Glossary
Phi Silica
Phi Silica is Microsoft's on-device language model, which runs on the neural processing unit of Copilot+ PCs. Microsoft says it is being replaced by Aion Instruct, and its voice typing help names it as the download to watch while fluid dictation gets ready.
Updated October 16, 20264 cited facts, each traced to its source
Phi Silica is a language model that Microsoft runs on the device. A language model is the part of an AI system that generates text. Microsoft says Phi Silica is optimized for efficiency and performance on Windows Copilot+ PCs, where it runs on the neural processing unit (NPU) 1.
Phi Silica is a developer model, and it is being replaced
Microsoft's guide to Phi Silica is written for apps built with the Windows App SDK. The Phi Silica APIs are part of a Limited Access Feature 2, so access is gated for developers. A note at the top of the same page says that Phi Silica is being replaced by Aion Instruct, a new on-device model 3. If you are building on Phi Silica, start with the replacement guidance on that page.
What it means for dictation users
Phi Silica is not a dictation app you open. Its one documented link to dictation is in Microsoft's voice typing help, which names the AI Phi Silica model as the download you can watch while fluid dictation is getting ready 4. Microsoft does not say in that help how the model is used inside fluid dictation. Treat the download as a sign that the model is part of that feature's setup, not as a full description of how fluid dictation works.
Because the model belongs to Copilot+ PCs, a PC without an NPU is not the target for this download. Check the page for each dictation feature to see what runs on the device and what needs a connection.
Related terms
- Copilot+ PC: the class of Windows hardware where Phi Silica runs.
- Neural processing unit (NPU): the chip that runs the model.
- Fluid dictation: the Windows feature whose setup shows this download.
- Windows key + H (Win+H): the shortcut for Windows voice typing.
Guides that cover it
Evidence
Every number on this page, with where it comes from. Open a fact to see the exact words in its source.
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Related terms
- Dictation on Windows: Voice Typing, Voice Access, AppsThe topic this term belongs to
- Azure AI SpeechAzure AI Speech is Microsoft's cloud speech service. Microsoft now calls it Azure Speech in Foundry Tools, and its speech to text powers the online recognition behind Windows voice typing, which needs an internet connection.
- Copilot+ PCA Copilot+ PC is a class of Windows 11 hardware with a neural processing unit (NPU) that Microsoft says can run at 40 or more trillion operations per second. Some newer Windows AI features, including fluid dictation, require this hardware.
- Fluid dictationFluid dictation is a Windows feature on Copilot+ PCs that corrects grammar, punctuation and filler words as you speak. Microsoft says it is powered by small language models that run on the device, and that you can undo a correction by saying Revert or Undo that.
- Neural processing unit (NPU)A neural processing unit (NPU) is a specialized chip for AI-intensive processes, such as real-time translation and image generation. Microsoft says many new Windows AI features require an NPU that can run at 40 or more trillion operations per second, and a speech model can run faster on one.
- Number and grid overlays (Voice Access)Number and grid overlays are Voice Access views for clicking things you cannot name. Show numbers puts a number on each clickable item so you can say it, and Show grid divides the screen into numbered tiles that narrow the mouse pointer step by step.
- Online speech recognitionOnline speech recognition sends your voice to a cloud service that turns it into text, instead of processing it on your PC. Microsoft says voice typing on Windows uses online speech recognition, which Microsoft describes as giving more accurate recognition.
- Speech pack (Windows)A speech pack is the set of on-device language files that Windows needs before it can recognize speech in a language offline. Microsoft says you must download a speech pack for a language in Voice access before you can use it, and the first download needs an internet connection.
- Voice Access (Windows)Voice Access is the Windows 11 feature that lets you control the whole PC and write text by voice, not only dictate. Microsoft says it works without an internet connection after setup, and it replaces Windows Speech Recognition.
- Voice typing launcherThe voice typing launcher is the Windows voice typing setting that lets you start dictation quickly when the cursor is in a text box. Microsoft lists it among the settings you open from the gear icon in the voice typing flyout.
- Windows key + H (Win+H)Windows key + H, written Win+H, is the keyboard shortcut that starts voice typing in Windows, so you can dictate into the text box where your cursor is. Microsoft says it does not start voice typing while Voice Access is running.
- Windows on ArmWindows on Arm is Windows running on processors that use the Arm instruction set instead of x86 or x64 chips. It runs native Arm apps and many unmodified x86 and x64 apps, and Microsoft says Arm-native apps give the best performance and battery life.
- Windows Speech API (SAPI)The Windows Speech API (SAPI) is Microsoft's interface that lets Windows apps use speech recognition and text-to-speech without building the speech engine themselves. Windows.Media.SpeechRecognition is a separate API for command and control in Windows Runtime apps.