January 11, 2020

The conundrum of Nikon Z6 ProRes RAW

Zoomed and scaled view of a zone plate test shot (Nikon Z6, photo mode - no video!)

Ever since Nikon and Atomos annonced the availability of the Apple ProRes RAW codec for Nikon Z6 (and Z7) cameras back at CES 2019, the more curious-minded among us were puzzled.

How can that be? Not the part that a RAW codec via external recording over HDMI was made possible. That part of the announcement sounded fairly straighforward. After all, a RAW codec only contains one channel per pixel. So, even at 12 Bit, there should be ample of bandwidth left in the HDMI specification - even when going beyond 4k. To be precise, HDMI 2.0 supports up to 18 Gbps. a raw 30fps 4k (UHD) 12bit signal is 2.78 Gbps. Even 6k raw adds up to 6.26 Gbps. So enough bandwidth indeed.

The puzzling part was that Atomos and Nikon announced the external raw feed would be 4k. Apple ProRes RAW is compressed but unbayered pixel data directly from the sensor. But it is known that the Nikon Z6 does a full ~6k sensor readout to downsample its video feed. The Nikon Z6 has no 4k raw data! How can that be? We're eventually going to answer that now!