AF’s Weblog

February 25, 2011

Much Ado About Dithering

Filed under: Mastering — Tags: , , , , — audiofanzine @ 10:31 am

It’s a dirty job to go from high-res audio to 44/16, but someone’s got to do it.

The ultimate form of digital audio used to have a 16-bit word length and 44.1 kHz sampling rate. Early systems even did their internal processing at 16/44.1, which was a problem — every time you did an operation (such as change levels, or apply EQ), the result was always rounded off to 16 bits. If you did enough operations, these roundoff errors would accumulate, creating a sort of “fuzziness” in the sound.

The next step forward was increasing the internal resolution of digital audio systems. If a mathematical operation created an “overflow” result that required more than 16 bits, no problem: 24, 32, 64, and even 128-bit internal processing became commonplace. As long as the audio stayed within the system, running out of resolution wasn’t a problem.

 

Nowadays, your hard disk recorder most likely records and plays back at 24, 32, or 64 bits, and the rest of your gear (digital mixer, digital synth, etc.) probably has fairly high internal resolution as well. But currently, although there are some high-resolution audio formats, your mix usually ends up in the world’s most popular delivery medium: a 16-bit, 44.1kHz CD.

What happens to those “extra” bits? Before the advent of dithering, they were simply discarded (just imagine how those poor bits felt, especially after being called the “least significant bits” all their lives). This meant that, for example, decay tails below the 16-bit limit just stopped abruptly. Maybe you’ve heard a “buzzing” sort of sound at the end of a fade out or reverb tail; that’s the sound of extra bits being ruthlessly “downsized.”

Dithering to the Rescue

Dithering is a concept that, in its most basic form, adds noise to the very lower-level signals, thus using the data in those least significant bits to influence the sound of the more significant bits. It’s almost as if, even though the least significant bits are gone, their spirit lives on in the sound of the recording.

Cutting off bits is called truncation, and some proponents of dithering believe that dithering somehow sidesteps the truncation process. But that’s a misconception. Dithered or not, when a 24-bit signal ends up on a 16-bit CD, eight bits are truncated and never heard from again. Nonetheless, there’s a difference between flat-out truncation and truncation with dithering.

Now let’s take a closer look at dithering…

Dithering Rules

The First Law of dithering is don’t dither a signal more than once. Dithering should happen only when converting a high bit-rate source format to its final, 16-bit, mixed-for-CD format (and in the years to come, we’ll probably be dithering our 32 or 64-bit internal processing systems down to 24 bits for whatever high-resolution format finally takes off).

For example, if you are given an already dithered 16-bit file to edit on a high-resolution waveform editor, that 16-bit file already contains dithered data, and the higher-resolution editor should preserve it. When it’s time to mix the edited version back down to 16 bits, simply transfer over the existing file without dithering.

Another possible problem occurs if you give a mastering or duplication facility two dithered 16-bit files that are meant to be crossfaded. Crossfading the dithered sections could lead to artifacts; you’re better off crossfading the two, then dithering the combination.

Also, check any programs you use to see if dithering is enabled by default, or enabled accidentally and saved as a preference. In general, you want to leave dithering off, and enable it only as needed.

Or consider Cubase SX, which has an Apogee-designed UV22 plug-in. Suppose you add this to the final output, then suppose ou add another plug-in, like the Waves L1-Ultramaximizer+. This also includes dithering, which defaults to being enabled when inserted. So, check carefully to make sure you’re not “doubling up” on dithering, and disable dithering in one or the other.

Dithering dans Cubase

If you insert dithering in Cubase SX, it defaults to being enabled. So if you use this, make sure that any other master effects plug-ins you add do not have dithering enabled (in this screen shot, the WAVES dithering has been turned off). Or, disable Cubase’s dithering section and use the other plug-in’s dithering instead.

The best way to experience the benefits of dithering is to crank up some really low-level audio and compare different dithering and noise-shaping algorithms. If your music has any natural dynamics in it, proper dithering can indeed give a sweeter, smoother sound free of digital quantization distortion when you downsize to 16 bits.

To read the full detailed article see:  All About Dithering

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