System Configuration : System Setup : Configuring Pro Tools System Settings : Hardware Buffer Size

Hardware Buffer Size
The Hardware Buffer Size (H/W Buffer Size) in the Playback Engine controls the size of the buffer used to handle host processing tasks such as processing with host-based, or “Native” plug-ins (AAX and RTAS).
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Lower Hardware Buffer Size settings are useful for improving latency issues in certain recording situations or for improving certain system performance problems:
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On all Pro Tools systems, lower settings reduce MIDI-to-audio latency (such as when playing a virtual instrument live and monitoring the instrument’s output). Lower settings can also improve screen response or the accuracy of plug-in and mute automation data.
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On host-based Pro Tools systems, lower settings reduce all input-to-output monitoring latency on any record-armed tracks or Auxiliary Input tracks with live inputs.
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On Avid HDX and Pro Tools|HD systems, lower settings reduce monitoring latency that occurs on tracks that have one or more Native plug-ins. Lower settings can also improve the accuracy of MIDI track timing on systems without a MIDI interface that supports time stamping. Lower settings also improve MIDI track timing on tracks using MIDI virtual instruments that do not support time stamping.
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Higher Hardware Buffer Size settings are useful for sessions that are using more Native plug-ins for playback. These settings allow for more audio processing. They can also be useful to reduce errors on machines that require a higher buffer size.
To change the Hardware Buffer Size:
1 Choose Setup > Playback Engine.
2 From the H/W Buffer Size pop-up menu, select the audio buffer size, in samples.
3 Click OK.