Elgato TURBO.264 HD - Manual (Page 11)

Turbo.264 HD will not take a file with a smaller
resolution, and scale it up to a larger resolution. For
example, if you add a iPhone-sized file to Turbo.264
HD, and choose the 1080p export format, then the
resulting file will not be 1080p - it will still be iPhone
resolution.
Turbo.264 HD Manual
11
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Encoding Formats