What Guitarhacker said - choose ASIO, and make sure you're using the Focusrite for both in and out. If ASIO doesn't work properly, try opening the ASIO control panel and increase the buffer size bit by bit until it works properly. If it still doesn't work, try upgrading your driver from Focusrite.

MME is an older driver type - it has much higher compatibility and is more stable, but has a lot of latency. WAS or Windows Audio is newer and lower latency, but somewhat less compatible. ASIO generally has the lowest latency, but has the downside of requiring a specialized driver, and only one music app can access the ASIO driver at any given time.

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