Audio Dithering: What You Need To Know
Dither is one of the least understood words in the music-production lexicon, but it's an essential ingredient when working with digital audio, and one that you really should understand if you're involved in mixing or mastering.
So, what is dither? It's a form of low-level noise that is intentionally added to a digital audio file as it's rendered to a lower bit depth. The concept of dithering might seem counterintuitive, but it's an effective process. Dither noise actually masks what's called "quantization distortion," which causes noise and artifacts in digital audio. Before we go into the specifics, it's useful to quickly review some basic concepts about digital audio.
Sample Rate And Bit Depth
The sampling rate governs how many samples your converter captures in a second to digitize the audio, and is critical in terms of the frequency response. The general rule, based on the Nyquist Theorem, is that you must have a sampling rate that's greater than or equal to twice the highest frequency of the audio. Otherwise, distortion called "aliasing" can mar the sound.
To capture frequencies up to 20kHz, which is the high range of human hearing, the lowest sampling rate commonly used is 44.1kHz (the rate in the CD spec), but these days, rates of 48kHz or higher are also common. The higher the sampling rate, the greater the precision for reproducing the frequency range.
The bit depth (also referred to as "word length") refers to the size of each sample, in terms of the number of zeros and ones it can contain. It quantifies how precisely the amplitude, and therefore the dynamic range — which in this case can be described as the difference between loudest sounds that can be captured and the noise floor — will be represented when digitized.
The higher the bit-depth, the more precise the description of the amplitude will be. When you compare 16 and 24-bit audio, the difference in resolution is pretty striking. Sixteen-bit audio can represent up to 65,536 discrete levels of amplitude, whereas 24-bit has the capability for 16,777,216 levels. In terms of dynamic range, each bit is equivalent to 6dB. In 16-bit, that gives you a 96dB (16x6).In 24-bit it's 144dB (24x6).
When your DAW or audio editor quantizes the bits when converting from 24-bit audio to 16-bit audio (or from 32-bit fixed point to 24-bit, or any other such reduction), there are significantly fewer steps available to map the amplitude levels to. As a result, the rounding can cause something called quantization error (aka "quantization distortion" or "truncation distortion"), which manifests itself as low-level noise or distortion.
Your music usually masks it, but with 16-bit audio, you can sometimes hear quantization error on quiet passages, or when the music is fading in or out. The bottom line is that it's a form of distortion and you don't want it to be present in your music. Some engineers say that even if you don't actually hear quantization distortion, it makes the music sound harsher overall.
Dither To The Rescue
Another term you'll see in software and plugins that offer dithering utilities is "noise-shaping." That refers to what's essentially EQ for dither, which is designed to change the frequency characteristics of dither noise so that it's in a range that's least audible to the human ear.
All the Waves L-Series Limiters, such as this L3 Multimaximizer, are equipped with IDR technology for adding dither.
Because limiters are typically the last stage in the mastering chain before dithering, most mastering-quality limiters have built-in utilities for adding dither. The Waves Limiters all feature IDR technology, which stands for Increased Digital Resolution. It lets you add dither for a variety of bit-depth targets. You can choose from two different dither types.
Type 1 is designed to remove any quantization distortion from your audio. It's comprised of dither noise that's modified by noise-shaping. Waves recommends it for 16-bit and 20-bit processing. By adding the Type 1 dither, the 16-bit master that you're bouncing down to, will sound like it's actually 19-bit. In other words, you're gaining three bits of perceived resolution thanks to the IDR dither.
Type 2 adds less noise to your signal but doesn't completely eliminate the quantization distortion the way Type 1 does. So, the choice between Type 1 and Type 2 comes down to whether you want to completely eliminate distortion or get rid of most of it while adding less noise.
The Shape Of Noise
In this screenshot of the Waves L3LL Multimaximizer, you see the various IDR noise-shaping options.
If it's not practical or you don't have time to audition the different options, Waves offers the following suggestions: Use Type 1 with Normal noise-shaping for CD mastering. Use Type 2 with Ultra to add the least amount of noise to files 16-bit and greater.
Type 2 also supports "auto-blacking" which won't add dither to any silent sections of the audio. If your audio does have such a section, choose Type 2. To get the maximum resolution, choose Type 1 with Ultra.
Whether To Dither?
However, if you're bouncing your mix to a data-compression codec like MP3 or AAC, dithering is not necessary. They're both formats that introduce artifacts into the signal that dither won't remedy.
The best thing you can do when encoding lossy codecs like MP3 and AAC is to use the highest bit-rate you can and still achieve the file size that's required for your streaming platform. Don't confuse bit-rate with bit-depth. Bit-rate measures transmission speed for streaming. You'll most often see it expressed in kbps (kilobits-per-second) or mbps (megabits-per-second). The higher the bit-rate, the better the quality, and the larger the file size.
· Only dither when you render your audio to a lower bit-depth.
· Don't dither before converting to MP3 or AAC.
· Always dither if you're creating 16-bit files for a CD from a 24- or 32-bit mix.
· You don't need to dither when going from 32-bit floating point to 24-bit (because 32-bit floating point doesn't have a higher bit depth), but you do from 32-bit fixed point to any lower bit depth.
· If you're preparing 24- or 32-bit data for a mastering engineer, don't dither. Let your mastering engineer, who's an expert, handle it.
· Make sure that dithering is the last step in your processing chain. Never insert a processor after it in your DAW or audio editor.
· Try to avoid dithering twice, because the effect is to add more noise to your audio. That said, if you go down in bit depth twice during a project (which is somewhat unlikely unless you start at 32-bit fixed point), you should dither during each of those conversions.
· If you're confused about the various dither options offered, go with the basic default offered by your dithering software. If you're feeling more ambitious, compare the results with different dither and noise-shaping options selected to see which works best for a particular song.
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