Dithering necessary?

Discussion in 'Mastering' started by David Henderson, Jul 11, 2011.

  1. David Henderson

    David Henderson Active Member

    I've found that when a recording is made in an actual acoustic environment, dithering isn't necessary because the natural noise floor serves as a dither. Does anyone else share that view?

    I quit using dither when I found that the noise it adds is often audible. I use Waves L1, which does have fancier types of dither that are inaudible, but the user is cautioned to make sure that the program is never re-dithered or else there may be audible artifacts.

    Of course, a recording made in an abstract setting such as digital reverb fading to silence absolutely must be dithered, but that's the exception in most of my recordings.
     
  2. bouldersound

    bouldersound Real guitars are for old people. Well-Known Member

    Dithering is for covering up the artifacts of truncating from one word length ("bit depth") to a lower one. If you don't shorten the word length then you don't need to dither. Whether the low level signal in the mix is natural or artificial doesn't really matter.
     
  3. David Henderson

    David Henderson Active Member

    Well, naturally I have to truncate a 24-bit file to master a CD. But my point is if the natural noise floor is sufficiently high, which is not very much, dithering doesn't seem necessary. What am I missing?
     
  4. bouldersound

    bouldersound Real guitars are for old people. Well-Known Member

    The point you're missing is that truncating interacts with the existing low level noise to create unpleasant artifacts. What was random before now has a pattern as a result of the truncation. So you have to add a bit if noise to re-randomize that low level information and hide the artifacts.
     
  5. David Henderson

    David Henderson Active Member

    OK, thanks for the explanation. Now I have lots of questions. Dithering has always been a mystery to me.

    1) What do people do to dither the track without adding audible noise?
    2) If any artifacts do occur to due to truncating, will dithering it later smooth out the artifacts?
    3) If some tracks in a compilation use higher-order dithering (which Waves says should never be redithered or else it may cause artifacts) and some don't, does the mastering engineer have to evaluate each track and possibly dither each one separately?
    4) If a track is dithered and then faded in/out, does that fade now have to be dithered?

    Obviously dithering has always been a mystery to me.
     
  6. bouldersound

    bouldersound Real guitars are for old people. Well-Known Member

    1) Dithering adds noise at the very bottom, literally the lowest possible level that can be represented by 16 bit audio.

    2) That's what it's for.

    3) I don't know. There are some interesting issues there. For example, it might be good to go back to 24 bit files, but are they available with the mastering effects (eq, limiting etc.) or would you have to go back to unmastered mix files? I would lean toward remastering from the mix files if the end product was a compilation. I think redithering would generally not be good.

    4) I suspect the fade algorithm would take care of it. What you don't want is a fade or decay that gets truncated without dithering.
     
  7. Mo Facta

    Mo Facta Active Member

    The "natural noise floor" of 24 bit audio is quantization distortion, which manifests down around -144dBFS in a hypothetically perfect system. However, typically, the quietest of digital systems will exhibit noise floors of around -120dB or so, because of their analog components (preamps, microphones, etc) and their inherent self-noise.

    From Wikipedia:

    "In 1981, researchers at Ampex determined that a dynamic range of 118 dB on a dithered digital audio stream was necessary for subjective noise-free playback of music in quiet listening environments."

    What this ostensibly means is that we have plenty of dynamic range in undithered 24 bit audio to work without dithering until we get to the mastering phase, upon which time we apply dither to remedy the possible distortion gained by truncation of the signal down to 16 bits. In the real worlds, there are a few caveats, however.

    FWIW, it is not uncommon for certain high end converter/PCI card combinations to apply dither as a matter of course to signals during recording and often along applying noise shaping. Of course, this can be turned on and off, as you would expect with high end gear.

    Applying 24-bit dither to 24-bit audio should not exhibit obvious noise and should only raise the relative noise floor around 3dB. The advantage gained from this is that signals traversing below the noise floor will attenuate smoothly and with linearity, without any truncation distortion. However, unless you're recording very dynamic music, like jazz or classical, this is probably not necessary.

    16-bit audio, on the other hand, benefits from the effects of dither (and subsequentially noise shaping) quite immensely. This is because the 96dB of DR afforded to us can be quite insufficient to accurately represent some styles of really dynamic music, as described above. In that case, dither should be applied, to ensure signals attenuate smoothly below the noise floor. If the right noise shaping algorithm is applied, the resulting signal should sound more like 18 bits.

    While this is mostly true, I would say that if you are working in 16-bit, dither, along with noise shaping, can help you retain resolution immensely, especially with things like artificially generated reverb.

    FYI, here's a little more info on dithering that I posted on another forum (because I don't feel like writing it all again):

    Cheers :)
     
  8. David Henderson

    David Henderson Active Member

    Thanks, Mo. That's a lot of good infomation. The natural noise floor is was referring to is ever-present ambient noise in an actual acoustic space. I record most live stereo concerts in 24-bit and mix them to CD, and I never hear artifacts from not using dither in that case. Crowd noise would cover up any artifacts, but I don't hear it if the room was empty, either (such as a recording session). I don't like extreme dynamics and tend to control the high and low extremes in the mix, so maybe the "low" level is way above the norm where one would expect to hear problems. I know what truncation grunge sounds like, but I seldom have a problem with it.
     
  9. Mo Facta

    Mo Facta Active Member

    Ok, I see what you mean. But yeah, it would just mean that the intrinsic noise in your room is above the noise floor of your digital system. Most quiet rooms still exhibit 40dBSPL of noise. It would have to be a REALLY quiet room for you to hear quantization distortion on a decaying signal.

    Cheers :)
     
  10. David Henderson

    David Henderson Active Member

    ah-HA! That's what I was getting at. Most of the recording I do is in less-than-ideal environments, so I guess I've able to get away without using dither.
     
  11. bouldersound

    bouldersound Real guitars are for old people. Well-Known Member

    I think with most pop music the only places it really matters are the fade ins and outs.
     
  12. Michael Fossenkemper

    Michael Fossenkemper Distinguished past mastering moderator Well-Known Member

    It's a bad habit to do without at the final stage. Hopefully you won't always be working on less than ideal projects.
     
  13. David Henderson

    David Henderson Active Member

    Most of what I do is live concerts, and I'm always afraid of locking in a type of dither that will cause problems if any tracks are selected and re-dithered for an album. Is that a reasonable concern?

    For commercial albums with a low ambient noise floor that are intended to be the final version, absolutely I agree that dithering is necessary. But what if the ambient noise is louder than the dither would be - why add more noise?
     
  14. bouldersound

    bouldersound Real guitars are for old people. Well-Known Member

    Give the client unmastered mixes at the original sample rate and word length to use for such things. Pulling tracks from a CD "mastered" by you is not a very good idea for a number of reasons.
     
  15. David Henderson

    David Henderson Active Member

    Boulder, that's a perfect solution for a perfect world. I live in the real world, where a client suddenly gets the idea to make a highlights CD of concerts recorded over the last 10 years, and not all, if any, recorded by me. If I gave clients 24-bit original recordings of concerts on DVD, they would likely immediately lose it (or toss it) after finding they can't play it in the car CD player; it doesn't matter how much I explain, they don't get it. I keep hard drive archives for some clients, but I can't be a warehouse for every concert I record. Better to leave concert one-offs undithered, and save dithering for albums.

    And yes, I get it that you don't approve of me "mastering" my own recordings. This ain't Telarc, this is High School Band concerts.
     
  16. bouldersound

    bouldersound Real guitars are for old people. Well-Known Member

    Then I don't think the client will care or notice if it gets redithered, making all this angst a bit pointless. It's probably going to be ripped to 128k mp3 anyway.
     
  17. David Henderson

    David Henderson Active Member

    I'm not on an ego trip, I just want to know:

    What if the ambient noise is louder than the dither would be - why add more noise?
     
  18. bouldersound

    bouldersound Real guitars are for old people. Well-Known Member

    Because dither has to be added after truncation. It's the interaction of truncation with low level audio information that causes the distortion.
     
  19. David Henderson

    David Henderson Active Member

    I can't believe that I'm so stupid that I can't understand this. 24-bit can capture a dynamic range down to the -127dB practical limit. 16-bit lops off everything below -96dB. There aren't any analog mics I know of with a higher S/N ratio than 80dB, and most concert halls have an ambient noise level of -60dB or so (from the loudest bass drum whack to an empty hall). After limiting peaks and managing levels, the truncation happens a full 50dB below the ambient noise floor. Why do I need dither?
     
  20. bouldersound

    bouldersound Real guitars are for old people. Well-Known Member

    Even waves that peak at 0dBFS have to cross zero. That's where the distortion happens. You just don't hear it because it's masked by the loud sound. But when the levels are lower they don't mask the distortion so well.

    [Mods, the automatic wiki thing sucks. Why does a generic reference get changed into a link to a product I don't endorse? At least give me a cut of the profits.]
     
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