System Configuration : System Setup : Configuring Pro Tools System Settings : Number of Voices

Number of Voices
(Avid HDX and Pro Tools|HD Systems Only)
On Avid HDX and Pro Tools|HD systems, the Number of Voices setting in the Playback Engine lets you control the number of available voices and how those voices are allocated to DSPs in your system. For example, the default number of voices on a Pro Tools|HD 1 system is 48 voices, using one DSP (at sample rates of 44.1 kHz or 48 kHz).
Changing the number of voices affects DSP usage, the total number of voiceable tracks, and overall system performance.
Depending on the session sample rate and the number of Avid HDX or Pro Tools|HD cards in your system, there are different choices for voice count. For voice limits on different Avid HDX and Pro Tools|HD systems, see Playback, Recording, and Voice Limits with Pro Tools HD.
To change the Number of Voices and DSP to allocate for voicing:
1 Choose Setup > Playback Engine.
2 Select the number of voices and DSPs to allocate for voicing by selecting a value from the Number of Voices pop-up menu as follows:
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Select minimum voice numbers if you are using high-bandwidth PCI or PCIe cards (such as video capture cards) along with your Avid HDX or Pro Tools|HD cards. These settings place the lightest processing load on each allocated DSP chip, but generally require more DSP chips be dedicated to voicing and mixing (leaving fewer available for plug-ins).
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Select medium voice numbers when your Pro Tools|HD cards are in an expansion chassis, or when you are using other PCI or PCIe cards along with Avid HDX or Pro Tools|HD cards. These settings generally provide an optimum balance between number of chips needed for voicing, and the processing load placed on each.
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Select higher voice numbers when your Avid HDX or Pro Tools|HD cards are the only PCI or PCIe cards in your computer, or when you are using an expansion chassis to run higher track counts (such as 64 tracks at 96 kHz) and you want more voices per DSP (such as 16 voices per DSP at 96 kHz). These settings use fewer DSP chips for mixing (leaving more available for plug-ins) but place the highest processing load on each.
3 Click OK.