Mixing : Mixdown : Using Dither : When to Use a Dither Plug-In

When to Use a Dither Plug-In
You should use a dither plug-in in any situation where you are reducing bit depth, for example, when mixing down to a 16-bit file with the Bounce to Disk command, or when sending your final mix to an external digital device that records at 16-bit resolution.
A dither plug-in is necessary even when mixing down to 16-bit from a 16-bit session. Even though 16-bit sessions use 16‑bit files, they are still processed at higher internal bit depths.
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Pro Tools|HD systems use 24-bit audio input and output signal paths, and internal 48-bit mixing and processing
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All other Pro Tools systems use 24-bit audio input and output signal paths, and internal 32‑bit floating point processing for mixing and audio processing
For this reason, whether you are working with a 24-bit session or a 16‑bit session, the following are recommended:
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When mixing down to a 16-bit destination, use a dither plug-in on the main output.
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When bus recording a submix to a track in the same session, do not use a dither plug-in on the destination track.
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When mixing down to a 24-bit destination, do not use a dither plug-in on the mainoutput.
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When mixing down to an analog destination with any 24-bit capable interface, do not use a dither plug-in on the main output.
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During normal recording and playback, bypass any dither plug-in on the main output.