When to Apply Dithering?
There are a number of choices of dithering to pick from, and dithering is actually appearing in far more places than ever before, making it even easier to misuse and abuse this critical stage of sound production, reduction, and reproduction. If uncertain about what you’re doing with dithering, the best thing to remember is simply this:
Do not apply dithering to anything unless you’re at the VERY LAST STAGE of production, as you’re transferring your material to the format you will be listening to it in. In most situations, you only want to apply dithering once, and the time to do it is just as you’re going to the CD that will be the Master CD for duplication during Mastering.
A good rule of thumb is to apply dithering only when you absolutely MUST reduce the bit-rate of the material you’re working on. An ideal scenario when working in a DAW like Pro Tools (or any others) is to work in 24-bit and to mix down to 24-bit as well. There is no need to go to 16-bit until you want to make a CD to listen to your music, but that CD should be made from 24-bit masters that every DAW is capable of producing. There are far more choices of dithering and far more flavors of dithering than most are aware of, and the point of the following discussion by Bob Katz is to help point out why dithering can often be the most critical stage of production and mastering. Improper dithering can vastly change the sound of your material, and improperly or repeatedly applied, can introduce clipping, noise, artifacts, and all kinds of unpleasantness to mixes you worked so hard making perfect.
What is Dithering?
Noise-shaping is a method of improving the signal-to-noise ratio of dither, resulting in an SNR more like the higher resolution source. It can’t be perfectly done – if we could fit 24 pounds into a 16-pound bag we wouldn’t need the bigger bag.
Noise-shaping curves are tailored to the limits of human hearing. For 16-bit output, the object is to keep the total power of the curve at 16 bits, but to lower the noise in the areas where the human ear is most sensitive. In critical bands it is possible to lower the noise to 20-bit levels.
In Figure 1, an FFT illustrates the dilemma at three ninth-order curves: Meridian’s shape D (blue curve), POW-R dither type 3 (yellow) and Waves Ultra dither type 1 (green, and from its new L2 Ultramaximizer). Notice how each manufacturer follows the same general shape, but each one chooses fine variations on that shape. Noise masks signal in the same frequency band. Thus, in the frequency ranges where the dither noise is reduced, more of the source music and ambience is revealed (unmasked).
Different Kinds of Dithering
The best thing to remember is this:POW-R is short for Psychoacoustically Optimized Wordlength Reduction. POW-R dither is owned and licensed by the POW-R consortium LLC, a group formed by four independent audio companies Millennia Media, Lake DSP, Weiss and Z-Systems. The goal was to produce the most sonically transparent dithering algorithm possible. The process the consortium came up with is not just dither with noise-shaping, but a patent-pending approach shrouded in secrecy.
POW-R is similar to other wordlength reduction schemes in its use of a random noise signal as part of the signal/quantization error de-correlation process. How this noise signal is generated and what its spectral characteristics and statistical properties are, differentiates POW-R.
POW-R dither comes in three flavors intended to fit different kinds of music. The POW-R consortium will license the algorithm non-competitively to its own members and to additional licensees, including notable DAW, processor and converter manufacturers.
Dithering in Use
That’s the theory. My skepticism turned to wonder – my first reaction when I hooked up the POW-R box was “is this thing on?” I thought I was listening to the source! I rechecked my connections, inspected my scope and switched around until I was sure there was no wrong connection. Apparently the POW-R dither avoids major sound stage shrinkage and tonality changes associated with other dithers.
There are three types of POW-R dither. Previous experience has shown me that rock-n-roll music frequently benefits from a gentle noise curve or a UV-22 approach; I think these curves cover up some of the evils in the source and tend to be less edgy sounding on brashy material. Classical music tends to benefit from the ninth-order curves, like Meridian shape D, which has greater depth and width yet has a bit of a sheen to it that makes it unsuitable for bright music.
Because each dither affects the tonality of the sound, I integrate it into the mastering process.
Ideally, a dither should be tonally neutral, with the 16-bit final sounding like the 24-bit master. In the FFT of Figure 2, note that POW-R type 1 (blue) is a near-Nyquist dither second-order curve, which resembles UV-22 in its shape. Type 2 (green) is a slightly steeper fifth-order curve, which resembles Meridian type E. And type 3 (red) is the severe ninth-order curve, like Meridian shape D or the new Waves Ultra curve.
As a serious dither connoisseur, I expected to hear meaningful differences between the POW-R curves. But again, I was wrong. Using a delicate 24-bit classical piano solo recording, I could not reliably distinguish the three 16-bit curves from each other or from the 24-bit source! This has never happened before. I usually notice some degradation and a meaningful difference between any box’s curve shapes.
I tried an excellent recording of a country rock group, which has a lot more high-frequency information than the classical piano. This recording came in on 1/2″ 30 IPS analog tape and I had previously mastered it using various digital processors and Meridian D dither. Starting with POW-R type 3 (the sharpest curve), I was amazed at the clarity, depth and lack of congestion in the music.
With this music, I was finally able to hear the smallest difference between the 24-bit source and the 16-bit dithered version; it’s hard to describe, the 16-bit being a bit more closed in but a respectable facsimile of the original in all respects.
I then tried POW-R type 1 and heard a bit more degradation compared to type 3, but nothing to write home about. I certainly can’t use POW-R for tone control; it may soften the sound, but more subtly than its predecessors.
Basically, after a few tests, I decided that the way to use POW-R dither is to stick with the best-sounding curve; POW-R Type 3. This is not the dither to use if you need to cover up some evils in your source.
POW-R neither disguises nor enhances any part of the spectrum.
We can also conclude that the effect is music-dependent; complex stereophonic music appears to expose dither problems more than simple. As a control, I compared the POW-R against my reference dither, Meridian’s shape D. I had originally chosen shape D over other dithers because it revealed more of the source and added a little bite, which was desirable with this music. So I was concerned that POW-R’s tonal neutrality might produce a more recessed sound. I found that POW-R reveals so much more of the source that I no longer needed the Meridian’s artificial edge – I was obviously using the edge to make up for the its lack of resolution. Instead, the transparency of the original source carried the day.
I really started noticing the congestion of the Meridian algorithm, yet the Meridian’s resolution is slightly better than other dithers. It’s all a matter of degree, but it didn’t take long for me to conclude that the POW-R dither is better than anything else out there. Negligibly different from the source, it’s an incredible achievement.
Putting this in perspective, many people are insensitive to the virtues of the different dithering systems. What order of magnitude of differences are we talking about? If you are inclined to music that has depth, space, clarity and purity of tone, you will recognize the qualities provided by a superior dither. A high-resolution monitor system is useful to distinguish the sound of subtly different dithers from each other and from the source.
Examples of White Noise Dithered in Various Ways
Below, you’re not looking at anything other than Noise Floors of the music in relation to frequency. In other words, using the POW-R3 as an example, the noise floor, after dithering, sits at about-140db across most of the sound spectrum, but drops to -165db around 3K, and sharply increases to about 105db from 12K to 20K. Since our ears are not too sensitive to noise in lower frequencies, very sensitive around 2K â€“ 5K, and and less sensitive at higher ones, the noise curves are created to “hide” the noise created from reducing word-lengths from 24-bit to 16-bit in places our ears don’t hear (WOW!).
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Bob Katz “POW-r Dither“