a speaker's bass like no other instrument, and some of the stops produce very complex sounds.

                              Comment


                              • #45
                                Before I even bother to reach for the text book, let's apply some common sense. If you have a bi-wired pair of speakers, unhook the biwire link feeding the tweeter and play organ and then guitar. You'll notice that a) it hardly matters whether the tweeter is connected or not when playing organ b) if definitely very much matters if the tweeter is connected playing acoustic guitar. That's because the guitar has a complex harmonic structure which extends far beyond the plucked note.

                                That proves to my satisfaction that the guitar has a wider range in the region that we are most interested in for comparative testing i.e. 3-6kHz. It's not relevant what happens down at low frequencies for this sort of comparative test because the problem with all digital (linear or compressed) audio coding systems is never how it handles the bass (where the digital resolution is highest and the ear's sensitivity to distortion the lowest) but how it handles the middle and high frequencies where the digital resolution is lowest and the ears sensitivity the highest.

                                In other words, you could probably get away with 6-8 bit resolution in the bass, but you'll need 16 bits where the ear is most sensitive (3kHz). So MP3 will easily be good enough to handle the coding of an organ. It would be a trivial matter to make an MP3 recording of an organ sound acceptable even at (I dare say) not 320kb but 96kb, or even less to code the bass adequately*. Probably less because the sound is virtually monophonic, so could probably be dropped to 40-50kb. Try it yourself!

                                *It may be necessary to increase upwards from 96kb depending upon whether the organ music is concentrated in the bass or has more fiddly higher-register content.
                                Alan A. Shaw
                                Designer, owner
                                Harbeth Audio UK

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