HD I/O 
Whether you work with music or sound for picture, HD I/O gives you professional - grade audio quality, high performance conversion, and the flexibility to integrate with a variety analog and digital gear, so you can quickly adapt the interface to meet whatever need comes your way.
A sonic improvement over its audio interface predecessors, HD I/O features premium A/D and D/A conversion, so you can achieve higher audio fidelity, extensive dynamic range, super-low jitter, and the absolute lowest possible latency in your sessions. With certain configurations, you also get built-in sample rate conversion and a soft clip feature for worry-free recording. And using Curv, a new built-in soft-knee limiter that catches even the fastest transients, you can track hotter signals and smooth out inconsistent levels from incoming sources.
HD I/O comes in three configurations, so you can choose the interface that best suits your connection needs. What's more, you can expand your analog or digital I/O by simply installing an HD I/O option card into any empty bay. HD I/O 16x16 Analog cannot be expanded, as all four bays are occupied.
The perfect choice when you need a balance of analog and digital I/O for your music or post-production projects, with the option to expand your I/O.
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The ideal choice for music productions—get the maximum complement of analog inputs and outputs (can be customized by replacing a card, but not expanded).
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The ultimate choice for post-production studios, and ideal if you need to interface with a digital mixer, dubber, or other digital infrastructure, and want the option to expand the I/O.
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As your needs grow or change, you can easily add more analog inputs, analog outputs, or digital I/O by installing an option card. You can install one option card in HD I/O 8x8x8 and up to two in HD I/O 16x16 Digital. While HD I/O 16x16 Analog comes with the maximum four cards, you can customize your I/O by swapping out any card.
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HD I/O and all other Pro Tools, HD Series interfaces completely integrate with Pro Tools|HD, so you get maximum performance, dependability, and control of your inputs, outputs, and routing right from your Pro Tools software interface. You'll also get higher production quality and greater workflow flexibility than with previous Pro Tools interfaces. And if you want to use your interface with Pro Tools| HD and other DAW software, you can do that too, as all Pro Tools|HD systems support Core Audio and ASIO drivers.