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Dither: Still needed with 24-bit recording?

Discussion in 'Recording' started by annannienann, Nov 6, 2005.

  1. annannienann

    annannienann Guest

    I have been reading a lot of articles explaining dither (because I didn't understand what that setting did in my sequencer program) and I now have a good understand about what dither is but none of the articles have cleared this up?

    When recording at 24-bit does dither need to be used if the dynamic range is small and the peak levels around -6Bd?

    Secondly, just thought of this whilst typing, can anyone tell what the term "32-bit float" means?


    Thanks everyone.
     
  2. audiowkstation

    audiowkstation Active Member

    Dither is used when converting from 24 bit to 16 bit. 24bit has much more dynamic range than 16 bit. 32bit float is a "type" of word length. You have fixed point and floating point. "Float" is obviously, floating point.

    digido.com has some wonderful articles to read about digital. It is a great place to absorb knowledge as well as the "rane" glossary at http://www.rane.com

    or better yet...

    http://www.rane.com/digi-dic.html

    If you go over that glossary at the rane site, you will have forgotten more than quite a few people in this business know!

    Have fun with it!
     
  3. annannienann

    annannienann Guest

    Thanks for the feedback, I have a lot of time for reading at the moment because my vintage monitor system has..erm..stopped working...well the right speaker has and mixing with one speaker is easier said than done. I know what the problem is the volume potentiometer right channel have broken but I have not got a replacement, so reading it is. Thanks
     
  4. dwoz

    dwoz Guest

    I have to slightly disagree on audiowkstation's comment about dither.

    Certainly true, dither is used in word length conversions.


    But, what dither actually is, is the addition of a bit of "noise" in the least significant bits (LSB) of the sample word. The PROBLEM with digital is, that at the bottom, you have to either be ON or you have to be OFF, there's no SORTOFONSORTOFNOT bit. what happens, is that the threshold where the last bit turns on or off results in a small amount of error in the conversion. This error manifests itself as distortion, or noise. Also called "quantization error/noise" Without dither, this noise ends up being highly correlated with the signal iteslf, which as it turns out, makes it ugly and noticeable. Adding that bit of dither noise at the very lowest levels, makes the quantization error UNCORRELATED with the signal, which, as it turns out, sounds much nicer to the ear.

    So, having said that, dither is present at ANY conversion into digital, whether at the original conversion from analog, or at subsequent conversions.

    A digital conversion happens every time you mess with the file in any way...such as when you move the "fader" in your DAW, or apply a plugin, or mix.

    So, dither comes into play ALL THE TIME, not just when you're doing word length conversions (i.e. 24 bit to 16 bit)


    Now, on to 32 bit floating point:

    floating point math is simple to grasp...its when the number of digits to the right of the decimal point can change. For example, if I divide 10 into 5, I get a result of "0.5", or one place to the right of the decimal point. if I divide 10 into 6, then I get "0.66666666666...forever....".

    In a floating point system, that uses a digital word that is 32 bits wide, the biggest (actually, smallest) number that can be represented is 2^32 (two to the thirty-second power), AFTER the decimal point. So, our example above becomes "0.66666666667"

    In fixed point math, the number of places to the right of the decimal point stays the same. So, in our above example, divide 10 into 6 becomes, say "0.6667"

    The long and short of it? that irrational numbers are much more accurately represented in floating point.

    and again, 32 bit simply means that the word length is 32 bits.

    My guess is that you're referring to something like the internal mix precision of a DAW...where it is performing all its calculations in 32 bit floating point math, then converting back to the original, say, 24 bit word length of the file being processed (using dither!).

    With 24 bit files, there are many who argue that the inherent system noise level is adequate to perform the dithering, but there are many who disagree...so take your pick. The underlying hypothesis is that randomized quantization noise (as in, with dither) is not NEARLY as cumulative as correlated quantization noise (without dither), over the course of a significant number of operations.

    dwoz
     
  5. annannienann

    annannienann Guest

    Thanks you are right, I did get the term "32-bit float" of a DXi plug-in and you explanation fits it perfectly so I am guessing your right there.

    my average noise when recording is -50dB which i think is more than adequate for the the A-D conversion but dither will be needed when mixing into lower bits and when running signals into plug-ins with a higher float than their output. Have I got that right?
     
  6. ghellquist

    ghellquist Member

    Generally (and there are always exceptions) there is not much use in adding dithering except at the very last step going from 24 bits (or float) internal resolution down to 16 bit for a CD.

    Very few, if any, recordings are truly 24 bit signal anyway, even the very best AD converters come in at 20 bits or less of true signal. The rest is some kind of noise and in my experience that takes the role of the dithering noise. So no need to add even more noise, even if it is at -140dBFS level or so.

    By the way, if you have only one monitor, try mixing everything mono. It is a trick used by quite a few engineers: mix til it sounds good in mono, positioning in stereo afterwards. If you can make it sound good in mono, it will often sound better in stereo, the other way round is not always true.

    Gunnar
     
  7. audiowkstation

    audiowkstation Active Member

    This has always been my understanding!
     
  8. dwoz

    dwoz Guest

    Go tell that to all the plugin developers...you'll save them a boatload of extra work that they are apparently performing needlessly, and the plugs will have a much shorter development cycle.

    basically, if you ain't dithering, you're truncating.


    dwoz
     
  9. audiowkstation

    audiowkstation Active Member

    My plugs work in 24bit mode and what I am working on is 24bit and internally, everything stays in 24bit until I drop to 16 bit.

    I think the moral of all of this is to KNOW your system and how it works. If each channel of a DAW is doing word length conversion while using plug ins, you need to know. Dither on top of dither on top of dither is not a good scene. Know your flow chart and signal path.
     
  10. IIRs

    IIRs Well-Known Member

    I don't think that is quite correct. You imply that dither noise merely disguises the quantisation distortion in a way that is subjectively more pleasant: my understanding is that dither actually preserves information that would otherwise have been lost below the LSB level. ie: it reduces quantisation error, it doesn't just disguise it..
     
  11. dwoz

    dwoz Guest


    close, but not quite! What you're referring to is that valid signal can actually be discerned below the level of the quantization noise. This is not quite the same thing as saying that the signal is preserved below LSB.

    Also, yes indeed, as you say, the quantization error is decreased, because statistically ANY application of dither noise will result in less quantization error WRT the input (i.e. "information") signal. (or rather, what I should say, is that the quantization error of information signal + dither will never be worse than quantization error of information signal alone) But, the main benefit is the un-correlation (is that a real word?) of the quantization noise to the information signal.


    and, finally, audiowkstation: what do you suppose is the internal precision of some of those plugs you're talking about? Some of 'em are 32 bit, some are 48 bit...as such, when they output, they either have to truncate back down to 24, or dither down to 24.

    AS you note, the more stuff you do, the more you tangle up those lower bits...this is one of the ugly sisters hidden in the closet of digital processing.

    Digital copies are pristine...digital processing is anything but.

    As has been noted, this may be so much tilting at windmills, but it may also be the difference between "whack" and "sack".

    ain't this stuff FUN!!!!!!


    dwoz
     
  12. annannienann

    annannienann Guest

    See I just can't have this kind of meaningful convercation with my friends at school, most of them get chest pain and the mere mention of something like dither and the die shortly after of bored. I have draw out my chain and got it all to 24-bit, pretty neat.
     
  13. IIRs

    IIRs Well-Known Member

    Isn't it? I don't see the distinction.. :?
     

  14. Check that to say dividing 3 into 2 (2/3) and Dwoz will be absolutely correct.

    Kids, there's bunches of dither going on all over your DAWs, trust us on this one.

    ~S
     
  15. dwoz

    dwoz Guest

    where's my damn proofreader!!!!!

    OHH YEAH. sent her out for coffee.


    dwoz


    WHERE'S MY DAMN COFFEE!!!!!!
     
  16. dwoz

    dwoz Guest


    my bad, I misspoke.

    "valid signal can actually be discerned below the level of the dither noise".


    that's better.

    dwoz


    awrrright, you nailed me, mr "get yer terminology straight". here's a cookie :)

    dwoz
     
  17. Zilla

    Zilla Active Member

    dwoz is correct.

    Possibly some are confused by the following common misconception: that the terms "dither" and "word-length reduction" are synonymous. They are not. Dither is indeed part of the process used in reducing word length. But it is used in many other digital applications. Already mentioned was how distortion from a/d quantization error is tamed with dither. Anywhere there is digital processing, there is a high likelihood that some dithering is happening under the hood.
     
  18. IIRs

    IIRs Well-Known Member

    A/D conversion can be thought of as wordlength reduction, no? :wink:
     
  19. dwoz

    dwoz Guest

    Depends on how you look at things.


    If you believe that analog audio has "infinite" wordlength, then by all means, yes.

    If you believe that analog audio has "undefined" wordlength, then you've got a bit of a semantic conundrum.

    Does divide-by-zero result in an infinite result, or an undefined one? is there really any difference?

    dwoz
     
  20. IIRs

    IIRs Well-Known Member

    Whether you call it "infinite" or "undefined" is irrelevent: there is information below the LSB level that can be partially preserved using dither.
     

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